Fused glass thickness is one of the most important laws of glass fusing. Fully fused glass will always want to be about 1/4 inch (6mm) thick.
If you fully fuse one piece of 1/8 inch (3mm) glass it will contract and the edges will pull as it attempts to be a 1/4" thickness resulting in a misshapen piece of glass.
Firing two glass layers in a warm glass kiln should result in a fully fused piece with nicely rounded edges and smooth top surface. With two pieces the glass should reach a 1/4" thickness without major distortions. Two layers are needed to achieve a 1/4" thickness.
This is known as the two layer rule.
If, for example, you fully fuse three pieces of glass in a glass kiln then the glass will spread out as it seeks to reach a 1/4" thickness. It will no longer be three layers thick but will be longer and wider than the original pieces.
Tack or contour fusing the glass will result in a piece that will be close to its original size. Tack fusing will retain the basic shape whilst contour fusing will give you nicely rounded edges and at the same time retain the basic shape.
Glass contour fusing produces a very smooth result with two or three layer items and is an alternative to full fusing for some design applications.
Using three layers may be desirable in many designs as glass can be cut to cater for the spreading that will take place under full fusing conditions. It is not unusual to add clear glass on top of two layers to give depth to the glass piece.
A clear glass top can give rise to air bubbles forming which may be undesirable in the finished piece.
Sometimes when making glass jewelry, extra pieces are added to the top layer as design elements. Little squares or other shapes can be added to produce a desirable color combination. Pieces added in this way should be kept away from the edges, as the edges will begin to bulge as the jewelry piece fuses.
Adding small amounts of glass frit or glass stringers on top of a two layer piece should not result in any distortions.
Further important information on glass thickness can be found on the Glass volume control page.