1] Your piece is not smooth: You did not fire polish long enough or to a high enough temp.
2] Your piece is misshapen or has flattened out more than you wanted it to: You over fired, i.e. fired to to high a temp or for too long.
3] Your piece cracked after taking out of the kiln: You took it out of the kiln too soon (or you peeked) and it was thermal shocked. This is from room temperature to 1000 degrees. Lower your ramp speed.
4] Your piece cracked in the kiln: If it is an ‘S’ shaped crack, you heated too quickly; if the crack radiates in all directions from a single point then the kiln wash was not applied to the center of that center point.
5] Did you have metal inclusions? If so, you need to do the initial heating more slowly as the metal will heat more quickly. Fire to 350 degrees and hold for 10 minutes then heat up to full fuse as normal.
6] Perhaps you cooled the piece too quickly. From 1000 degrees downward – DO NOT OPEN THE KILN until it is under 500 degrees for small pieces (no larger than 7.5mm deep, 2” x 2” L, W), under 200 for large or thick pieces.
7] The crack follows along the line where two colors meet up: the two touching pieces are not compatible.
8] Unwanted Bubbles: Are caused by heating too quickly between 1300 and final fusing temp. Fire more slowly thru this stage to allow air to escape. If using a mold, make sure the hole in the bottom of the mold is open or it can trap air and form an ugly bubble at the top of your piece. For flat pieces – if you have uneven design elements between layers such as metal inclusions, stringer, frit or little pieces of glass, think about escape routes for the air. If you have an enclosed design then there will be no way for the air to escape. Cut your pieces smaller; smaller pieces (that comprise one large work) are less apt to trap air. Firing on fiber paper will also help eliminate air bubbles but does leave a somewhat undesirable texture on your glass (you may like it).
There are some principals of fusing that you simply need to know. I won’t bore you with the scientific hobbledy gobble though.
How do I get my pieces to be round?
Well you simply have to grind down all your bits of glass so they are round before assembling and firing…….. NOT….. I actually had a few students who had been taught that you needed to do that! The simple truth is that, unlike the women of today, glass has a burning desire to be round. It will round up all by itself if you give it the chance. However, you have to have enough volume of glass for it to do that. You will be enlightened as you read on…..
Should I use thin or standard thickness of glass (1.5mm or 3mm)?
Many people choose to use 1.5mm art glass on the bottom and top layers because they want to keep their pieces light in weight. While this may seem prudent, I don’t do it and no one has ever complained about the weight of my work. I DO use 1.5mm dichroic glass. I find that if you use 1.5mm art glass as the base your piece won’t have enough liquid volume of glass to round up nicely. In addition, if you are making a pendant the mandrel sinks into the base a little and the back of the channel will be too thin. I use 3mm clear on top for the same reasons – an adequate volume of liquid glass to round up nicely. In addition, 3mm clear on top lends a better optical effect. As with everything, you should experiment for yourself and see what you think.
Why is my piece larger/smaller than it started out?
OK, here’s the thing…. Glass wants to be ¼” thick. If you start out with a piece that you stacked up an inch or so then it is going to flatten out and get wider so it can be ¼”. If you start out with 2 thin layers of glass then it will pull in, rounding up, so it can reach ¼” in height. Rephrased – if it is less than ¼” when you start, it will round up to reach ¼”. If it is higher than ¼” it will flatten out and spread so it can shorten to ¼”. Think of it as baking cookies - you spoon a little ball of dough onto the cookie sheet and as it bakes it spreads out becoming larger around but flatter... same thing with glass.
Some glass is repelling.
When layering glass you need to know that you shouldn’t place dichroic coating to dichroic coating as the coating repels itself. UNLESS, you stack other pieces on top so they melt around and hold them together. Iridized coatings will also repel each other and dichroic coatings.
Stacking dichroic glass:
If you are going to stack layers of dichroic glass one on top of the other, always stack UP the rainbow, i.e. Red is on the bottom, Orange next up with Violet on top. Dichroic glass will change in the kiln and, while red seems fairly transparent, it will be very opaque once fired and you will not see the glass underneath. (Rainbow = Red, Orange, Yellow, Green, Blue, Violet).
Naming conventions for dichroic glass:
Dichroic glass colors have 2 names, i.e. magenta/green. The first color is the transmitted color, or the color you see when you hold clear dichro up to the light…. It’s the color that goes thru the glass. The second color is the reflected color, the surface color. The reflected color is the one we mostly care about when fusing jewelry. It is VERY important that you understand this so you can buy the colors you want and make the pieces look the way you want them to. Magenta/Green is GREEN not magenta! Green/Magenta IS Magenta!! Specialty colors and Premium colors only have one name like Salmon or Candy Apple Red.
Figuring out which color name is what color!!
Probably the single most difficult thing when using dichroic glass is learning which color is which and training your eye to know the unfired colors before you make your pieces. The colors shift, some manufacturers’ colors shift more than other. When using CBS dichroic glass, the only colors that shift much are the golds and reds. See the color chart below as I’ve annotated it to indicate which colors shift.
Which side is the dichroic side?
It’s hard to tell which side is coated sometimes so it takes practice. There are a few methods for doing this. Take any sharp, pointy object and hold the point to the glass. If the reflection of the point touches the actual point then that is the coated side. If there is a space (equal to the depth of the glass) between the point and it’s own reflection, then that is the non-coated side. Another method is to tip the glass at a 45 degree angle. If you can see the edge of the glass thru the surface then it is most likely the non-coated side. Note that the color you see reflected at this angle closely relates to the after firing color as well. Some colors are harder than others. In every class I have at least one student make a pair of earrings with one up and one down….sometimes I even do it myself .
Permission granted and written by Nancy Tang.