• First of all you are looking through the transparent to a reddish metal so that will always influence the color of the transparent – especially the light ones. But in addition, when you are firing transparents on copper you need to do two things: melt the enamel and eliminate the red oxide that forms on the copper. Red oxide precedes the black oxide that we know and love as “fire scale”. It forms before the grains of enamel have fused to the copper beneath. In order to “get it out” you need to fire the enamel long and hot enough to dissolve the oxide into the enamel. I fire copper pieces at 1500° F. for about 3 minutes for jewelry pieces or longer for larger pieces.
• If the coat of enamel is too thin on a copper piece, areas where there was insufficient enamel to cover will turn black. Correct this by overcoating with another layer of the color. Also red and orange opaque enamels often react with the copper beneath and form these black spots. This can usually be avoided by firing a coat of undercoat white or flux on the copper first. The black spots may also be caused by trivets or firing racks that are covered in scale which pops off when in the kiln. Periodically clean the fire scale off by filing or soaking in a pickle such as Sparex #1.
• There are many causes for bubbles and pits in enamels: using too much holding agent, applying a coat of enamel that is too thick, impurities in the metal underneath gassing out when the piece is fired, using enamel particles that are too fine – or too large, not getting the enamel grains to “settle”. When I use a holding agent like Klyr-fire in conjunction with unfired enamel I thin it out 5 parts water to 1 part holding agent. Some enamelists use a 50/50 solution. Always use as little as necessary both in quantity and in strength. Several thin coats of enamel fired separately will produce much fewer bubbles than one thick coat. Use a combination of grain sizes. Grains between 200 and 80 mesh in size make a good mix. The smaller grains can nestle down in the spaces between the larger grains. Use some means to settle the grains and smooth out the layer of enamel. This can be accomplished by tapping in the enamel with a metal tool or by vibrating the piece. Running a knurled tool along the edge of the metal base will do this. Don’t overdo this though or enamel will pull in toward the center and leave the edges bare. If this happens push it out toward the edges again.
• Realize that we are asking two very different materials, metal and glass, to “get along”. With the heat of the kiln and the subsequent cooling, the metal and glass expand and contract at different rates. There are some ways to mitigate warping: dome the metal piece, construct a piece that is close to equidistant from its center e.g. a circle or square rather than an oval or long rectangle, counter enamel (I strongly recommend this), use thicker metal with a thinner layer of enamel. Or if you are dealing with a large or elongated piece use several trivets to support it in as many places as possible or paint the counter-enamel with Scalex so that you can fire the piece directly on the firing rack without a trivet (the Scalex prevents the enamel from sticking to the rack) or fire the piece flat on Lava Cloth or some material that will not adhere.
• 24K gold does not produce any chemical color changes in an enamel but both copper and silver and lower karat golds can. Copper tends to make transparent reds, pinks and oranges, turn dark and unrecognizable. Copper may cause multitudes of bubbles in other transparents such as turquoise and chartreuse. Copper may also lead to black spots in warm colored opaque enamels. Undercoating with a flux or white undercoat will usually solve this problem. Warm transparents on fine silver will often look more yellow than you expect. Many opaques on fine silver will have this same reaction e.g. grey, purple, white etc. This color change is caused from silver salts being released from the silver upon firing which colors the glass. Undercoating with a flux for silver will help avoid this reaction as will fewer firings and lower temperature firings. Also if you use silver cloisonné wires the enamel touching the wires will be discolored. Some enamelists use gold foil under the reactive enamels and gold cloisonné wires to eliminate this effect.
• This probably will only occur when silver wires are used on a copper base. If the wires sink through the undercoat and touch the copper in the kiln an eutectic alloy is produced. An “eutectic” is a phenomenon wherein two metals mixed together will melt at a temperature lower than either of the metals alone. This is a good thing when making a solder but not so much when making a cloisonné piece. Undercoat with a harder (higher firing ) flux and/or keep your firings low e.g. 1350° F. Because of the lower temperature you will need to fire longer.
• One characteristic of enamel is to bunch up or gather in the center of a cell or the center of the piece. Try to fill in more heavily around the periphery so that when it is fired the enamel will even out though this gathering process.