Effect of Air Space Around Shelves

The Bullseye research on annealing thick slabs indicates that it is important to have a 50mm space between the shelf and the kiln walls. This is to assist even distribution of the air temperature above and below the shelf.

I decided to learn what the temperature differences are between ventilated and unventilated floors of kilns. The recording of the temperatures was conducted using pyrometers on the floor of the kiln and in the air above the kiln shelf. The pyrometer above the shelf was at the height of the kiln’s pyrometer. The recording was done during normal firings of glass. The graph below shows temperature differences during a typical firing.

The blue line indicates the air temperature, the orange line the floor temperature and the grey line the difference in the two over the whole firing. Each horizontal line is 100C

The next graphs show in more detail the differences between having no significant space and another firing with space between shelf and kiln walls.

Horizontal axis legend:

1.  = 300°C
2.  = Softening point
3.  = Top of Bubble Squeeze
4.  = Top temperature
5.  = Start of anneal soak
6.  = start of first cool
7.  = start of second cool
8.  = start of final cool
9.  = 300°C
10.  = 200°C
11.  = 100°C
12.  = 40°C

The general results are that there is a greater difference during the rise in temperature and a reducing difference in floor and air temperature during the anneal cool. However, there are significant differentials at various points during the firings.

Space between the shelf and kiln walls:

• Smaller temperature difference is experienced on the heat up.
• Floor stays hotter than the above shelf air temperature during the anneal soak.
• This difference gradually equalises during the anneal cool

Without space between the shelf and kiln walls:

• Significantly greater difference on heat up is experienced – over 100°C cooler than ventilated floor area.
• Floor temperature is less than air until the final cool.
• During the anneal soak the floor temperature difference becomes larger than at start of anneal. This seems to be the consequence of heat continuing to dissipate through the kiln body, while the air temperature above the shelf is maintained at a constant temperature.
• The difference between the air and floor temperature gradually reduces during the anneal cool as the whole kiln and its contents near the natural cooling rate of the kiln.

 

This appears to indicate that space between the shelf and kiln walls helps to equalise the temperature during the critical anneal soak and first stage of the anneal cool. This will be particularly important when annealing thick slabs.

These tests were done in a kiln of 50cm square. It is likely that the differences would be greater in a large kiln, making it more important to have the air gap between shelf and kiln wall. Smaller kilns and thinner glass seem to be less affected by these differences.

Note that the air temperature and shelf temperature differences in these firings maintain the same character whether the floor has good circulation or not. The shelf temperature lags behind the air temperature throughout the heat up.

The fact is that floor and air temperatures are nearer each other with air space around the shelf. The difference reduces during the bubble squeeze and the top temperature soak. The difference in temperature on cool down is small. During the anneal soak and cool, the shelf tends to be a few degrees hotter than the air temperature.

There was no difference in the amount of stress in the glass in these tests on a small kiln whether there was a gap or not between the shelf and the kiln walls.

Implications for kilns with multiple shelves

Those using multiple shelves in a single firing load should take note of the implications from this. It is important to have significant ventilation between layers to get consistent results from firings.

The ideal would be to have larger than 50mm/2” gap around the upper shelf. Possibly 100mm/4” would be a good starting point. This would allow sufficient heat circulation to compensate a little for the lack of radiant heat from the elements.

If you have a really deep kiln and are using three shelves, the ideal would be to start with a 50mm/2” gap around the bottom shelf. Then a 100mm/4” gap around the middle shelf and finally a 150mm/6” gap around the top shelf. This will assist the heat to circulate to the bottom layer.

 

There are greater differences in temperature between the floor and above shelf air temperature when there is no ventilation space around the shelf. This is especially the case during the anneal soak.