Texturing and bedding agents

Texturing & bedding materials, the what and how

These are materials applied to kiln shelf hearth or mould to give texture to glass. They must be able to withstand glass firing temperatures, so will be from that class of products known as refractory materials. They should be free of organic matter to avoid out-gassing or staining of the glass.

They must not readily stick to glass, although some adhesion is to be expected at higher temperatures.

They are usually used in powder form so that they can be applied by sieve. Sieving is the most effective way of achieving reasonably uniform distribution.

They should preferably not undergo change if they become damp or wet; should revert to their original form when the moisture is dried out.

Over time, kilnformers have used a wide range of materials for this purpose. They include alumina hydrate, calcined alumina, kaolin, plaster, sand & gravel, whiting (calcium carbonate).   

These materials differ in chemical composition, appearance and in structure; as well as in the particle size range in which they can be readily obtained. Each will be discussed separately, but first some general comment.

What one expects from a texturing agent depends much on the scale of the work being done and the extent of critical examination to which the fired piece will be subject.

Fineness of detail is nowhere as important in a large panel to be viewed from afar as it is in a small plate or window hanging which will be closely examined.

Often seen on TV are conveyor belts depositing materials such as woodchips, coal, iron ore, in stacks with sloping sides. In some cases the sides will be steep; in others more flat. This is variously called the slumping angle or the angle of repose and gives some indication of how well the material can stand up under load; the steeper the angle the better it will do so.

When a powder is sieved onto a hearth or kiln shelf the top surface will be uneven, with some points higher than others. It is on the high points that the entire mass of the glass must be supported.

When the glass is placed on the sieved powder it will flatten the tops of the higher stacks until the full load is supported.

Particles which are tough with a sharp angular structure will support the load with less flattening and loss of detail than will softer, more rounded materials.

Of course, the care with which the glass is placed in position can greatly affect the outcome. Steadiness of hand, lack of tremor or sideways movement of the glass can all reduce the extent to which the stacks are ‘pushed over’ as the glass is placed in position.

Use of suction cups can greatly improve the outcome; as well as eliminating the ‘finger tip’ marks which often impair the edge formation.     

For the small kiln operator with shelves which can be removed and dusted down after a firing, any of the available materials can be used.

The large kiln operator lacks the luxury of being able to do so. Attractive as the idea  of having a range of options may be, the effort & time involved in removing one material to make way for the next will usually render it uneconomic.

Having a bit of fine powder mixed in with a coarse grit is no problem, but the reverse can be trouble indeed.

Particle size

This can be denoted in either of two forms;

• As sieve size through which the material will pass, or
• As particle size measured in microns.

Mesh or sieve size is denoted by the number of holes per unit measurement, so the larger the number the finer the particles; thus 100 mesh or 300 mesh with 300 mesh being the finer or smaller.

Measurement in microns denotes the size of the holes in the screen in millionths of a metre; so the smaller the number the finer the particles; thus 150micron or 50micron with 50 micron being the finer or smaller.  

Imagine a tiered series of sieves or screens of different mesh size, arranged so that the coarsest mesh or hole size is at the top and the finest at the bottom, with intermediate sizes in between.

A particular mesh size product will contain all that which passes through the coarser screen and collects on the next finer screen. It is not one size but a collection of many sizes and the mix is identified by the size of the screen through which it passed.

There are numerous International or National standards in use, some conflicting slightly one with another. The following table gives some idea of how the mesh and micron scales compare.                                                

A particular material will not be available in all, or even many, of these sizes. The range is too broad and the market in Australia is too small.

100 mesh/150 micron is a common size, as is 300 mesh/50 micron. Note that, 50micron, not 53. With such a wide range of sizes mixed in, who’s going to haggle over a small number. But then, maybe it was a 50 micron sieve and not the international standard size at all.

Sources of supply/Packaging

Most of the refractory materials can be obtained from potters suppliers in bulk or in repackaged quantities of various amounts down to 1/2kg for some items.

Bulk packs will be what can be safely lifted and retail packs will reflect the cost of the repackaging.

Storage

Properly stored, most of the materials can be used over and over again; and will last indefinitely. They must be kept dry and free of contamination. If using more than one material they should also be kept free of cross contamination.

For the small kiln operator, two or four litre ice cream containers make admirable receptacles. A small wire strainer about 50mm to 80mm (2” to 3”) in diameter kept in each container makes for speedy application. Brush the used material back into the container and stir before re-use.

If using a brick hearth, be careful not to brush up loosened brick material with the powder; it will often be coarser than the texturing powder.

Purity, calcining

Most of the materials are produced in large quantities for commercial and industrial applications. The quantities used by kilnformers are but a drop of water in an ocean.

Those sourced from the earth by quarrying can contain vegetable matter (organic matter), whilst others can contain chemically combined or absorbed water, which can be removed by a process called ‘calcining’; the process of removing organic matter and chemically combined water by heating. The removal results in reduction of mass.

Calcining is sometimes done in bulk and the product is sold as calcined, as with ‘calcined alumina’. In other cases the value of the product will not justify the added processing cost, so the material will be sold in its natural or raw state.

This is the case with many of the refractory materials used by potters. The calcining takes place during the initial or ‘bisque’ firing of the ware and results in considerable shrinkage of the product. There can also be emission of gases.

Now to some of the materials

Alumina hydrate

Fine white powder. Sharp, supports load well. Commonly available in 50 micron grade.

Due to the loss of chemically combined water, the material shrinks during the first firing. The hydrogen atoms combine with oxygen to form water vapour at temperatures around 300°C (570°F). It will have all dissipated before it can have any effect on the glass.

The material loses about 30% of its mass during this dehydration process. No difference will be noticed between new or re-used powder when sieving.  

However, a fully compacted layer of new powder will form an attractive spider web like pattern on slumped glass. Just make a thick layer of new, unused powder, press the glass firmly into it so that there is 100% contact, remove any ridges around the edges and fire it.

After the first firing the material becomes a pure form of calcined alumina.

One peculiarity which has been noticed is that whilst the 300 mesh 50 micron product works well when fired to low 800 degrees, 100 mesh 150 micron powder does not stick to the air side of float but does stick to the tin side.

Calcined alumina

Depending on the supplier, may be available in 240 or 300 mesh. Both work OK. Also available in 100 mesh, but this is not recommended as it has been found to sometimes stick to the tin side of float; but not to the air side. 

Kaolin

Off white to pale brown clay-like alumino-silicate material. Often changes colour to pale pink after firing.  Used as ingredient in battwash. 

Sightly gritty feel when first rubbed between fingers but becomes smoother when extra force applied.

In Australia is often sold under the Eckalite brand, with a series of grades dependent on the level of iron impurity. Cheapest grade is Eckaglass, followed by Eckalite 2 with Eckalite 1 the most pure.

Available in other, cheaper forms including ‘china clay’ and ‘ball clay’. Not calcined, so have considerable shrinkage. 

Plaster

Sets irreversibly if wetted, so must be kept absolutely dry. Use only building products free of organic additives, preferably material labeled as potters plaster. Do not use handyman type patching products as these can contain cellulose and other additives.  

Whiting

Calcium carbonate. In powder form it has been widely used wherever leadlighting is done and is a logical first choice for the leadlighter venturing into kilnforming.

It decomposes at 825°C (1515°F), giving off carbon dioxide gas. In spite of this, and the risk of the gas causing large bubbles to erupt in the glass, it is still often used as a bedding powder.

Soft, collapses easily under load. Feels like talcum powder when rubbed between fingers.

Changes colour to fawn after firing. 

Can sometimes cause a cloudy effect somewhat like tin bloom appearing on the glass. As this also appears like incipient devitrification, some believe that whiting promotes devit.

Product labeled ‘whiting’ may become difficult to find. This is due to small demand in Australia. Products labeled ‘calcite’ are also calcium carbonate and are suitable alternatives for leadlighters but should be used with caution by kilnformers.

Sand & gravel

Some will bond fairly readily to glass, so the effect of a coarse grit can be achieved by sieving a thin layer of one of the fine powders over the top of the sand or gravel as a separator.

Whatever its source, the material should be washed and dried before use.

One material available from some garden suppliers and nurseries is packaged in 20kg clear bags by Cement Australia and labeled as ‘washed sand ideal for rendering, bedding of tiles, concreting and sandpits’ with a bar code number 9311808 935264.

It has been found to be dry, well washed and free flowing, and suitable as a texturing material or as a sand bed in deep bed sand hearth applications.

It may be available from outlets such as Bunnings, but other washed products from that outlet are nowhere near as suitable.

Coarser gravels from garden and building suppliers require much preparatory washing, and testing to ensure they don’t outgass and break down during firing.

Other materials may sometimes be considered, but they are unlikely to have advantages over those already mentioned. Whilst materials such as talc (talcum, hydrous magnesium silicate) are cheap, they are usually not calcined so will contain organic and other foreign matter.