Building 'Theophilus' Kiln', Part 1 - The Structure

I'm having quite a lot of fun this evening playing with clay. It's difficult to call it anything but play -- I have minimal clay training or experience. I think it will enrich my master's degree thesis, however, if I construct a medieval-style kiln to understand that experience and process. 

I've only found historical information on two kilns -- One is the kind documented in Theophilus and which has a recreated example in the Centre International du Vitrail just next to Chartres Cathedral, in France. The other is a late Gothic/early Renaissance brick kiln from Amsterdam, documented in a manuscript in Antwerp which was transcribed in 1974 and analyzed by Joost Caen in his doctoral thesis and related publications. Of the two styles, the earlier is more appropriate for my formal research but I'm fairly sure I'll have to try the later model for SCA purposes before too long.

"Chapter 22. The Kiln in Which Glass is Fired

Take some flexible canes and fix them in the ground in a corner of the building with both ends aligned to form a series of arches, a foot and a half high, the same in width, and a little more than two feet long. "

The skeleton of this project involved a number of necessary modifications. I was building this kiln in my garage, and while 10-cane could probably handle the impact, I am just not strong enough to get them into the concrete floor. Further I'd love to be able to take this to SCA events or similar educational activities, though at this point I probably can't lift it alone. From the start some kind of platform was planned. I used hardwood/birch plywood as the base and attached four corner brackets on the bottom. At the time I thought I could use the brackets' holes to string rope for lifting, and now I laugh at the folly of youth. They do serve their secondary role of table legs quite well, with no apparent strain yet. 

The top surface of the board I covered with thinset (cement) and then covered it with stone tile. I'd like to avoid doing all this work again for a couple of years at least, and I hope the stone level between the kiln and wood will add some longevity to it.

I have some limited experience working with clay through the Chicago Mosaic School's fantastic classes [Go check them out, no I am not paid to say this], so initially I threw a bit of clay down, tried to practice wedging, building clay, really playing with it to get the sense of how strong it is in different ways. This wasn't related to the project in any way, but I mention it because it inadvertently gave me observations later along that I would have missed. 

And for 30 blissful minutes I was a 12-year old with Legos and Play-doh

I asked friends for suggestions where to buy basketweaving canes, and this lead to the generous Donna Weeks coming way out of her way to give me a few hours of basketweaving education AND spare materials. I knew nothing when I placed my own order before this demonstration, and figured size '10' canes (the largest) would be the most appropriate choice. Every basketweaver who asked, I told this to, and universally everyone was shocked. I understand this now, I'm pretty sure size 10 cane is produced by folks from the rebar plant in their free time. Much good came from this lesson, however, and several hours of wasted time avoided.

I had intended to stick the ends of the canes into clay, and allow the clay to hold it in place. "Allow" as though I have any real say in the matter, when in fact the 10-canes were giving the orders. The clay was not going to hold the canes in form (yes, soaked). I continued with the idea of making a "basket upside-down" and made a wooden shape to hold the ends of the canes, which stood in perfectly for packed-dirt in my glazier's shed outside Sées... oop, sorry, I'm back. Here came an important lesson: When I said I made a wooden shape, that's probably barely legally defensible. I used a chunk of MDF I had in the corner of the garage, without thinking about it at all. Cut an oblong horseshoe-shape out, drill holes just big enough for 10-cane. Those canes, which had been soaking in water. Onto a bed of clay, fresh out of a sealed bag. That's a lot of moisture hitting thin MDF from all sides.

So it began disintegrating very quickly and I had to start tying reeds together to try and maintain the shape.

Oh, also, I took the opportunity to practice basketweaving. Don't hold the results against my teacher!

So yes, use real wood.

"Then vigorously knead some clay with water and horse-dung in the proportion of three parts of clay to a fourth of dung. After kneading this very well, mix with it some dry hay and make long loaves out of it. "

I'm sure I'm not alone in that every once in a while I stop to examine my life and try to figure out what specific mis-step lead to voluntarily spending a nice fall Saturday trying to track down horse excrement for my latest mad engineer experiment. After contacting local horse ranches/therapy facilities, most of whom were more than willing to let me cart of their refuse, my friend Kate came through with an offer to assist a few days sooner than the horse facilities could, which meant a whole weekend more I could get done. I took a plastic tote (15 gallon size, I think) lined with an industrial garbage bag, got to meet the horses, and drive off into the distance with a considerable amount of road apples. Trunk apples?

Three parts of clay to a fourth of dung. The interesting thing is when measurements are given in "parts" it's usually because volume and weight measurements are interchangeable (think baking, where many ingredients are so similar). I could not say the same about pottery clay and horse leavings. I have a possible side-project of testing both volume and weight compositions, if I can figure out the best way to do such a test.

I'm going to talk a bit about making and working with this clay mixture. However, since I know the subject can cause a visceral reaction to some people with sensitive stomachs, everything from this paragraph to the next picture should be black-on-black text. You can read it, if you want, just by highlighting it.

Edit: Ok, that didn't work. FROM HERE:

The before-mentioned Donna came up with the great term cloop, a portmanteau of clay and poop that is much less evocative to the imagination. Some people seem to consider this a form of cob (as in cob ovens, cob buildings) but I'm not finding highly consistent usage. Cloop, it is.

If this whole idea is leaving you with doubts, let me assure you it is not remotely as bad as you would expect. The faint smell in the close vicinity of the tub disappeared in a day. The first several days I worked on it, I wore latex gloves which inevitably tore up within ten minutes. Eventually I gave up and took relief that I had filled a bucket with water beforehand I could use to scrub a millimeter off my hands before going back inside. Nothing about it is reminiscent of dog, cat, human, etc. waste, and really it feels and looks like something you'd buy at the garden center, like a fine mulch. 

I went with the idea to go by volume. It seems widely-accepted that the organic additions of dung and clay are to build pockets of air to change the clay's thermal properties. Volume seemed more natural for that choice. I did not use any modern measuring tool and just did it by visual approximation.

For the longest time I would tear out a fist's worth of clay, flatten it into a flat disc (it took me seven tries to find a phrasing that was in no way connected to food). I then added the equine component... and started mashing it. I think that the most effective technique I found, at least initially, involved rolling the cloop into tubes, folding them in half, and repeating the process in order to get many layers. Then just a bit of random squishing. I didn't decide this until later, but the easiest way to add a reasonable amount of hay was to put hay down on a workbench and just dredge the cloop like breading (Oops, sorry). Kneading it in with no particular technique yields a sturdy ball or log of material quickly. I noticed that after an hour of making these I stopped really measuring it all out. The cloop has a certain visual quality to it, and the hay adds so much rigidity, you can quickly just get a feel for your normal. 

I did try, one time, to mix it en masse in a bucket. PLEASE, do not try it. The only problem with hand mixing it as above is that your hands will get tired quickly. I did find that taking larger (say double) portions and kneading it on a covered workbench like dough (sorry). It felt like it went quicker without straining my weak nerdy limbs. 

Thanks to my initial goofing off with the clay, I realized how different the two are in properties. When I made a slab of plain clay, it dried while I worked with it and was not easily indented hours later. When I made the base slab of cloop, it was easily indented 36 hours later (both were uncovered).

To HERE.

Cloop!

While I was working on the base, I was plagued by a beast which is famous for having plagued monk-scribes.

"Cover the arch of canes inside and out to the thickness of a fist and leave a round hole in the center of the top, big enough to put your hand through. Also make three iron bars a finger thick and long enough to reach through the width of the kiln. Make three holes for these on each side of the kiln so that you can put them in and take them out whenever you wish."

At this point, it's a building montage.

This is the view peering down into the chimney which can be seen in other pictures in the back of the roof

The walls are between 2" and 3" in thickness everywhere I measured. The cob oven bakers in my life said they aim for 2½" to 3½", and beadmaking furnace friends said at least 2", so I feel like this is a solid range. 

I'm trying to let this boulder dry naturally for a few days, occasionally hit with a heat gun, before I follow Theophilus' last step:

"Then put fire and wood into the kiln until it is dried out."

For Part 2, I plan to write-up firing the kiln (including its door) and tests on how long it takes to heat up, likely using various fuels, etc. I'd like to have a fairly predictable firing idea even without modern measurements, and I don't have 4-6 free years to fake an apprenticeship and learn the timing by trial and error. I have 4 high temperature thermoprobe sensors I can use to take some measurements. 

For Part 3, I plan to document firing a piece of glass in it, or several sample chits using period formulas which I'm incorporating into my MA thesis.

Why and How Glass Breaks

I spent far too much time last week contemplating why we apply pressure on the opposite side of a score to break glass, and had to do a few more ‘GLASS IS NOT A LIQUID’ screeds. I decided this was worth a blog post. THE SCIENCE OF WHY AND HOW GLASS BREAKS.

1 - What Glass Is!

Please, repeat after me, GLASS (clap emoji) IS NOT (clap emoji) A LIQUID (clap emoji)! In grade school we learned that there are three (or four) phases of matter, right? Bad news, Mrs. Thompson lied to you. Ok, didn’t lie, just grossly oversimplified. 4th graders probably won’t understand the degradation of neutrons.

Solid – A uniform and dense configuration of tightly-bound molecules gives us rock, wood, ham sandwich, our skulls, etc.

Liquid – Less tightly bound and it’ll slosh around in your whiskey glass

Gas – Freer still, and it’ll fill up a room or otherwise expand to fill a container.

You might have also gotten to Plasma, which is the bright flash of lightning during a storm that ruins your day if it hits you.

Glass doesn’t fit neatly into any of these, so over the years various stories have developed. It must be a liquid, it must be a solid, etc. The truth is there are a lot more than 3 or 4 phases of matter, and glass is in a category of its own. It’s an ‘amorphous solid’ but that puts it in its own category along with degraded neutrons and other more unique things.

1b – Glass is NOT A LIQUID, pt 2.

The amount of time it would take for a glass object to ‘melt’ is something like ‘the age of the universe’. There is a very common misconception that old window glass is thicker on the bottom because it has flowed that way. If that were true, we wouldn’t have glass objects from Ancient Egypt and Rome. People wouldn’t have heirloom glasses older than grandma’s house with the weird windows.

The reason there is a thicker part to the window is that older methods of making glass sheets (e.g. the crown and cylinder methods) produced a sheet of irregular thickness. It’s some kind of ‘common sense’ to put the thicker part down for stability, but it’s not a rule and contrary examples exist.

1c – Wait, you didn’t really explain what glass is

Glass is solid, in the common sense that it’s not a liquid, gas, or plasma at room temperature (on earth, etc etc). But its structure is very disorganized, which is why it has similarities to a liquid. If its molecules were more orderly you would have "devitrified glass" ("de-glassed glass"), which qualifies as a ceramic.

Because of that attribute, a sheet of glass has strains within in. Various internal pulls and pushes and twists, so to speak. When glass is annealed a great deal of the stresses are able to resolve themselves, but not all. This is also why glass doesn’t have a grain like wood.

[Update: I thought it might be good to note that while current science identifies glass as a separate phase of matter, earlier academic documents would refer to it as an "amorphous solid" and, going earlier still, as a liquid. I believe that was accurate (for the time) using a very scientific definition of liquid as opposed to the common understanding. However, "Glass is NOT a liquid!" is much more concise than "Glass does NOT melt into new shapes over the course of one or two hundred years because it is not a liquid in the layperson's meaning of the term which has been superseded anyway by scientific progress and the inexorable march of time!"]

2 – How glass breaks, unsupervised

As is usually the case, force likes to take the Path of Least Resistance through glass. If a substance has a grain, like wood, you may be able to guess how it would split if hit with a general force. But in the case of glass, there is no grain and the stresses are mostly invisible[0]. As a result if you hit a piece of glass the fracture lines will follow the path of least resistance taking advantage of these faults and weaknesses until it reaches the edge of the sheet.

It's interesting to note that research (source below) shows you can apply less force than is needed to run a score if you apply it for a longer amount of time, even as much as 50% less. The authors note that if glass is under strain, it REALLY needs to be protected or minor damage may destroy it.

3 – How glass breaks, supervised

All we are doing when we break a piece of glass is coaxing the path of least resistance to be a route we want. I’d say ‘choose’ but if you’ve spent more than five minutes breaking glass you know it’s more of a coaxing and/or tearful pleading.

A typical modern glass cutter has either a steel (cheap) or tungsten carbide (much better) wheel that is shaped like a chisel. Incidentally the angle of that ‘chisel’ MAY have an influence on the cutting. I had never ever heard of this before until someone mentioned it (from a Wikipedia article). That article refers to a book on automotive glass and says a more sharply angled wheel cuts thicker glass better, from 120° - 154°. I don't work with industrial glasses so maybe that is common knowledge to them, but 154° is oddly specific to me.

Bob Beranek; Ann Schuelke (1 August 2011). The Complete Guide to Auto Glass Installation. AuthorHouse. p. 336.

Both steel and tungsten carbide are harder than glass, and so they are able to bite into the surface of a sheet. Carbide is just much harder than steel, so it is more durable. Usually steel wheels are found on cheap cutters you get at a hardware store. Carbide wheels are what you would expect from a specialty store and most anything directed at the stained glass market. Carbide cutting wheels also almost always have an oil reservoir in the handle.

Why oil? Its more to do with the tool than the glass. The stress of scoring glass builds up heat, which could eventually mess with the tempering of the metal wheel. It won’t generally be enough to cause thermal shock in the glass, but little glass particles are also caught in the deposited oil and that’s a good thing.

Fun fact, researchers (source below) discovered that the pressure applied to your cutter has an S-shaped effect on the force required to run the score (that is, break the glass). Once you reach the minimum force needed, adding pressure (pushing down your cutter harder) actually increases the torque needed to break it... until you blow past that zone in which case more pressure makes it easier again.

3b – Bringing it together

When we ‘cut’ glass we are actually introducing a known fault line into the glass in the hope and expectation that, when flexed, the path of least resistance will follow our score. And we all know that, sometimes, it doesn’t. Maybe a curve was too sharp and the PoLR was a straight line. Maybe a seed (bubble) was present that coaxed the line askew at a point, and then the PoLR is to follow the new angle instead of bending back to the score. Those invisible stresses can also be at fault, because glass is a temperamental mistress and lives off our tears. Wait, what? </Bitter Glazer>

4 – So, why DO we press from the opposite side of the cut.

It bothered me that it made sense in my mind, but I couldn’t explain it in words. I actively chewed on this for the better part of a week, thinking in terms like a race track. If you’ve seen one, you probably noticed that the starting line is staggered, and the closer to the center you are the further back you begin. This is because the center ring is smaller, and shorter, than the outer rings so that racer has to be further back to get the same distance. I was trying to leverage that rationale to explain breaking a flat sheet of glass and it didn’t quite work until I stopped thinking about a donut-shape.

 Not my best drawing, but showing that running pliers (or your thumbs) applying pressure on the cut effectively applies pressure to squeeze the score line. You might get the glass to break but it will be uncontrolled and unguided and probably isn't following your score. Note that the score is magnified A LOT in this drawing.

Another little sketch showing a greatly magnified score under no pressure, some sort of light pressure, and finally breaking pressure.

Incidentally this CMoG blog post has an even better graphic of this I found two days later, using foam.

5 – Bonus: Why your hand position is so important when cutting

I originally learned to cut glass by holding my cutter like a pencil. It's not quite that relaxed or the wheel wouldn't be able to contact the glass, but similar. I also noticed my accuracy was never where I wanted it to be, and I was always grinding pieces, finding new ways they didn't fit my pattern, grinding other areas of the piece, and eventually throwing it into scrap. After I began teaching with Molly I learned from her the "peace sign" grip that is pretty universally called for in professional-grade books.

  

My hand holding my favorite  cutter in a comically relaxed pencil grip, a much more likely pencil grip, and finally a 'peace sign' grip. The camera was not straight on, but you can still clearly see how the first two are further off-center than the last.

 I haven't found solid research into this, but I strongly believe that when you hold your cutter off-center, the score itself (and the associated "microfractures" if you believe that idea) are also at an angle. Least resistance being what it is, I suspect it would be more like a J-shaped curve than an actual diagonal, but you end up with a cut that is not perpendicular to the surface of the glass, what I know as a shelf [1]. Shelves are ridiculously sharp and seem to disappear if you even bring them near a grinder. I think that this is why my accuracy was poor for a long time;

You can see the 'shelf' on this piece, right above the little bump. It took me seven tries to hold a cutter wrong and get this piece

Holding the cutter off-center isn't going to give you a shelf every single time, or I would never have lasted this long in the hobby, but for me there was a major reduction in shelves and an increase in accuracy when I learned to use the better grip. Shelves are ridiculously sharp and will cut through your copper foil and your fingers effortlessly, not to mention (again) mess with the size of your piece. Probably a half-dozen times I have cut myself on a shelf-y piece and watched the delay as my skin realizes it's been cut, recovers from its shock for a second, and THEN starts bleeding. 

Allegedly neurosurgeons use obsidian scalpels because glass can be 100x sharper than steel. I believe it can be sharper than steel but about a year ago when I tried to investigate this, it seemed these scalpels were only an experiment.

6 - Really Cool Bonus: Glass Heals

I had heard Molly talk about this fact one summer while we were teaching. I'll be honest, I didn't believe it. How can a glass score heal and become more difficult to run if you let it sit for a few days? While doing other research, however, I found an academic paper from 1937 (Info below) that tested and proved the concept.

Their research showed that if you scored glass and let it sit, the amount of force needed to run that score increases over time, maxing out around 16 days. By the third day it's done the vast majority of its 'healing', however. Their hypothesis is that over the course of days the innate stresses in the glass will work themselves out a bit and reduce the effectiveness of your work. 

In conclusion, glass has a number of odd behaviors that bring to mind the expression "wibbly wobbly timey wimey", we work with the glass and beg it to do what we want, and windows aren't melting because GLASS IS NOT A LIQUID.

--------------------


[0] - There are polarized filters you can look through that will show the strains, but I don't THINK they are practical for non-scientists and I'm pretty sure they aren't the same thing that lets you tell which side of float glass hit the tin.

[1] - I am finally reaching that age where I can't quite remember whether I came up with this term myself or if I learned it from Norma/Danielle at GlassLink a decade+ ago.


Sources:

https://www.scientificamerican.com/article/fact-fiction-glass-liquid/

https://www.scientificamerican.com/article/is-glass-really-a-liquid/

https://en.wikipedia.org/wiki/Phase_(matter)

https://en.wikipedia.org/wiki/State_of_matter#Glass

https://io9.gizmodo.com/the-glass-is-a-liquid-myth-has-finally-been-destroyed-496190894

http://users.monash.edu.au/~ralphk/glass-cutting.html
(I'm not fully convinced on several concepts Mr. Klimek writes about, such as using spit to help run a score or the idea of microfractures, and we now know that glass is not a liquid but the anecdotal information is entertaining.)

"Concerning the Cutting of Glass" by MERTON W. JONES AND JULIAN M. BLAIR
Journal of Applied Physics 8, 627 (1937); https://doi.org/10.1063/1.1710352

https://blog.cmog.org/2015/06/03/part-2-why-does-glass-break/

Research tips and tricks for SCAdians!

[Moving this here, since Facebook keeps deleting this post. I'm telling people how to find the information without stealing, and they claim I'm telling people how to violate Intellectual Property!]

--- Google "Dorks", advanced operators for Google searches and many other engines. You probably know making something "a quote" tells Google to return that exact phrase. You can also control results by the website, or phrase in the address, filetype, and more. For example, I'm very tired this morning of seeing pinterest:
"search term" -inurl:pinterest
This will return results with "search term" where the address does NOT include 'pinterest'. This is handy for removing ALL of the domains (.com, .ca, .co..uk, etc)
You can also search a specific site with "site:" or exclude it with "-site:"
Only want a specific filetype? Use it's common Windows extension, such as "filetype:pdf" to only return results within PDF files.
Others:
Related:domain.com (for finding websites 'related' to another)
cache: (for finding Google's last copy of it, see also Wayback Machine below)

--- Google Reverse Image Search, at https://images.google.com/, lets you search by uploading an image. Trying to find the source of a picture for attribution or to get permissions? Try and upload it here. If you use Google Chrome as your browser, you can also right click the image and just hit 'S' and this is done automatically.

--- Wayback Machine, provided by Archive.org, allows you to enter a website and see old cached copies of sites which may have been offline for years. A precious bookmark or unique site go down? Try this! The website also hosts a large amount of public domain content like books. I use it several times a week.

--- Academic Publishers pretty much look like a scam to me. I was trying to track down an article and one major publishers site would sell a copy to me for $25, another for $10. The website for the actual journal it was published in offered it as a free download without even a login. If you are not blessed to have well-connected academic or librarian friends (I won't name you for your sakes, but you three know who you are and I love you), many times you can get free access to massive academic databases by using the computer at a library or college campus. It may even work just using the wifi anywhere on site. If you are near an academic hospital, it may even work there, too.

One local academic library has great access, but no ability to save/email documents. I asked the Librarian if I could print the most important two and she said "no one prints here anymore, use all the paper you want".

On that note, many libraries which may seem closed are not if you ask nicely. Libraries associated with colleges and museums may easily give you access upon request. The academic library I just mentioned is in a museum, but an e-mailed request gets you a free pass to go ONLY to the library inside.

Your Public library may also provide patrons with database access (Ebsco, Jstor, etc). Check the website or consult your local reference librarian-- you might not even have to be at the library! Academia.com is also a good source for papers.

As a last resort, I have had great success just e-mailing the author. They are almost always allowed to give you a copy, even if it's only otherwise available through a paid publisher. I spent weeks trying to get a copy of a doctoral dissertation, including offering to pay fellow SCAdians to physically go to the university and scan their archive copy. The author kindly e-mailed me his second-to-last draft, the only copy he had on hand.

--- Speaking of Libraries, I've also had repeated good results e-mailing a Library and saying "You have this rare book and I would really like these 4 pages from it, is there anyway I can request photocopies? I'll gladly pay your xerox fees, postage, etc." has always gotten a reply with a PDF attached.

--- The term "Independent Researcher" carries a lot more power than "obsessive nerd with a burning passion for this subject" trust me on this. Seriously, though, many academic websites recognize "Independent Researcher" as a status/position, It's what you are, it's dignified, use it!

--- Storing your documents online is very handy. Someone (Jen? Dena?) pointed me toward Mendeley, an online 'citation manager'. Except it also lets you store and organize the entire documents too. I currently have 500 research papers and ebooks organized and searchable, and available anywhere with internet access, for free.

--- Did you know Interlibrary Loan (ILL) can be arranged even out of state? I didn't until recently. It all depends on your library, but you may just have to pay postage for the privilege.

--- Google Scholar: You can filter by content types, subscribe to alerts when something new matches your favorite search, and tons more! (my most-hit search involves a particular set of includes and excludes for digital preservation, for example.) http://scholar.google.com (From Dena)

This is not an exact match for all-museum searches, but you can do CC0 image searches here: https://search.creativecommons.org/ - Europeana is a huge collection of digital images that are largely European museum affiliated? (From Aurora)

A Glass Chemistry Primer

[Work in Progress! Likely revised often in the next week. My tables were broken when published, I will revise them shortly - Brynn 8/7/18]

Components of Glass
Formers - The material that makes up the bulk of the actual glass. Most commonly this is Silica Dioxide (SiO2), as in soda-lime glass. Aluminum fluoride and zirconium fluoride are also options.

Fluxes - As in other trades and crafts, fluxes lower the melting point of the former(s) involved. Feldspar, soda, natron, and potash are all fluxes.

Stabilizers - These materials impact a variety of attributes of the glass, like ability to withstand weathering. Limestone (calcium carbonate) is a stabilizer. Sodium and Magnesium are water-soluble and can leach out in water without stabilizers.

Colorants - Optional components to add (or remove) color and opacity to the glass. Mostly metal oxides. Gold, silver, copper, nickel, cobalt, iron, and many others act as colorants. Lead acts as a decolorant. Tin and antimony are opacifiers.

Fining Agents - Optional compounds to remove bubbles, e.g. arsenic trioxide.


Types of Glass
Soda-Lime - The type we encounter every day, roughly 90% of all glass produced. Silica’s very high melting point is lowered by the presence of soda (Sodium carbonate). Different formulations are used for flat (or float) glass vs other containers.
Potash-Lime/Forest glass - Potassium Carbonate (potash) is used instead of sodium cabonate (soda). The potash comes from burning inland plants, hence the term Forest glass. The ashes are soaked in water (leached) to extract the needed substances.
Borosilicate - Glass with a very low Coefficient of Expansion (around 33), which is very resistant to thermal shock. This makes it useful for cooking ware, scientific glass equipment, and artisan smoking vessels.
Fluoride glasses - Glasses which do not use silica as the predominant ingredient. Aluminum fluoride and zirconium fluoride are both options. This glass is usually used for advanced engineering and scientific applications, such as fiber optic cables.
Lead glass (Lead “crystal”) - Originally glass with a very high lead content (potentially over 50%, higher than the ~30-40% silica content). This glass has a brilliant reflective quality and is very clear, making it the best choice for cut ‘crystal’ glassware. Due to concerns over the lead content, modernly barium, zinc, and potassium oxides may be used instead of lead.
Flint glass - An early form of lead crystal. 4-60% lead, flint was used as the source of silica originally, giving it the name.
Fused quartz/fused silica - Silica fused into glass without fluxes. This requires extremely high temperatures, which were not attainable until modern technology (1650°C/3000°F). This is often used for engineering/scientific needs, though rods are also used by lampworkers.
Milk Glass, Vaseline Glass, Carnival Glass - Mostly names based off specific colors, rather than actual differences in the glass.
Porcelain, thermoplastics, glass-ceramics, etc. - There are many other things which, in strict technical terms, are glasses. They are not what any of us laypeople think of as “glass” however and are ignored for our purposes.

Specific Glass Chemistry Concepts
Striking Colors - Colorants are added to the batch of raw glass, and at the high temperatures involved they disperse well throughout the material. The colorants are able to bond with other elements in the batch and, when quickly cooled, stay in those compounds. The glass is often clear when leaving the factory.

Once the color is introduced to a torch or furnace, the temperature is hot enough to cause the colorants to break free from the compounds and form oxides or colloidal particles. Colloidal particles mean the colorant is dispersed throughout the glass without actually bonding with/truly becoming part of the glass (clouds, mayo, milk, and many other things are colloids too). This temperature is also cool enough to prevent the colorant from forming the original bonds at the factory (and/or is cooled more slowly).

Silver glass is a specific form of striking color. Particles of silver are colloidal particles in the glass. At the working temperature, larger chunks form and cause the glass to get beautiful metallic sheens.

Coefficient of Expansion (CoE) - The CoE of a glass is something that often scares people. It is a value indicating how much the glass expands when heated, and generally speaking all glass artists need to know is that you can’t join two glasses which have more than a 1-point difference in CoE. Glaskolben (a glass bulb-and-tube used to make ornaments, which borosilicate lampworkers may also call a ‘point’) that are commercially available have a CoE of 89, but are still compatible with CoE 90 glasses which are commonly available at art glass suppliers.

This value is actually an average! The CoE is tested at a number of temperatures, generating a curve. The number glass artists (and glass scientists) usually refer to is the average, roughly. While we usually refer to it by a number, in scientific terms this number is expressed in scientific notation, 10-7 per degree (in Kelvin). It’s further muddled by the fact that the change is expressed as a fraction of the length, rather than a typical unit like millimeters.

CoE 90 = 90 x 10-7 K-1 = 0.000009 change per degree Kelvin
(The space between 100 degrees Fahrenheit is roughly 55 degrees Kelvin, for comparison)

Trivia: Ceramic artists and scientists use the same values, usually expressed in terms of 10-6 x K-1, so CoE 90 in “glass” would be CoE 9.0 in “ceramic” shorthand.

But HOW is it measured? - With a “push rod dilatometer”. A sample of a material is fixed in place, a rod with a known CoE is placed against it, and a VERRRRY sensitive device is on the other end of the rod. The sensor has to be able to detect very tiny changes. As the sample is heated it will push on the rod and thus the sensor. Then “math happens” to correct for the reference material’s CoE. Fused silica/quartz rods are often the reference.

Example Coefficients of Expansion:



Satake Glass
(Used in Japanese lampworking)
CoE 120
Effetre, CiM, Vetrofond, etc
(Common in western lampworking)
CoE 104
Spectrum & Wissmach fusible glass
CoE 96
Bullseye & Uroborus fusible glass
CoE 90
Typical window/float glass
CoE 82-87
Borosilicate glass
(e.g. Pyrex pans)
CoE 30-33

List of Colorants - Below is a list of common colorants, and the basic color(s) they give to glass. I’m using the default “Crayola 12” colors rather than trying to describe the different shades, such as the green from Iron verses that from Chromium. Most of these only need to be added in tiny amounts to color glass. .001% Copper Oxide can impart an emerald green, 2% Cerium Oxide gives a light yellow.

Some of the decolorants below are also colorants. This works because the resulting colors balance/cancel each other out (e.g. Manganese brings in purple which ‘fills out’ the green from iron impurities). Sunlight could change Manganese over time, turning clear glass into purple!


Red
Copper-Tin, Cadmium-Selenium
Amber/Brown
Copper, Nickel, Sulfur
Yellow
Uranium*, Cadmium, Sulfur
Green
Iron, Chromium, Uranium*
Blue
Copper, Cobalt
Purple
Manganese, Neodymium
Black
Iron, Manganese, Cobalt, Lead
White
(Opacifiers really)
Tin, Antimony, Arsenic, Bones**
Pink
Gold, Erbium
Decolorant
Lead, Manganese, Cerium, Sodium Nitrate








* - Yes, Uranium as a colorant is as bad an idea as you think it is! Mostly discontinued
** - Bone ash, specifically. Yep, skeleton bones!


Sources:
https://www.cmog.org/article/chemistry-glass
https://en.wikipedia.org/wiki/Fluoride_glass
http://www.compoundchem.com/wp-content/uploads/2015/03/The-Chemistry-of-Coloured-Glass.pdf
https://en.wikipedia.org/wiki/Forest_glass
Introduction to Glass Science and Technology, 2nd Ed. Shelby, J.E.
http://www.tainstruments.com/wp-content/uploads/BROCH-DIL.pdf
https://www.ima-na.org/page/what_is_feldspar
http://www.chemistryexplained.com/Ge-Hy/Glass.html
http://www.compoundchem.com/2015/03/03/coloured-glass/
https://www.lehigh.edu/imi/teched/GlassProcess/Lectures/Lecture04_Shelby_ColoredGlass.pdf
https://www.mountainglass.com/tips-and-tricks
https://sha.org/bottle/pdffiles/TheColorPurpleLockhart2006.pdf
http://www.bullseyeglass.com/what-are-striking-glass-colors.html

Vitreous Paint Experiments (Malachite)

I bought a small bag of malachite chips to grind into pigment, to go into the William/Isolde scribal gift box. As I was having fun smashing it, it occurred to me that we do have greenish pigments, and I wondered if malachite might not compose one.

So, I smashed it and smashed it until I got cramping fingers and crossed eyes (Actually, not that long or difficult). I got out my handy 325 mesh sifter, as in theory that's the level Reusche grinds to, and sifted the malachite until I had a handy stash in an old spice bottle.

Because I am for scientific experimentation, I wanted to control everything except one variable. I used some Reusche clear glaze base so I could see what the malachite would do on it's own. I realize, I don't know the ratio of glass (or glass components) to oxides in pigments; One period text would say 2 parts glass to one part copper. I made four test chips to compare different ratios, 1:1 through 1:4.

I mixed them on an ad hoc glass palette with a muller. The muller, being glass, seemed the easiest to clean. I noticed how easily the pigment mixed, much like working with commercial (Reusche, Fusemaster, etc) paints and stains. The very fine mesh size seems to promote fluidity. I used a bit more water than I would for "real" but with this small quantity of paint it was rather difficult to get the right water content (I guess I could have tried to drip it off an eyelash or a cat's whisker, but neither myself nor Zod were willing to cooperate with that). The water won't change the performance of the paint, only how it handles on the brush, so having too much water shouldn't impact the results.

I weighed them on my mini digital scale to get the ratios. Although it does .1g increments, the floor seems to be .2 grams. That was what I used as a "unit", so the 1:1 chip is .2 grams of malachite to .2 grams of clear base. The 1:4 is .2 grams of clear base, and .8 grams of malachite. I wrote my name on the chips, for some reason the first thing that entered my head, to test the line work. I then smeared paint on the bottom block of each chip to show various values. I also marked the ratio at the top.

I then fired the chips on my standard vitreous paint schedule, which matches the range for the clear glaze base. The next morning I was fascinated to see significant change in pigment. What I was aiming for was something akin to "Grey green" pigment, a modern sample of which is here placed next to the chips:

The malachite DEFINITELY darkened. To my eye, it also seems more faintly blue than green. Azurite, a deep blue twin of malachite (both are copper (II) carbonate rocks) will turn into malachite when weathered, but nothing in my research shows the reverse. I did find that azurite when heated turns into copper (II) oxide, a black powder. That oxide is then used as a ceramic pigment, making blue as well as green, black, pink, red, and gray colors. I suspect that's what has come into play here, the malachite likely also forms the oxide which would account for the blueish hint and the darker color. I am not a chemist, however, and I couldn't easily find a reference to what happens when you heat malachite. [Edit: A friend reposted my link to get the attention of some chemistry-buff friends, and one Liz pointed me at this link. Malachite does turn into copper (II) oxide.]

I first picked up the 1:4 chip, and immediately noticed the paint flaking off onto my fingers. My fingertips were tinged grey/black/blue. I found, using a wooden skewer, that the 1:3 chip was also very easy to scratch paint from. The 1:2 I could leave some trace, but not much. The 1:1 completely resisted the stick like Reusche paints would. Looking at the reflected light, the 1:1 chip also looked much like a dozen other test chips I've made; the paint is completely glassy and adhered to the test chip. I fire my vitreous paints to the high end for that effect, so this is expected. The other three showed a rough, grainy texture I associate with previous experiments that had too rough an oxide.

             


                                                         


Lessons Learned:

• Yes, a randomly selected mineral MIGHT make a usable vitreous paint!
• Something near a 1:1 ratio is probably idea to bind the pigment to the glass, though 1:2 was also serviceable. My tests of the period formulas are 1:2, and were very similar.
• It would be wonderful to find out what actually goes into Clear Glaze. My normal secret trick is to check the EU vendors, who seem to list MSDS's that US vendors do not. Unfortunately Peli doesn't include one for Clear Glaze. Reusche gives them out if you make a special request in writing, whereas Peli just has them on their website. I rather suspect this is because the MSDS sheets rather give away the secrets. 

Two Mistakes: Hamsa

I had the idea to make a Hamsa. The idea persisted, and turned into a large flop. So, I remade it. Which turned into a second flop.

Enamels are difficult.

I ended up making nearly the same design, but pieced. I was mad and determined, at that point.  I started with a design I found online; I found it on several websites, in several ways and places. I couldn't determine an original creator, etc and chalked it up to a generic cultural motif. Having learned much from Estelle's scroll, I thought I planned it well. I used the more stable blue, and skipped the idea of using silver stain.

Attempt 1, I painted on the black line work, then added a mat of blue enamel. In the past, again for one of Estelle's badges, I got a phenomenal sapphire blue right off the bat. That gave me incorrect assumptions about how easy it can be to use!

This happened over a year ago, and Facebook isn't helping me work out the order. I know at one point I discovered I had two different blues on hand, with no recollection of the opaque one. So much so that I went back and found the order and made sure I had consciously ordered it (which I had).

The next piece, I kept adding blue trying to get the Sapphire hue I wanted. After a few coats the enamel turned opaque and off-color, a grayish tone to it. I still don't know why, but I decided I needed to get it right from the start.

The next attempt, after doing the line work and the enamel I decided I wanted to shade the background and went back, to add more black mat. That tanked the whole piece as well, as shown below.

Ugly and wrong color, above!

 My "mistake" in going back to vitreous paint. Guess blue enamel isn't as stable as I thought.

A closeup of the weirdness that ensued. I suppose too many layers of enamel flattened out and blurred the black lines beneath, like layers of glass can displace one another when fusing.

The final piece I successfully made and framed, to get a Hamsa out of my mind. I've not included a picture of the pattern to retain my blog's G-rating, as each piece was named with a different profanity, instead of the customary numbers or symbols.

The bottom right corner was 'bullsh*t' if I recall. It was easy to cut, that was just it's nickname.

Lessons Learned:

• Be sure you are using the glass paint you think you are using!
• Blue enamels are also touchy. All enamels are semi-evil.
• Weird things can happen if you apply them too thickly (?) or fire them too many times
• It looks like many layers can displace one another between firings.

Glass and Gold and Gilding

I've signed up for a couple of mosaic classes in April and May. It reminded me I wanted to try my hand at making Byzantine-style tesserae, and then a friend's Facebook post kicked me into high gear. She had much better success than I did and shared some of her wisdom. My test piece:

Two pieces of clear fusible (Bullseye CoE 90 Tekta clear) with one layer of gold foil sandwiched

Rhode Kephalaina let me know in her sample chips, marked 1 and 2, thats the sheets of foil. So, chip '1' has four layers approximately, and '2' with 8. My samples above are 4" square, not 1", and had one layer. It's not exactly ugly! It's just not the beautiful gold glass expected. The nicer parts of mine are where the foil doubled on itself (see Lessons learned, below...) I know now to fix it, though, thanks to a conversation with Rhode.

Ive also fallen for verre églomisé. Predating Rome, pretty much, this art (gilding glass and painting the back black) was practiced through the Roman period into modernity. It gets its name from an art collector 200 years named Glomy. It turns out it's not super difficult!

Sorry it's sideways. I can't quite remember how the technique entered my awareness, either through researching mosaics or mirroring. A little gelatin, some gold leaf, and some glass. A fine needle to scratch it up, and black paint. I'm going to teach a class on it at Pennsic this summer, which is exciting to me. I've tried a few types of leaf, a few tools to transfer it to the size, and will be picking up a second (larger) gilder's tip this week. It's beautiful to look at and I'm excited to see how I can integrate it into stained glass and mosaic work.

Probably To Be Continued...

Lessons Learned:

• Transfer foil is a lot better to work with for verre églomisé.
• Tiny creases are almost unavoidable with loose leaf, but the gelatin size flattens them out as it dries. The end result isn't perfect but it is much better than what you start with.
• TURN OFF YOUR CEILING FAN. Many people remark that gilding can be done at your kitchen table, and they are quite right. But when you bought a book of loose gold leaf and have the fan on medium, you are going to make a kaleidoscope of tears and gold for a moment. 
• Making Roman Gold, as Rhode is trying, or Byzantine Tesserae as I am, costs a bit! The leaf is not terribly cheap, though you can find it reasonably. She speculates gold foil, not leaf, would work better but it runs $75/5 sheets. 
• Don't use imitation gold. I did this before a year or two ago, without entirely realizing. It was an aluminum-based product, I believe. It turned horrible colors and crinkled up under the glass. 
• I wondered at using silver to do this. I'm told it can work, but my experience with silver stain says it certainly cannot. Ken Leap's book shows an example of firing a piece of solid leaf, and it made a dark amber stain at just slightly higher temperatures than I use to fuse. Further, I've used ground silver leaf in a period formula at much lower temperatures, and it made a light lemon yellow. I'd think 'silver' would necessarily be platinum leaf, which I have not priced or looked at.