I fired a one-layer piece of glass and it shrank. What did I do wrong?
This result relates to the thickness that glass, under kiln forming circumstances achieves. The combination of gravity and viscosity lead to this effect. As the glass becomes less viscous (more runny), the surface tension is greater than gravity and so it becomes thicker at the edges. This additional glass is supplied from the edges and to some extent from the interior. The glass in the middle becomes thinner, allowing in certain circumstances bubbles or holes to appear.
This illustration from Fusedglass.org shows the effects of gravity, which is related to mass, and viscosity. The lack of mass means the surface tension allows the glass to draw up to be come thicker, forming the classic dog boning appearance.
Knowing why this occurs allows you to take come precautions, when firing single layer pieces, to help prevent the shrinkage, often known as dog boning.
Fire larger. You can cut the glass larger than the final piece will be. After firing, you cut it down to the size you want. You may have to do a bit of cold working to get a rounded edge to the glass before any further processing.
Fire lower. You can fire at a lower temperature for a longer time. You will need to observe to determine when the glass begins to shrink. Either stop the temperature rise and soak there for a time, or reduce the temperature a little and soak for as long as needed to get the surface texture wanted.
Fire oval or circular pieces. With these shapes the shrinking is not so obvious, as it occurs all the way around. With rectangular pieces, as the glass shrinks, the corners become thick more quickly and so do not shrink as much, giving that dog bone appearance. Rounded pieces become thicker all the way around more evenly and the shrinkage is not so obvious. However, you still get thinning in the interior which can lead to holes or bubbles, so observation is still necessary to prevent excessive thinning and bubble formation.
Fire thicker. The real prevention is to fire two layer pieces as that is the thickness at which viscosity, surface tension and gravity are in balance. So the glass does not change size at kiln forming temperatures.
Alternatively, you can cold work the edges back to straight parallel edges. This can be done by hand grinding or by machine.