Why does wood crack when it dries?

Differential Shrinkage – To get right to the point, wood cracks due to differential shrinkage! Differential shrinkage occurs because the outer fibers of the cover dry out first and begin to shrink. However, the core has not yet started to dry out and shrink, and consequently the core prevents the shell from shrinking. Thus, the shell is in tension and the core in compression. With the stresses of the shell and core pulling in opposite directions, the wood fibers break and a crack forms. The larger the wood member, the more stress is placed on the wood member. Examples Let’s take an example of a 1×8 board versus an 8×8 lumber.

• The 1×8 board is only an inch thick, so it dries out pretty quickly. Efforts are minimal and only a few, small controls on the surface may result.
• Much larger 8×8 (8 in x 8 in) lumber takes a long time to dry, which could be up to six months. The interior of the wood will remain moist for months, while the exterior dries to a moisture content well below 20 percent. The stress that sets within the 8×8 will eventually result in a crack of ½ inch or more on one face of the wood. This crack will go all the way down the center of the wood and will usually only be on one side.

I’ve seen people cut a round “lily pad” off the end of a log so they can use it as a cutting board in the kitchen. It looks good green, unseasoned, but after a few weeks it develops a large circular crack in the piece. Over time it is discarded as unsightly. After throwing it away, they wonder what happened to the wood that allowed this to happen.

This is just another reason why wood materials should be properly dried before being used in anything other than exterior use, such as fence posts, landscape lumber, or raw fencing. If they are used where their final moisture content will be 15% or less, they must be dried prior to milling and installed at their final end use. In this way the tension cracks, of seasoning, can be aligned in such a way that they are hidden or discarded. Crack Prevention: Proper Drying and PEG Techniques What can be done to prevent this curing degradation in wood? With large quantities of lumber, boards or lumber, proper drying can only be achieved by following the many details that make up the entire process. This can be done with an oven or with air drying, but many details must be followed to obtain the desired results. The purpose of this brief topic is not to outline what must be done to properly dry wood. This will be covered in another article.

For small and fairly expensive items such as carvings, another method can be used to prevent degradation due to cracks and crevices. This is done using a chemical called polyethylene glycol-1000, or PEG for short. This material looks like a block of paraffin in the solid state, but it dissolves in water. The correct method of using this material is to soak the newly carved piece of wood in water for a month or more depending on its size. After it is completely saturated with water, it is placed in a solution of PEG and water. It is kept in this solution for several months for best results. The solution should be lukewarm and can be kept this way with a fish tank heater. After the carving is fully saturated with PEG, it should be dried slowly in a cool environment and out of sunlight. The result is a wood carving that will not shrink when it dries and therefore will not crack.

The physical explanation of the above process is as follows: PEG can only be transmitted through wood if the wood is fully saturated with water. When PEG enters cells, it fills the cell walls and the cell lumen located in the center of the cell. When the wood dries it cannot shrink because the cell walls are now filled with a solid (PEG) and cannot shrink. If wood can’t shrink, it can’t crack or split! As a final note, I used this method on a piece of sycamore that was buried in a clay pit for almost 6,000 years. I had it converted to a gun stock and after 30 years of use the gun stock looks like new without the slightest crack on any surface.