Wednesday, 10 June 2009

Combing Glass

This process is done at relatively high temperatures for fusers – around 925C. It consists of pulling or dragging the surface of the hot glass to produce a marbling effect.

A batt washed ceramic shelf is the best surface. You can use fibre paper on your shelf, but you must be careful to avoid raking deeply enough so that you pick up the fibre and drag it into the glass.

Make a boundary with 10-12mm fibre board on your shelf. You can use strips - for the most efficient use of the board – or cut a shape from a sheet. If you are using strips, fasten them together with wire staples. This will resist the glass flowing at the edges.

Place the glass into the space created by the fibre board. You can place 10mm strips on edge to form very tight lines, you can shingle glass to give broader lines, or you can place the glass in a more random way to give quite different effects.

When shingling or placing glass randomly, it is often best to cut a sheet of iridised clear to lie on the bottom to ensure you have enough depth of glass. Placing the iridised side of glass down toward the shelf provides an additional release, and can give added interest to the back. Anything less than 8-10mm thick leaves the glass pulled away from the edges in the direction of the combing.


The initial temperature rise can be fast because the glass is made up of narrow strips. No bubble squeeze is required for the vertical or shingled strips, as there are easy ways for the air to escape. Randomly placed glass should have a bubble squeeze at around 650°-675°C for 30 minutes at least. Otherwise you can fire at about 300°C/hr to 925°C.

You need to programme a soak at that temperature for approximately 120 minutes. This soak allows you to do the combing and have the kiln recover temperature without needing to re-programme. When the combing is finished you cancel the soak after the kiln recovers to 925°C (which allows any peaks generated during the combing to settle down).

Allow or programme the kiln to cool as fast as it can to the annealing temperature and soak for 120 to 180 minutes. Set the annealing cool at 30°C/hour down to 450°C, then 60°C/hour to 370°C and finally at 200°C/hr to 21°C.


Safety first. You must do you combing in a kiln that has a safety switch to turn the power off once the lid is opened a short way. If your kiln does not have such a safety device you should reconsider your desire to comb in your kiln. Many say you can overcome this by switching your kiln on and off at the wall socket. However, in doing so you also turn off the controller, making it necessary to re-programme your kiln each of the several times you have to reach into the kiln with your raking tool. This is essentially impractical.

The combing tool is a pointed steel rod, bent at right angles to the shaft - often called a rake. The shaft should be of wood to avoid holding a hot metal rod in your hand. Stainless steel rods are best as mild steel can spall and leave flakes of metal in or on the glass. The rod and wooden handle should be soaked in water while the kiln is heating up. The wet wood will not char so quickly as the dry. This bucket of water should remain beside the kiln so you can cool the metal point, when it begins to stick.

The second bit of safety. You will need to wear gear to protect yourself against the heat. A full face visor is important as the heat will singe you hair. You need to have heat resistant gloves. You need to have heat resistant sleeves to go over your arms. You should wear only natural fibres - cotton and wool are best, as they smoke before bursting into flame, giving you some warning that things are just too hot. An assistant to hold the kiln door/lid open while combing is advisable. And the assistant should have the same heat resistant gear that you have.

You begin to do the combing at 900°C. The glass will be soft enough to be pulled by a gentle stroke across the surface of the glass. Avoid digging into the glass. That will pull or push a gather of glass ahead of point. This leaves a characteristic droplet shaped mark in the glass at the end of the stroke. It may also go deeply enough that the kiln wash or fibre paper that is underneath the glass will be pulled up into the glass. Only light pressure is required to do the combing.

You will only be able to do a few strokes with the rake before the temperature of the glass falls and the glass resists movement. When the glass becomes difficult to move, it is time to close the lid and let the temperature recover. You will have to do this numerous times, until you have the look you want.

Another limitation is the speed that the rake metal heats up. When the metal becomes hot, it sticks to the glass. Whenever the rake is not in use, it should be in the bucket of water cooling off, and re-wetting the wooden handle.

You can comb the glass in any manner you wish. To get the traditional feathered look, you need to alternately pull and push the rake to give chevrons in opposite directions. Experienced people sometimes use two rakes – one to pull and one to push - at the same time. You can also rake diagonally across the sheet and even across the previous rakings. Some experimentation will show which effects you like best.