I recently made a statement about the effects of various dam materials on the scheduling. This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board. I decided to test these statements. I found I was wrong.
I set up a test of the heat gain and loss of the three materials. This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams. The dam materials were laid on the kiln shelf with thermocouples between. These were connected to a data logger to record the temperatures.
The schedule used was a slightly modified one for 6mm:
300°C/hr to 800°C for 10 minutes
Full to 482°C for 60 minutes
83°C to 427, no soak
150°C to 370°C, no soak
400°C to 100°C, end
The data retrieved from the data recording is shown by the following graphs.
· The dam materials all perform similarly.
· This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case).
· There is the curious fall in the dams’ temperatures during the anneal soak. This was replicated in additional tests. I do not currently know the reasons for this.
· The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.
· From the second cool to the finish, the dams remain hotter than the air temperature.
Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air. This graph is to assist in investigating how significantly different the materials are.
This graph is initially confusing as positive numbers indicate the temperature is cooler than the material being compared and hotter with negative numbers.
As an assistance to relating the ΔT to the air temperature some relevant data points are given. The data points relate to the numbers running along the bottom of the graph.
Data Point Event
1 Start of anneal soak.
30 Start of 1st cool (482°C)
45 Start of 2nd cool (427°C)
65 Start of final cool (370°C)
89 1st 55°C of final cool (315°C)
At the data points:
· At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.
· At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.
· At the end of the 1st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.
· At approximately 450°C the air temperature becomes less than the dams.
· At 370°C the hottest dams are approximately 17°C hotter than the air. The ΔT between the dams is 10°C.
· The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool. The difference gradually decreases to around 8°C at about 120°C.
· Ceramic and fibre dams loose heat after annealing at similar rates – generally having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.
· Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre. Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do. This means that vermiculite is the most heat retentive of the three materials.
· Dams will have little effect during the heat up of open face dammed glass. The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass. Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.
· The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the glass by the dams to give unequal temperatures across the glass with the risk of inadequate annealing. I suggest the soak should be extended to that for glass of 6mm thicker than actual to account for this.
· The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board. The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.
Based on these observations, I have come to some conclusions about the effect of dams on scheduling.
· There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.
· The lag in temperature rise by the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C can be used.
· The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.
· The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool.
· Ceramic dams perform the best of the three tested materials.