Sep 14, 2019

Sap Removal

One of the advantageous processes is to replace the sap of green wood logs with water: the sap removal, in other words the elimination of sap. In particular, its interest is to eliminate starch from the sap that would subsequently be eaten by insects and destroyed by fungi: the woods are therefore better preserved. The sap removal is done by flotation before debit or with steam after debit.

By flotation before debit

This process is used mainly for light woods, softwood in particular. It is more difficult to use with heavy woods, such as oak; the logs may sink. Two methods may be used:

  1. a) Hot water flotation: the sap is soluble in hot water; however the temperature must not be too high, otherwise the sap coagulates. This process is not really used.
  2. b) River or pond flotation: the wood is immersed in running water for a period of time 15 days to 1 month or more, or in standing water for 1 to 2 months. The wood is then stacked for drying.

As the water evaporates faster than the sap, a certain acceleration of drying occurs. But the practice shows that there is only acceleration at the beginning of drying. The gain of drying time is therefore illusory. But a log of softwood retained by flotation has the advantage of being wetter at the time of sawing; it is therefore easier to saw; then it is sufficient to use a saw having a large enough step.

With steam after debit

This is called steaming. The boards are stacked in a well-sealed room called an oven. The steam is introduced in such a way that the temperature is about 80°. Wood softens, tissues distend and are penetrated by the steam that drives the sappy water. The wood is dried when the water, coming out of the oven, is no more stained.

The boards are taken out of the oven and exposed to air for 1 to 3 months: it is the wiping; they are stacked then in the stores.

In conclusion, the sap removal makes it possible to have wood that dries faster at the beginning and that are rid of a large amount of nutrients that normally attract insects and cause the growth of fungi. Thus, they are better preserved than dried wood naturally. In addition the time saved during drying is not appreciable if one wishes to obtain by this process a very dry wood.

Drying in hot and humid air

There is another artificial drying process: drying in hot and humid air.

You have observed that wet soil dries quickly in summer. The reason is very simple: dry and hot air absorbs moisture. However, if the air is hot and humid, after a thunderstorm for example, the soil dries more slowly. Also, you may notice that laundry can dry very quickly on some days without sun but with dry air, faster than when the air is warmer but strongly wet. This drying process is based on these remarks: the speed of drying depends not only heat, but the hygrometric state of the air.

A stream of warm, damp air passes between the boards and absorbs moisture from the wood. Thus, the rate of evaporation is tempered by the high humidity of warm air and wood does not tend to split as it would if the air was dry. In practice, we begin the ventilation by almost cold air and becoming wetter as it becomes warmer; then, as the wood becomes drier, the warm air becomes less and less humidified.