Thermal expansion question


I have a question for any pipe smokers in the group. It is often claimed that too much carbon build up inside the pipe bowl--the cake--can cause the bowl to crack. It seems to me the only way this could happen is if the thermal expansion of carbon is significantly greater than that of briar wood. Can anyone confirm or deny this?
Bradley
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I've not smoke a pipe for over 30 years now; but I never heard of that being a problem. I never let the cake build up all that much either I guess. Anything is possible, but in reality, wouldn't the cake also expand to the center of the bowl and relives some of the stress if it did expand more than the bowl?
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[...]

No. If that were the case wood when shrinking would not develop cracks but compression zones...
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Dr. Juergen Hannappel http://lisa2.physik.uni-bonn.de/~hannappe
mailto: snipped-for-privacy@physik.uni-bonn.de Phone: +49 228 73 2447 FAX ... 7869
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Juergen Hannappel wrote:

Everyone know that a disk expands. A hole in a sheet expands just like the disk does. At the molecular or atomic level the reason for the whole expansion is also clear.
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The "cake," so far as I recall, was unburned tobacco residue. The briar merely charred on breaking in, thereafter the burning tobacco was contained within the "cake," which was periodically thinned to maintain capacity and avoid the taste of partially burned tars.
Also, in spite of assertions otherwise, expansion takes place in the direction of least resistance. If the coefficient of expansion were radically different, the "cake" would flake off on its own.
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snipped-for-privacy@yahoo.com wrote:

Well if the cracking is true, and it probably isn't, it would be due to thermal expansion for two reasons. First, the thickness of the carbon is relatively thin so it won't expand much, regardless of the expansion coefficient. Second, the thermal expansion of carbon is significantly less than that of the wood.
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snipped-for-privacy@yahoo.com wrote:

Consider that the inner portion of the bowl, whether lined with carbon or not, operates at a much higher temperature than the outer portion does. A thick carbon cake offers the advantage of insulation, allowing a somewhat higher temperature differential. The carbon cake is more dense than wood, and most likely doesn't have much resilience (it's brittle), thus has nowhere to go but outward when heated. Since the wood bowl constrains the carbon cake, a thick cake could crack a weak wooden (or meerschaum) bowl when hot. But lots of other things crack pipe bowls, too. Briarwood makes good pipe bowls in part because it is resistant to cracking.
So even if briar and carbon had identical coefficients of expansion, the carbon's higher temperature and lack of resilience could result in a cracked bowl. But mine usually crack for other reasons.
Dale
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If heat is the primary culprit, then an uncaked bowl would be much more likely to crack than a bowl lined with a carbon cake since the cake provides insulation. But in a bowl without cake, burnout will occur before cracking.
The specific claim I am questioning is that a thick cake is a cause of cracking a bowl. I'm not sure I understand your claim that given equal coffecients of expansion the higher temperature of carbon along with lack of resilience could cause a crack in the bowl. Are you saying that the heat is what causes the crack and not the thickness of the carbon?
Bradley
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