Molds from fiber blanket
Ceramic fiber blanket doesn’t have any organic binders that need to be burned off, so you don’t have to pre-fire it to “shrink” it. It doesn’t need kiln wash straight off the roll since it does not have rigidizers in it that will cause the glass to stick.
The blanket can be cut into shapes or crumpled if you have some free-form idea in mind.
A rigidizer won’t burn off; it burns into the mold material and makes it harder. The resulting mold material will also be more brittle and should be handled with some care, ie., never pick up the mold by the edges or with a piece of glass on top.
The rigidizer can be brushed on or sprayed on. Yes, some users will soak the blanket in the rigidizer and then squeeze out the excess. (Once the rigidizer is hard, it really can’t be used, so pour out what you need and store the remainer in its bottle by squeezing out excess air and putting the whole shebang into a plastic bag.) Depending on how you are shaping the mold, you’ll want to decide on an appropriate way to dry the mold. (If you are using glass, heat to 500 very slowly to avoid thermal shocking the glass. If you are using clay, it should be dried and preferably already fired.) Avoid using anything as the pattern that cannot be heated to 500 degrees F safely.Air drying will work, but it will take days.
Heating to 500 degrees F makes the mold rigid and allows you to remove the pattern. Let the mold cool down, remove the pattern and cure the mold further in a kiln at 1100 degrees F. (The mold won’t be susceptible to thermal shock, so it can go into a kiln already heated to 1100. Use tongs.)
When all the mold release and black matter has burned away and the mold looks white and clean, it is ready. The mold will not need to be annealed. It takes about 3-5 minutes of air cooling before you can handle it with your hands.
Rigidizers are collodial silica products or sodium silicate. Because it is a silica (think of it as a cousin to glass), the mold will need kiln wash.
Because fiber blankets add texture to the glass, you can sand down the high spots. Usually 100 grit sandpaper works fine. Add rigidizer and the “dust” from the sanding to any holes or dimples in the mold. Re-apply rigidizer to the sanded areas, cure the mold at 1100 degrees F again. Apply the kilnwash.
The rate of heating the kiln will depend on the complexity of the shape of the mold and the thickness of the glass. In general, you can use slumping temperature guidelines at this point for setting the kiln cycle. I would suggest that you follow the guidelines for similarly shaped molds and keep careful records.
Molds I like get their own boxes and can be used several times with careful handling.