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Solar Kiln Project:  

Kiln commentary

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Last updated on 11/05/2016
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How significant is the difference in performance between the solar kiln and the simple wood stack in terms of drying wood?
At first glance the temptation is to say: not a lot. Over the week just 5%, that is 20% - 15%, in the case of the wood freshly introduced into the drying process and an even smaller 2.5% (11.4% - 8.9%) in the case of the wood that had already been drying for a week. 

But that is to ignore a key point. The loss of weight is, as I understand it, almost all water. That makes it important to regard the shrinkage delivered by one drying process in relation not so much to the original weight of wood but to the shrinkage produced by the other process.

Let me explain. If a 1000g block of wood reduces in weight in the kiln by 20%, it is losing 200g of water. If the same block in a wood stack shrinks by 15%, it is losing 150g of water. The first process is stripping 33% more water out of the wood than the second process.

Likewise in the second week the kiln may appear to be only extracting a measly 2.5% more water than air drying but is in fact 28% more efficient.

By the way that is not to downplay the practice of drying firewood in a wood stack. It is a pretty effective method but in my experience will struggle to deliver wood with a very low moisture content over a a single summer.

But ultimately what matters in all this is whether the kiln can in a relatively short period - say four to five weeks - bring the water content of wood down below 20% where it can be compared to conventionally kiln dried wood. 

Whether the solar kiln can yield this result remains to be seen. The new few days look quite unsettled. I expect both processes to have a relatively modest impact on moisture content. 

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