Fusing glass in molds for jewelry: This is not an easy subject to get information about, so I'm sharing what I've learned. (This only applies to small jewelry size pieces and molds as larger sizes may differ.)
First you will need: a mold of the appropriate size for jewelry, some glass frit (crushed glass) and, of course, a electionically controlled kiln. Oh yes, and you MUST have some kiln wash - and the only one I have found to work is Primo Primer by Hotline. The stuff that came with the kiln for use on shelves just doesn't work on molds.Glass frit comes in 4 different sizes: powder, fine, medium, and coarse. I do not use powder as it is too easy to breathe into your lungs (respirator required). Also, the size will vary from one company to the next. One medium may equal someone else's coarse. You will have to experiment. Try to buy only 2 or 4 ounces the first time as a one lb. jar is going to last a long time; and if you are like me, you will want several different colors. The Warm Glass website has small jars in all sizes and colors. Also note that frit comes in both transparent and opaque. They have a very different look when fired. I usually mix the 2 together, as I did in the above photo.
Molds are available several places, it's just a matter of finding the shape you desire. Check the overall measurement to make sure it will fit in your kiln (if it's small like mine). Then get the inside size of the mold itself to be sure that it is the right size or what you had in mind. I bought some that were for pendants and if filled to the top with frit are very heavy. Above is the photo of a group of the large sized ones.
A note about molds. On some of them -especially ones with designs - the bottom of the mold is the top of the piece when you take it out. This means that the top (really the bottom) is rounded and will not be very flat. This can be a problem. I have refired the pieces without the mold to a tack fuse to get the bottom to flatten out a little for gluing. This can also flatten out the top where the design is if you fire it too hot.
The process: Brush off any dust from the mold. Mix the primer according to the instructions. (Don't breathe the dust.) This is usually 1 part dry mix and 4-5 parts water. Put in a jar that can be sealed and shaken. It tends to settle and must be stirred well each time before using. Using a soft brush, coat the inside and top of the molds with the primer. Let dry a few hours and repeat until you have at least 4 coats on. Be sure it is dry before adding glass. Adding the glass - I use a small metal measuring spoon and tweezers to put the glass in and push it around. If you're trying for a pattern of any type, I would usually put a solid color in the bottom and arrange the pattern on top of that. You can add clear to the overall top if you desire. It's fun to experiment!
The fusing: The important part is next. These molds are made of ceramic and as such must be heated slowly or they will crack. I've had a couple crack when I was using the little micro-wave oven kiln. They sell those small molds to use in it, but don't do it. The microwave heats much too fast. You must ramp up your real kiln temperature slowly - here's my schedule: (obviously your's may vary).
300 degrees per hr to 1000 - hold 20 min.
400 degrees per hr to 1250 - hold 20 min.
600 degrees per hr to 1550 - hold 30 (large molds can go to 1600, hold 40)
full to 1000, hold 40 min.
100 to 900, hold 40 min.
100 to 750, hold 20 min.
400 to 100, & off. (don't peek until cool!)
When finished - turn the mold upside down and hopefully the pieces will fall out. If not use a soft spatula or similar to pry a little. There will probably be kiln wash stuck to the bottom. Scrub it off with scouring powder and a toothbrush. Admire your new jewelry pieces.