Flat slumping

What flat slumping entails

Flat slumping is where a pattern or texture is made on or into the kiln shelf or hearth and the sheet of glass placed on it, so that when softened by heat it will slump under its own weight to take up the pattern or texture.

As the glass softens, an imprint of the pattern will be made in the underside of the glass. The extent to which the pattern or texture will be reproduced will vary with:

• The boldness or other characteristics of the pattern.
• The temperature of the glass.
• The thickness of the glass.

It should be remembered that, unlike rolled glass where pressure is exerted on the sheet to force the pattern into the surface, here the only force is that of gravity acting on the mass of the glass: the thicker the glass, the greater the mass.

A ‘lazy’ or rippled pattern requires little surface deformation whilst a ‘chunky’ pattern, or bold relief will require much more. This latter calls for more ‘heatwork’ to be done on the glass: either by higher soak temperature or longer soak time, or both. In the image shown above, the letters GLASS would have required more heatwork to form the bold edges of the letters than would have been required if only the fainter textured background was being imprinted.

Consider a piece of glass placed on a small block on a shelf as in Fig 1 below.

Fig 1

With limited heatwork it will slump over the block, as in Fig 2, forming a ‘lazy’ curve at A and maintaining reasonably uniform thickness throughout.

Fig 2

Because the glass is now a fluid it will, like water, try to find its own level. Continued heatwork will allow the glass to flow and conform more intimately to the shape. It will flow and the thickness will become less uniform.  The glass on top of the block will flow sideways and downwards and there will be tight conformity of the glass to the top edge of the block, at B in Fig 3.

Fig 3

Should the block be higher than the thickness of the glass, then if sufficient heatwork is done, all the glass can flow off the block and a hole will be created, as  in Fig 4.

Fig 4

Surface tension will come into play. Should the glass be below equilibrium thickness it will roll back from the hole to increase its thickness and increase the diameter of the hole.

Creating texture or pattern

Embedded detail

A flat textured or patterned slump can be created in many ways, including;

• By sieving fine powder onto the hearth, so that a fine ‘hill and dale’ effect is created. Using sieves of different mesh size can give variety, as can the way in which the sieve is tapped or vibrated to cause the powder to fall.
• More uniform textured effects can be produced by using ceramic fibre (CF) paper as an underlay.
• By pressing objects into the powder or by dragging fingers, feathers or other objects through the powder.
• By using cutout shapes of thin CF paper.

Because these patterns have low relief (are not very ‘high’) there will be little effect on the top surface of the glass and that will remain fairly smooth. The thickness of glass will not be greatly affected.

The contrast in texture between a 1mm thick piece of ceramic fibre paper and a thin layer of ‘hill and dale sieved powder’ can be most effective whilst barely affecting the overall thickness of the glass.

Adding bold or coarse texture, either by sieving a coarser material or piling the fine material higher, as in fig 5, can call for more heatwork to reproduce the entire pattern. Substantial thinning and flow of the glass is needed to pick up the CF paper texture in its entirety.

Fig 5

When the glass slumps over a sharp edge it will form a clear line at the top of the slump (A) and a much less defined curve at the bottom (B), as in Fig 6.

Fig 6

Slumping ‘into’ a circle can give a different effect to slumping ‘over’ a circle. Consider  figs 7A & 7B —the greatest distortion of the glass will occur at ‘X’, in one case inside the circle and in the other outside. it.

The greatest effect on transmitted or reflected light will occur in this region, so one circle will appear larger than the other, even though they are the same size.

Fig 7A

Fig 7B

Using more ‘chunky’ items to create the relief will cause more distortion of the top surface and require more heatwork to smooth or level it out, as shown in figs 8A & 8B.

Fig 8A Fig 8B.

 The glass must first flow downwards and then sideways from the high points as it becomes more fluid. All of these patterns will be ‘embedded’ in the underside of the glass and it will be thinnest where the pattern is boldest.

Embossed detail.

Features which are to stand proud of, or to be ‘embossed’ onto the surface of the glass must be carved or otherwise imprinted into the supporting surface so that the glass will flow downwards into the pattern.

Fig 9

The feature can be carved or cut into flat sheet material such as Fibreboard, Ceramaguard or Calcium Silicate board, so that the board becomes a mould. As was seen previously, the greater the heat work done, the more the glass will flow to create a level top surface. In this case it will flow from the sheet into the depressed feature. The feature will become thicker than the parent sheet, rather than thinner which is what happens with embedded detail.

Some commercial operators use a sand bed hearth with a layer of fine sand up to 12mm deep covering the entire hearth. An edging on the hearth allows the sand to be screeded smooth after each firing, allowing new designs to be embedded,

Using frames on hearth or kiln shelf.

A temporary arrangement can be made for use on brick hearths by using strips of fibreboard pinned to the brick surface using fine wire pins or brads. They can be hammered in easily and the hole will be filled by bedding powder when the pins are removed. Cut the strips reasonably wide (50mm or so) and give them one or two coats of colloidal silica and they will have a long life. If cut long and arranged as in fig 10 they can create deep beds of whatever size required. 

Fig 10

A bold pattern can sometimes create problems when fitting a slumped piece into a a solid frame. Should such a  feature coincide with an edge, then difficulty can be experienced in mounting the glass, or fitting a bead. It is fairly obvious that, to avoid this, one should position the feature so that it is kept away from the finished edges or cut lines.

One way of doing this is to make a frame as described above with the inside marginally smaller than the size of the opening into which the glass will be mounted, fill with bedding powder and then embed the design. When the glass is placed on the frame the edges will always be flat, irrespective of how bold be the design.

Remember, it is important that all the surfaces which may be contacted by the glass, are coated with battwash or otherwise appropriately treated.

Fig 11

For kilns with shelves, one shelf can have border strips permanently attached as shown in Fig 11. In this case the shelf is for a Riley GS-1 and the strips are cut from a silimanite batt and cemented in place.

Temperature for slumping

As well as the faces of the glass, changes will also be taking place to the edges. Most of the soda-lime glasses will slump and take up surface texture at temperatures between about 700°C  and 800°C (1290°F & 1470°F). 

As has been seen, at the lower end of this range the glass will be stiff and will just ‘settle down’ onto major supporting features, whilst at the higher end it will ‘flow’ sideways as well as downwards.At the lower end the edges may be square, have barely lost their sharpness, whilst at the higher end there can be rounding and even some undesirable downward flowing of the glass. The longer the soak at any of these temperatures the more of the settling or flowing will be done.

The trade-off between two desirable objectives, texture detail and edge development, can  be resolved in two ways; either by high soak temperature for a short soak time or by a lower temperature for a longer time.

To illustrate how these apparent conflicts can be harmoniously resolved, paint decorated objects such as small dishes and coasters made from 4mm to 6mm float are often shaped textured and decorated in a single firing of about 30 minute soak at 780°C (1440°F ) in Riley Top-Hat kilns. Different soak temperatures and times may be needed with different types and brands of kiln. 

Texturing agents

Over time, kilnformers have used a varied collection of materials to support and to impart texture to the glass during a firing. They include plaster, whiting (calcium carbonate), kaolin, sand, calcined alumina and alumina hydrate.

For more on where and how to use these materials see 13.Texturing and bedding agents.