To make a meniscus mirror I need a precisely-shaped thing that will stay the same shape even at 600C. On Jan 31, I machined out a new piece of wood, sprayed it with polyurethane, and then poured in furnace cement!

...but several days of drying later, I couldn't get the cement out of the mold. It stuck to the wood too well. In attempt #1 I used 3 coats of polyurethane, but in this I only used one. Eventually the wood started warping, so I figured I'd try using a hacksaw to try to cut out the wood, but even that didn't help. Maybe the edges were holding it in place? Nope. The wood was practically bonded to the cement.

So... how can I remove this furnace cement (which cures at high temperatures) from the wood?

I decided to set it on fire.

After a few weeks of drying, the wood was very warped and I couldn't get the furnace cement out; perhaps it had bonded to the wood. What's the difference between wood and furnace cement? Cement is heat resistant, and wood is very flammable.

So I started a fire. I grabbed some sticks and some newspaper as kindling and piled it on, then lit it on fire inside a barbecue. Turns out I suck at starting a fire - I didn't have enough small sticks to sustain a fire for a while and catch the large wooden mold on fire. It burned decently, but didn't get hot enough to catch the wooden mold on fire.

After all that... I ended up with lots of ash and some blackened wood that STILL wasn't separated from the furnace cement.

Also, the cement itself, which was supposed to resist the heat, didn't resist the heat! It sprouted a giant pocket in the middle like naan. I thought I got rid of the moisture by drying for weeks... what happened?

Turns out... I was using the wrong type of furnace cement all along. I need "castable" furnace cement - otherwise it won't let the water out. I was using "furnace cement" designed to seal over cracks, and in the fine print it said "spread a thin layer". Thin meant "less than 1/4" I suppose. Water and air couldn't get out of my 3/4" blockt, and when I set it on fire it expanded and caused those bubbles.

So I need to buy some castable furnace cement. I did some searching and it's only available in 50lb boxes for $70. Aaaaargh