Once you’ve mastered the basics of fusing bottle bottoms on molds, you can expand your glass practice and add colors to your designs. There are 3 main ways to add colors to fused bottle bottoms: Frits, Enamels and Mica. All of these have different processes, so we’ll take a minute and run through the best practices for each.
Frits
The very first thing is, you’ll need frits that are compatible with bottle glass. The frits that are designed for use with float (window) glass are the closest match, and typically work very well. I use Armstrong Float Frits and have been pleased with the results.
Frit Process:
After coating your bottle bottom mold with kiln wash (carefully, so you don’t fill in the details), add color frits to the mold. I use a very soft brush to carefully brush the stray frits into the mold without disturbing the kiln wash. Once this is done, you can add your bottle bottom and fuse normally.
Things to know:
Frits work best on light colors of bottle glass, simply because they are difficult to see against the darker glass. The exception, of course, is opaque white, which shows up very well. The frits won’t fuse perfectly smooth. Because you are pressing the frits into the bottom of the mold, under the fused glass, they do retain some of the frit texture at a regular fuse. If this is a deal breaker for you, you can experiment with a longer hold, or slightly higher top temperature, do be aware that this may increase the chances of devit though, so make sure you are in a testing mindset.
Enamels
Enamels are concentrated colors that melt at much lower temperatures than frits. I’ve used two different types of enamels for my examples, Thompson medium expansion enamels for float (window) glass and Reusche enamels, let’s look at them separately:
Thompson enamels mature in the 1400-1450 range, which is right about a slump or tack fuse. This means that enamels must be applied AFTER fusing and then fired again. I like to fire my bottle bottoms to a full fuse, then apply the enamels, either in an enamel base, or by sifting over the glass. Enamels can be stuck onto glass using hairspray or fusing glue. Fire the bottle bottoms to a slump or slightly hotter temperature to get a glossy finish. I often slump the pieces at the same time, combining two different processes in one firing.
CAUTION: Do NOT put enamels into the mold before fusing, due to the lower melting temperature, the enamels will stick to the mold and then cause cracking as the bottle glass sticks to the enamels (which are sticking to the mold).
Reusche Enamels fire at an even lower temperature. These enamels are very fine and much more like paint than a powder. Reusche Enamels will need to be mixed into a carrier medium prior to use (there are many available) and painted onto your glass after it has been fused. The bottle bottom can then be fired again using a slump firing to achieve a high gloss finish.
Mica
Mica is a superfine powder that can be added to the surface of fused glass to give it a metallic shine. The key point is to ETCH the fused glass before adding mica, to give the mica something to stick to. The mica needs texture or ‘tooth’ to settle into. After adding the mica, fire the glass again to a slump temperature (again, you can slump into a mold at the same time). The excess mica will wash off (this is normal) and leave a metallic water color type of finish. Mica looks best on dark glass.
Quick Reference Guide to Adding Frits, Enamels and Mica to Fused Bottle Bottoms
| Product | Process | Firing |
| Frits (Armstrong Float Fire) | Add to mold | before fusing glass |
| Thompson Enamels | Add to fused glass | After fusing, then slump to set |
| Reusche Enamels | Add to fused glass | After fusing, then slump to set |
| Mica | Add to fused glass | After fusing, ETCH, paint, then slump to set |
