The mystery of kiln-formed glass explained

All the fused glass art on this web site is made using the process of Kiln Forming including such techniques as Fusing, Slumping, Carving and Casting. These techniques are generally referred to as “Warm Glass” techniques because they are carried out at temperatures below the full melting point of glass (Hot Glass working) and above the temperature of Cold Glass working. At Garden Glass Works, we do, however, also make use of “Cold Glass” working techniques in our finishing processes, because they add an extra dimension to the finished piece.

Warm Glass Techniques

Fusing

Joining pieces of glass together by slowly melting them in a kiln at temperatures of around 760 °C to 815 °C. Using the lower end of the temperature ranges “sticks” the glass pieces together, whilst using the higher end of the temperature range (full Fuse) “melts”  one piece into another.

Slumping

Heating glass to the point where it is “soft” and gravity and the weight of the glass pull it down into a mould to let the glass take on the shape of the mould.

Carving

Similar to slumping, but carried out at full fuse temperatures and using “relief” moulds to create deep shapes and patterns within the glass.

Casting

Similar to fusing and carried out at full fuse temperatures but using very thick layers of glass or crushed glass pieces in a mould which “melt” together to form a very solid, very thick casting.

High Firing

The glass is heated to a temperature between “Full Fuse” temperature and melting point, somewhere around 925 °C, to make use of the unique properties of the glass to flow and move at these temperatures. Some unique patterns and colour combinations can result from high firing the glass, such as those produced by “pouring” different colours of glass together or “raking” across the glass.

Fire Polishing

A technique of heating to soften the glass which is used to “polish” the glass after it has been cold worked in order to remove marks and scratches and return it to a shiny finish

Cold Glass Working Techniques

Cold Polishing

Shaping the edges or surfaces of the glass to smooth it or add a pattern. This process uses grinders or sanders, mechanically or by hand, to abrade the glass surface. It is normally used to remove uneven or sharp areas as well as for imparting a particular shape or pattern. After cold polishing for shaping or removing uneven areas, the glass is usually “fire polished” to return it to a shiny finish.

Etching

This technique allows the addition of designs or patterns to a glass surface, either using chemicals to abrade the glass surface or sand-blasting. Glass can be frosted and patterned using etching techniques.

Cutting

The process of cutting or trimming the glass to shape and is used on cold glass which has either been fused or not. A specialised glass cutter is used to score the glass and the glass is then pressured along the score causing it to split apart. For thicker glass pieces mechanical cutters such as tile or glass saws can be used. If the glass art piece is not to be fused again or slumped, then it is normally “fire polished” to return the cut edges to a shiny smooth finish.

Many glass art pieces will have been made using a combination of some or all of these techniques and variations of them to produce the final finished piece.

Glass Used in Kiln Forming (see Glass & Manufacturers for a selection of colours/patterns)

Art glass or kiln forming glass is manufactured especially for its colourful and kiln forming properties. The glass is made by adding chemicals to silica and heating it until it is molten. The molten material is then rolled or blown to produce the glass sheets. The sheets are made in different thicknesses and often, because they are made from hand-blown or hand rolled glass, the thickness of each sheet has variations, which adds to its propensity to transmit light in varying ways.

The glass is also produced in other forms, which are often specifically used in glass fusing. Crushed glass, known as “frit” comes in different sizes ranging from a powder up to quite large pieces. Thin shards of glass known as “confetti” and thinly stretched pieces of glass called “stringers” can all be used by the glass artist to produce unique and unusual colourful patterns and designs.

Most manufacturers produce either “opalescent” (opaque) glass and/or “cathedral” (transparent) glass. Some manufacturers of art glass products take this further by adding coatings to the glass to produce iridised finishes or a very special glass called “dichroic” glass.