Monday, 21 February 2011

Initial Rates of Advance

Almost everyone has fired the glass from cold too fast at least once. This provokes the question of how fast it is possible to fire glass. Of course, there are lots of variables which relate to the evenness of heat across the firing surface of the kiln. A number of factors will affect this. Among them are:

  • The size and shape of the chamber
  • Composition of the chamber – brick, fibre or a combination of both
  • Location of the elements - lid, sides or elsewhere
  • Distance of elements from the work
  • Spacing between the elements

This means that only general guidance can be given for you to use in gaining experience with your kiln, as your kiln is unique, even if it is one of a production series.

Now with all those cautions, some general guidance can be given. This table assumes that you have a top fired kiln and that the glass is about 300mm below the elements and that it is at least 50mm from the sides of the kiln. It also assumes that the glass has not already been fused.

First Segment Heating Times for Top Firing Kilns
by Number of even 3mm Layers and Size

100mm dia: 1 layer - 999C/hr || 2-3 layers - 999 || 4 layers - 750 || 5 layers - 600
200mm dia: 1 layer - 999C/hr || 2-3 layers - 500 || 4 layers - 430 || 5 layers - 330
300mm dia: 1 layer - 999C/hr || 2-3 layers - 335 || 4 layers - 275 || 5 layers - 215
400mm dia: 1 layer - 750C/hr || 2-3 layers - 250 || 4 layers - 200 || 5 layers - 155
500mm dia: 1 layer - 600C/hr || 2-3 layers - 200 || 4 layers - 165 || 5 layers - 120
600mm dia: 1 layer - 500C/hr || 2-3 layers - 165 || 4 layers - 135 || 5 layers - 105



First Segment Heating Times for Top Firing Kilns
by Number of Uneven 3mm Partial Layers and Size

100mm dia: 1+ - 850C/hr || 2+ - 750 || 3+ - 500
200mm dia: 1+ - 500C/hr || 2+ - 430 || 3+ - 335
300mm dia: 1+ - 300C/hr || 2+ - 250 || 3+ - 215
400mm dia: 1+ - 215C/hr || 2+ - 188 || 3+ - 158
500mm dia: 1+ - 177C/hr || 2+ - 162 || 3+ - 137
600mm dia: 1+ - 150C/hr || 2+ - 143 || 3+ - 120

If you are adding decorative elements to the basic disk, you need to consider the effect that the additional pieces have on the whole. Each piece of glass shades the pieces below it from the heat of the elements, so to keep the glass heating at the same rate throughout, you need to slow the rate of heating by using the suggestions for the next higher thickness. So, for example, a two layer piece of 300mm can be fired at ca. 330C/hr, but if you have decorative pieces spread across the base, you need to slow the rate of advance to 250C/hr.

Note that these rates should be taken as the fastest possible rates. They may be too fast for your kiln.