Unsolved Problems in Kiln-drying
1. Physical data of the properties of wood in relation to heat are meagre.
2. Figures on the specific heat of wood are not readily available, though upon this rests not only the exact operation of heating coils for kilns, but the theory of kiln-drying as a whole.
3. Great divergence is shown in the results of experiments in the conductivity of wood. It remains to be seen whether the known variation of conductivity with moisture content will reduce these results to uniformity.
4. The maximum or highest temperature to which the different species of wood may be exposed without serious loss of strength has not yet been determined.
5. The optimum or absolute correct temperature for drying the different species of wood is as yet entirely unsettled.
6. The inter-relation between wood and water is as imperfectly known to dry-kiln operators as that between wood and heat.
7. What moisture conditions obtain in a stick of air-dried wood?
8. How is the moisture distinguished?
9. What is its form?
10. What is the meaning of the peculiar surface conditions which even in air-dried wood appear to indicate incipient "case-hardening"?
11. The manner in which the water passes from the interior of a piece of wood to its surface has not as yet been fully determined.
These questions can be answered thus far only by speculation or, at best, on the basis of incomplete data.
Until these problems are solved, kiln-drying must necessarily remain without the guidance of complete scientific theory.
A correct understanding of the principles of drying is rare, and opinions in regard to the subject are very diverse. The same lack of knowledge exists in regard to dry kilns. The physical properties of the wood which complicate the drying operation and render it distinct from that of merely evaporating free water from some substance like a piece of cloth must be studied experimentally. It cannot well be worked out theoretically.