Kilnforming a platter

Detailed here are the steps in the making of a platter from 6mm thick float glass (window glass).

Step 1: Obtain the glass blank

A piece of float glass 400mm x 250mm x 6mm thick is required. It could be obtained from a local glazier, either cut to size or in a larger piece to be cut down. Glass merchants are expert at cutting glass and can meet one’s need with minimum hassle. Art glass outlets can often supply ready-cut blanks, not only in float but also in many of the specialised kilnforming or leadlighting glasses. 

Step 2: Preparing the glass

For the corners to form properly it’s desirable that the sharp points be removed. It can be done by diamond grinding, using a grinder especially designed for glass working. Just a small radius on the corners is all that’s needed.

It’s also good practice to lightly grind the edges, especially if they were chipped during cutting, or if the break was uneven. If one lacks a grinder, nipping the corners with special pliers called grozing pliers will do the job reasonably well. An oil stone can be used to clean up the edges if chipping is of concern. Don’t worry if the edges go frosty looking, that’ll disappear during the firing.

Step 3: Cleaning the glass

To produce a bright and shining fired piece it’s essential that the glass be absolutely clean and free of smears or other surface contamination before it goes into the kiln. Residue from masking tape or labels can be removed using methylated spirit or a solvent. Oil residue or smears from the cutting operation or from greasy fingers can be removed using a few drops of dish washing liquid in water.

The glass should then be rinsed in clean water and thoroughly dried by draining or by wiping using old towels or similar fluff free cloth. From here on, handle the glass only by the edges. Another good reason for grinding the edges - less likely to cut oneself.

Why this accent on cleanliness of the glass? Any surface contamination such as fingerprints, dust, fly spots, bits of glue, can be fired into the glass and can reduce the shine or even look downright unsightly.

Step 4: The Mould

The mould gives shape and form to the fired glass piece. Moulds can take many forms.This mould is made of fibreboard. There's lots on moulds and other ways to shape and texture glass in the premium content section 'Advanced kilnforming'..

Step 5: Preparing the mould

Glass will stick to most surfaces during a firing, so it’s necessary to use some sort of separator between the glass and the mould or support surface. A mixture of refractory powders in water is called a ‘battwash’.

It is usually applied as a weak solution by brush, using light even strokes so as not to build up too thick a coating nor too ‘ropy’ or streaky a finish.  A number of coats are applied, with each coat being dried before applying the succeeding one. The mould should be thoroughly dried before use.

Why is it called battwash?
Because the kiln shelf is called a ‘batt’ by potters, and they first used the wash coat to prevent their glazes (glassy coatings) sticking to the shelf (batt).

Step 6: Preparing the kiln shelf

When the glass slumps into the cut-outs in the mould it will touch the shelf and could stick to it, so the shelf also needs to be battwashed. The procedure is the same as for battwashing the mould.

Use a weak solution so there’s lots of water to be absorbed by the shelf. Apply the mix evenly and in thin layers, avoiding large variation in the thickness of the coating, or of excessive streakiness.

Apply a number of layers, brushing in opposite directions for each layer and allowing the previous layer to dry out before applying the next. When dry, smooth out any brush marks by very lightly rubbing with a soft cloth.

Step 7: Loading the kiln

The photo shows our platter being loaded into a Riley GS ‘Top Hat’ kiln. Top Hat kilns are opened by raising the hood off the hearth, in this case by means of a small jack and cables. For ease of access, the hearth on this kiln rolls out.

First, the shelf is placed on small props on the hearth so that heat can circulate all around it. To not do so, but to put the shelf flat onto the hearth, would cause such uneven expansion of the shelf as it heated that it could crack. Next, the mould is placed on the shelf. If desired, a small object or cut-out design can be placed on the shelf within the mould cut-out area to imprint a pattern in the glass when it slumps down onto it.

Finally, the glass is placed on the mould, making sure that the glass remains free of smears and fingerprints and that it is centered on the mould and the margins are even all round.

Check to see that the shelf is spaced evenly within the lines scribed on the hearth. This to ensure that the hood won’t strike the shelf as it descends. Push the hearth back into the kiln frame and against the back stops. Lower the hood by slightly opening the jack valve.

Step 8: Entering firing pattern in controller

The firing pattern is the instruction given to the controller as to how the glass is to be ‘cooked’. The level of complexity and accuracy of control varies widely with the type of controls fitted. In this instance the kiln is fitted with the most modern controller which can take the kiln through up to 9 steps of heating, soaking and cooling with extreme precision.

Other kilns may have a controller which approaches the cooking as if it’s a microwave oven cooking a chook. Just tell it the size of the piece of glass and how hot and fast one wants it cooked and it looks after the rest.

Still others, more suitable for pottery, may have a temperature indicator but only minimal control of how fast the kiln heats up.

With the controller shown, when the pattern number is selected just press the ‘RUN’ button to start the firing.

Step 9: Demoulding

All that’s now left to do, when the cooking is done, is to remove the glass from the mould and give it a good wash in soapy water to remove any battwash which may have adhered: then sit back and admire one’s handiwork.

Of course, like everything else in life, there are always qualifications. These are discussed at length in the sections on Glass and kilnforming.