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The true meaning of COE


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#1 yedrow

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Posted 28 December 2012 - 01:07 AM

Is there a cut and dry definition of COE? My Insight glaze calc gives a number. I understand that to be something like, for example, 7.25 x 10 -6, but the number given is simply 7.25. Does that number come from the percentage of expansion divided by the temperature change, or something to that effect? If not, can anyone tell me where it comes from? Other glaze calcs seem to have a different number, but I'm guessing that the zeros are just changed.

Joel.

#2 Marcia Selsor

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Posted 28 December 2012 - 11:21 AM

Joel,

I found a good tutorial as well as a simple ceramics explanation.
Here is a physics tutorial:
http://www.physicstu...and-contraction

Nelson's "A Potter's Handbook" has a simple definition in the glossary:Coefficient of Expansion is the ratio of change between the length of a material mass and the temperature to which it is subjected.

If you read both the Physics tutorial and Nelson's definition it may gel.

Marcia

#3 Mudlark

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Posted 28 December 2012 - 02:34 PM

Robert Fournier's Illustrated Dictionary of Practical Pottery ISBN9057034042 published by Craftsman House explains it quite well. For a more technical discussion you may try reading Chapter 6 of Ceramics Glaze Technology by J.R.Taylor and A.C.Bull ISBN0-08033465-2 or 0-08-033466-0 published by Pergamon Press. In practical terms you take the coefficient of expansion and move the decimal point 6 places to the left to give the rate of expansion per degree temp change. There is some disagreement for the values of some compounds in the Tables given by English and Turner and Winklemann and Scott which may be cause for confusion but potters being practical people just average them, which seems to work. Another source for information on this subject is Ceramic Whitewares by Rexford Newcomb, Jr published in 1947 by Pitman which may be available from a library.

#4 yedrow

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Posted 28 December 2012 - 11:03 PM

Coefficient of Expansion is the ratio of change between the length of a material mass and the temperature to which it is subjected.


Thanks Marcia, that is exactly what I needed to know! Thank you both for the help. I think this is coming together. I've just red an article by Sohngen and revisited Ron Roy and I think I have a grasp of this now. I'll put it in another post and feel free to tell me where I'm getting it wrong.

Joel.

#5 pricklypotter

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Posted 29 December 2012 - 01:34 PM

Is there a cut and dry definition of COE? My Insight glaze calc gives a number. I understand that to be something like, for example, 7.25 x 10 -6, but the number given is simply 7.25. Does that number come from the percentage of expansion divided by the temperature change, or something to that effect? If not, can anyone tell me where it comes from? Other glaze calcs seem to have a different number, but I'm guessing that the zeros are just changed.

Joel.


the answer is yes. it's a coefficient of linear expansion (ie how much length, delta L, as a part of the original length L is added with added temperature, delta T). see for example http://www.plainsman...m/data/H550.HTM or similar graphs is glaze chemistry books (i think there are some in MC6G)
a little twist in this: you'll notice the number you're getting is for the area where the curve is a straight line, and that the curve is not straight at all points (ie at certain temperatures your clay/glaze will lengthen MORE with temperature change than at other temperatures. this happens for example around the quartz inversion and around cristobalite inversion, which are temperatures at which the clay/glaze undergo a sudden and bigger expansion). as for the number of zeros, it doesn't matter as long as you omit the same number of zeros from all such figures. Like writing 'Neph Sy 35' in a glaze recipe and meaning '35%' or '0.35'




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