Draping glass over cylinders or similar shapes presents some ordinary problems in a problematic combination.
- · In general, the glass is a long rectangle
- · The glass is supported on a long thin part of the mould
- · The glass is usually high in the kiln
- · The mould is heated unevenly
- · The material of the mould influences the way the glass is heated
- · The characteristics of the glass interacting with the mould material
Especially in smaller kilns, a long rectangle will receive uneven heat. The short edges of the glass are nearer the sides of the kiln than the long edges are. This means that the ends nearest the sides are in relatively cooler parts of the kiln in a top fired kiln. It is the opposite in a side fired one.
Long thin support
A drape on a cylindrical mould means the glass is supported on only a long thin part of its substance. This further increases the temperature differential in the glass. The unsupported glass receives both radiant heat and heat transmitted through the air, allowing the unsupported glass to heat faster than where the glass is in contact with the mould.
Glass high in the kiln – the effect of placing glass on top of a cylindrical mould – heats more unevenly than on the shelf.
Uneven mould heating
The mould directly under the glass will be shaded from radiant heat, but will continue to be heated by convection of along the lower sides.
The two common mould materials are steel and ceramic. These gain heat at different rates. The steel generally heats more quickly. The ceramic is usually thicker, so with a greater mass, and the heat transfers more slowly through the ceramic than an equivalent mass of steel.
Glass is a good insulator of heat. This means that heat transfers to the mould supporting the glass more slowly than through the air.
The question becomes how to overcome or at least alleviate these limitations.
Relatively narrow glass sheets that extend near side elements will heat those narrow edges more quickly than on the long sides. Top fired kilns often have the opposite problem, as the short sides may be in the cooler part of the kiln. The usual solution is to reduce the rate of advance, or to baffle the hot parts. Either of these should work well in this circumstance.
The long thin support of the glass creates the problem of a heating differential. The glass may be in contact with half a centimetre of the mould all along its length. The glass and mould heat at different rates. The normal solution to this is to slow the rate of advance. The slower rate of advance can be combined with periodic soaks 100⁰C intervals.
Glass high in the kiln needs special care, as the heat is more uneven there than most parts of the kiln on the heat up. A general rule of thumb is that the radiant surface temperature given by the elements evens out at a distance from the elements. This distance is determined by the distance between the elements. The radiant temperature evens at a distance that is one half the distance between the elements. If your elements are 100mm apart, the radiant temperature will only be even 50mm below the element. Any glass closer than this will require slow schedules to overcome this uneven heating.
Uneven mould heating
As described earlier the mould will be heated by convection current of the hot air, rather than directly the radiant heat from the elements. To reduce this difference, the rate of advance needs to be slow.
Although there are other materials, steel and ceramic are the most common materials from which moulds are made. Steel gains heat much more quickly than ceramic. In the forms used for glass draping, ceramic has much more mass to heat than steel. Steel also transmits the heat more quickly. This means that a steel mould can give a hot line under the glass, and ceramic a cool line. Reduction in the rate of advance will assist in overcoming this differential heating.
Experience has shown that a very slow rate of advance to a soak of 20 minutes at 100⁰C will allow the temperature to equalise between the glass and mould. However, too fast a rise after that will cause thermal shock possibilities. So, increase the rate of heating by 50% to another 20 minute soak at 300⁰C. Follow this by a rate twice the initial rate to 500⁰C for another 20 minutes as a precaution. Then proceed to fire at a normal rate.
These precautions are not necessary on the annealing cool as the glass will be in contact with the mould.
Glass is a good insulator, so the heat passing to the mould will be less than through the air. With steel, this will give a hot line and with ceramics a cool line. Slowing the rate of advance will help reduce this differential. Experience has shown that placing a sheet of 1mm fibre paper over the mould will also help to reduce the effect of the temperature differences. You can place a sheet of Thinfire or Papyrus over the fibre paper to retain as smooth a surface as possible.
The best defence to the thermal shock of glass on a cylindrical mould is to reduce the rate of advance with periodic soaks to equalise the temperature. The addition of fibre paper to the cylinder is an added protection against uneven heating from a hot or cold spot on the mould.
But why does the glass break at right angles to the length of the mould?
I have talked of the long thin contact line between the mould and the glass. “Why does the glass not break along the length of the glass?” I hear you ask.
In thermal shock, the break will occur on the line of least resistance. In these cases that is on the short sides.