What is slumping?

Slumping is softening glass so that gravity can cause it to change shape or to take up a changed surface texture. 

It is done at a range of temperatures, depending on what other effects are required along with the ‘changed shape or surface texture’; but also on the dimensions of the piece. It is commonly accepted that it is done at a lower temperature than fusing, although that isn’t always the case.

Slumping is generally done using a single thickness of glass. Multilayer pieces are usually fused together in an earlier firing and then slumped in a subsequent firing at a lower temperature. This is so that detail from the first firing will be less affected in the second firing. When a slump and fuse are being done simultaneously the firing will be done at fusing temperatures, so that will be discussed later.

Similarly, where the edges of a slumped piece will be exposed and good edge development is required, firing temperatures will need be raised above normal slump values. This was the case with the platter shown in the Basic Kilnforming section of the website.

American literature, notably by Bryce Lunstrum, uses two terms to describe the process:

• Slumping where the glass is heated to a temperature close to its softening point and where the thickness of the glass remains fairly uniform.
• Sagging where the glass is taken to higher temperatures so that flow takes place within the piece and the thickness varies markedly.

Other authors use the opposite definition for the two terms. In Australia this distinction is rarely observed, and slumping is the term most commonly used. After all, there’s enough on which to disagree without introducing debate on where lies the boundary between the two.

NOTE. Softening point is defined as the point at which glass will bend under its own weight. It is included in the published data for most glass.

It is a technical term linked to a viscosity value.

It should not be taken to mean that glass won’t bend at any lower temperature. For example, the softening point for Viridian float is given as 737°C, whilst the same glass can be bent at around 600°C (1110°F) without impairing its surface texture or clarity. It may take some time, but it will do it under its own weight. More on this in Bending glass.

Types of slumping

There are three main types of slumping

• Flat
• Shallow into or over moulds
• Drop-through or similar deep slumping.

These are discussed in separate pages in this section.