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I have noticed that when I burn a wood log in my fireplace, the log will soon take on a kind of 'alligator' pattern, with stripes (cracks!) somewhere between half an inch to an inch apart. See picture:

enter image description here

I have 2 questions:

  1. Why are the cracks always straighht along the log, or perpendicular to it? Why not diagonal?

  2. Why is the spacing fairly consistent at between half to one inch : why is that? Is this because of something in the wood log, is it some kind of stable attractor, or ...?

P.s. Not sure what tags to use here ... Or if this is even the right site for this question ...-

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    $\begingroup$ Can you post a photo. It's hard to know exactly what you mean otherwise. $\endgroup$ – WetSavannaAnimal Nov 25 '16 at 3:26
  • $\begingroup$ @WetSavannaAnimalakaRodVance Ha, my fire just died out! Ok, I'll make sure to take a pic next time I have a fire. In the meantime, let's see if I find a representative pic online (I assume what I see doesn't just happen in my own fireplace! ) $\endgroup$ – Bram28 Nov 25 '16 at 3:33
  • $\begingroup$ Darn, I never know how to copy and paste on my device .. But Google 'burning wood log' for images and a bunch of these patterns will show up. $\endgroup$ – Bram28 Nov 25 '16 at 3:36
  • $\begingroup$ It is your question. It is not for us to look for photographs to find out what you are asking about. If you want an answer, you should do the work to make yourself understood! $\endgroup$ – sammy gerbil Nov 27 '16 at 1:33
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When wood burns, it oxidizes into carbon dioxde, water vapor, and carbon resdue. The carbon residue shrinks, which is why vertical cracks appear around the circumference of a log. The log gets shorter, and breaks into sections as the wood fibers are no longer strong enough to hold the shrinking log together as one solid piece.

Wood species have different percentage shrinkage values down to zero percent moisture content. For example, Table 2 of this article on wood and water relationships lists radial (shrinkage across the grain) and tangential (shrinkage around the circumference of the log) percentages for different wood species. The "alligator" pattern you noted will differ in different wood species.

The distance between cracks also may be affected by how rapidly the wood shrinks - how hot the fire is. If shrinkage is rapid, the ash residue may undergo greater stress than during slow shrinkage. Greater stress may cause the ash residue to fragment more fequently (less distance between cracks) than if there were less stress in a slower-burning fire.

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  • $\begingroup$ Thanks! But do you know why the spacing is as it is? Why not every 2 inches, or every one tenth of an inch? Why this fairly predictable half to one inch? Does that change due to circumstances? E.g. If there would be more oxygen available the spacing would decrease? But again, why? That is what I am trying to understand. $\endgroup$ – Bram28 Nov 25 '16 at 3:49
  • $\begingroup$ Ah, you worked out what the OP meant. As soon as I saw your answer, I did too! $\endgroup$ – WetSavannaAnimal Nov 25 '16 at 7:17
  • $\begingroup$ @Bram28: I expanded the answer to provide some possible reasons for spacing of cracks in burning wood. $\endgroup$ – Ernie Nov 26 '16 at 3:18
  • $\begingroup$ Oh, I see: the wood shrinks and therefore cracks, but it only requires only so many cracks to relieve the stress. That makes sense, thanks! $\endgroup$ – Bram28 Nov 26 '16 at 3:30

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