A quick note on terminology, as I was taught and use. The two broader surfaces of the came's "H" that are visible are leaves. The lead portion connecting them, which is directly between the pieces of class and cannot be seen, is the heart.
A less quick note on safety: Lead (and other heavy-metal) poisoning almost always happens because of ingestion, not inhalation. If you get a whiff of fumes while soldering and it irritates your nose or gives you a headache, it's because of the flux compounds (which generally are not serious, but check the MSDS for your flux). Think of lead like water. If you have a cup of water, is it billowing steam in your face? No, it's in liquid form. Even if you heat the water a bit, its warm or hot water. It is quite possible to heat lead and not generate lead fumes. If you are boiling the lead, then you have a problem! At the hottest setting of my stovetop I could barely melt the lead. Still, I ran the vent hood on high. I believe this process is very analogous to soldering (which is melting lead), and soldering is pretty safe for a hobbyist. I know professionals with decades of stained glass experience who do not have elevated blood lead levels. A little research indicates that the temperature at which lead generates fumes is 800-1000 degrees hotter than the melting temperature during casting. That being said, a hot plate outside will not do you wrong, either. Remember that any tool or container you use for working with lead is contaminated and must be washed. If, like me, you use a pan on the stove do not reuse the pan for food. I highly doubt you could get it clean enough.
I started with some wooden blocks that I believe are intended as part of crown molding. At the hardware store I verified they were relatively flush when squeezed together (such as by a clamp).
At first I tried free-hand carving the grooves. That sort of worked but I realized it was going to be impossible to keep them lined up the whole length without the leaf-grooves being wider than I wanted.
Going back to primary sources provided an answer. Soaking string in paint, laying them out, and then pressing the other board onto the first. This would create two sets of lines that should line up again. I found this to be fairly difficult, however, because I was trying to keep the lines a uniform distance, and the thick yarn made a fairly imprecise line to then follow. Not to mention it rolled a bit when I was trying to line up the edges of the block too (probably unnecessary).
This view of the clamped mold shows the difficulty I had with the period method.
I ended up using a precise rule and measuring in an inch from the edge of the block. I made a line, moved over about 3/8" and made another line. The idea was to provide a common reference point (the block edge) and, if my measurements were accurate, everything should line up.
I then used a handsaw to "connect" the lines on either end. This ended up more difficult than I initially anticipated because of the wood I used. It has a slight curve, so sawing the ends was easy but it took some patience to get the grooves in the center.
The boards after the grooves were sawn.
I had a mold for the leaves, but I needed to remove a little of the wood between the two grooves to put the heart between them. I tried several tools and didn't find an "easy" one to make this painless. First I tried wood carving chisels, which did almost nothing (they are a cheap set). Then I tried the thin edge of a large file, with minimal results. I then wrapped sandpaper around that thin edge and tried again with some progress. The only sandpaper I had available was relatively fine, however, so I think that impacted it.
Finally I used a set of smaller files, where I could use the crosscut broad surfaces, and they worked better than anything. I sat during a Blackhawks game and filed away material until I could feel a distinct difference. A bright light provided a good test as well:
When my former protege-brother (he was elevated, there was no falling out) taught me to cast with lead-free pewter, he taught me to use an electric crucible popular with sportsmen for making lead sinks. Another protege-brother had said that a cast iron skillet works just as well in a pinch. I purchased a cast iron skillet with nice little pour spouts for this purpose (If anyone comes over for breakfast I promise not to reuse it!). I placed a bit of scrap came into it and fired it up. And waited. And waited. Eventually, with the fire turned up as high as it would go without the igniter clicking, it worked.
As soon as I tipped the pan a little a lot of the above lead pooled together. The impurities are fairly visible and many "colors" appeared as time went on.
I couldn't take pictures of the actual pouring because at this point I required speed, accuracy, and two hands.
The top of my mold with some over-flow. It took less lead to fill it than I anticipated.
The bottom of the mold. I had set it on a block of soapstone so as to protect the surface of my stove from molten lead.
Prying the mold halves apart was often a bit of work, and the first time it required an implement or two to assist. The results were somewhat ugly, though identifiable to me. The pictures may be hard to see because of how shiny the came was. It washed out the "depth" I think.
I immediately remelted the lead and tried again. I know that slight alignment changes can have big effects.