Drying Wood

There are several benefits of dry wood samples rather than wet ones. A dry sample may be ground to a high surface finish and it is easier to store. If the wood is not deteriorated (fresh or waterlogged for not a too long time - in Scandinavia usually less than a few hundred years), cores and smaller pieces may be dried just in the air. The deformation will be approximately 10-20%, and that is usually not a problem for dendrochronological samples. But if the wood is deteriorated it is quite problematic. There are a lot of methods, many of them used by museums in order to conserve artifacts. Many of those methods, like PEG or Freeze drying may not be optimal for dendrochronological purposes, either because the wood structure becomes blurred or because the equipment is too expensive, and available only for a well founded institution like a national museum or a university. Methods which is useful for dendrochronological samples and manageable without too expensive equipment are needed.

The following text, originally an e-mail discussion, contains some preliminary suggestions. More experiences and suggestions are welcome!

When you try to dry wood, think "how to remove water?"!

Water-soluble solvents

One way is using Water-soluble organic solvents.

The principle is to replace the water with a solvent that is easier to remove afterwards than the water. E.g. first replace the water with ethanol and then let the ethanol disappear through a "drying process" which is faster than letting the water air-dry directly from the wood.

Three solvents are often used for this: alcohol, acetone, ether.

Water and alcohol are completely soluble in each other. Nearly absolute alcohol is hygroscopic which means that it probably extracts water from the sample. The same is true for water and acetone, but acetone dries up quicker. You may choose to use either one of the two.

On the other hand, ether is not much soluble in water and has no hygroscopic qualities. But you could use ether, which dries up quickest, to replace alcohol or acetone once the water has been washed away from the sample.

Question: You say “nearly absolute alcohol is hygroscopic which means that it probably extracts water from the sample”. The label of alcohol that I’m using say “alcohol 96% composition: ethylic alcohol 99,9%; Benzalchonium Chloride 0,1%”.

Answer: The label sounds dubious. 99.9 % is nearly absolute alcohol, 96 % is not (it contains 4 % water). Benzalchonium Chloride is a biocide and surfactant, why do you have it in the alcohol, it will probably remain in the dried samples? I think that 96 % alcohol should do the job, but 99.9 % will be faster. Though 96 % is cheaper as it is a natural azeotrope and can be made by distillation. Higher percentages require desiccants like zeolites which are quite expensive.

Question: After the bath in alcohol the sample is drying much faster (and with almost inappreciable deformation until now) than other waterlogged samples. I can buy purer alcohol in a laboratory supplier store, but it’s more expensive. For how long time should the sample be left in the alcohol bath?

Answer: You could exchange the alcohol one or two times. And wait to let the solvent do its job. There is an equilibrium forming between the water in the sample and the water in the solvent. There is no way to get more water out of the sample as there is in the solvent.

A cheep alternative to freeze-drying?

Instead of using a vacuum drier you could use a glass bottle with some desiccant in the bottom to get the samples really dry. Much cheaper! The desiccant can be regenerated (be careful though, the solvents used are highly flammable!).

Rosin or other resin to fill up

Rosin is sometimes named in relation with these drying operations - what's that?

Answer: Rosin (or some other resin) is soluble in alcohol too and used to "fill" pores in the wood, glue it together. It will of course make the sample unusable for C14 dating, as any other glue will do.

One suggestion if you want to enhance the surface of polished samples: apply a thin layer of furniture polish or white (bleached) shellac in alcohol. But make an experiment with a less valuable sample first! And do not use the wood for C14 dating afterwards!