Slumping small single-layer pieces
A different set of conditions are formed when slumping small pieces. It takes more time or heat to slump small pieces, because there is less weight to pull the glass down into the mould. Because there is less weight in a single layer than in a two layer fused piece, more time is required for the glass to conform to the mould. The exact amount of time and heat will have to be observed for each kiln and kind of layup.
If you are not concerned about the exact nature of the shape of the piece you can fire pretty fast and high. This will produce a curve, but probably not good conformation to the mould. Even if there is good conformation, there will be significant marking of the back of the glass.
The alternative strategy is to increase the heat. This can work, but leaves more mould marks on the finished piece. The increased temperature causes the forming to occur much more rapidly and may lead – if not closely observed – to over stretching or “bubbles” at the bottom of the mould.
If you want a relatively unmarked and slumped piece, you need to change strategy. You can take the glass up in temperature rapidly to ca. 600C and then use about 40C/hour to 677C. Begin watching from about 635C. This slower advance in the slumping range gives better control of the slump. Slower rates of advance give less distortion and less movement within the mould. It also provides a less marked piece, because the slumping is done at a relatively low temperature.
When the piece achieves the shape you want, record the temperature and advance to the next – cooling - segment. The recording of the temperature will give you information for future slumps of this nature.
On any slump that is new to you, either in terms of the mould or the thickness and size of the glass, you need to make records to help determine the optimum combination of conditions. You should of course, record each firing anyway.
You will find more information on factors affecting slumping at these locations: