Here is some descriptions of how glass fusing works.
Warm glass, glass fusing or kiln-formed glass is the working of glass, usually for artistic purposes, by heating it in a kiln. The processes used depend on the temperature reached and range from fusing and slumping to casting.
'Warm' glass is in contrast to the many cold-working glass processes, such as leaded glass. 'Hot' glass, glass blowing or lamp-working is the working of glass in a direct flame, such as for laboratory glassware and bead-making.
Warm glass working uses a variety of processes, according to the working temperature and the time the glass spends at this temperature. The glass becomes progressively softer, less rigid and less viscous with temperature. Kiln-worked glass responds slowly though, and so the amount by which this affects the glass depends on the time it spends at working temperature.
There are three main processes that I use, with variations within them. The broad process depends on the temperature, the variation within it depends on the time and also on slight variations of temperature. These processes are:
Fusing, the glass retains its shape, but becomes sticky and adjacent glass pieces join together.
Tack fusing is the joining together of glass, with as little change to the shape of the pieces as possible. Tack fusing may be used either decoratively, or to assemble a large piece of glass from multiple smaller pieces.
Where tack fusing is used to apply small decorative details to a larger piece, it is often desired to partially melt the small pieces so that they change shape (usually becoming more spherical, under the influence of surface tension), but without changing the shape of the carrier piece. This can be done by using an increased temperature, but only briefly. The large piece, of large thermal mass, heats up more slowly than the small decorations.
Full fusing is like tack fusing, but the temperature is higher so that the fused pieces begin to coalesce. In the complete case, decorative additions to a surface are absorbed entirely into it and the surface becomes flat again. It is usually done for decorative effect.
Slumping, the glass deforms in shape, becoming flexible but still retaining its approximate solid form. This is done into a mould.
Slumped glass is heated to the temperature at which the glass softens and begins to deform. It may either bend along a single curvature or, if heated sufficiently, may become elastic enough to stretch and curve to follow a compound curvature, such as a bowl.
Mould slumping begins with a sheet of flat glass placed above a ceramic mould. When heated, the glass slumps into the mould under its own weight.
Draping is a variety of free-fall slumping, where the mould former is placed in the centre of the piece and the outer edge falls under the heat. As this outer edge is unconstrained, it tends to fall in large folds. The edge is thus highly uneven, although a carefully draped piece may still retain perfect symmetry. For this reason draped pieces are often used as vases or wavey-edged bowls, but are difficult to use as a more functional vessel.
I also use a process called Kiln carving where I cut designs from fibre paper and by placing glass over it, the full fusing, an indented design is formed. This can then be slumped into a mould.
It is common for one piece to use several of these processes in turn.