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 in the shell dry first and begin to shrink. However, the core has not yet begun to dry and shrink, and consequently the shell is restrained from shrinking by the core. Thus the shell goes into tension and the core into compression. With the stresses from the shell and core pulling in opposite directions, the wood fibers break and a crack forms.  The larger the wood member, the more stress that is exerted to the wood member.

Examples

Let's take the example of a 1x8 board versus an 8x8 timber:

• The 1x8 board is only one-inch thick so it dries fairly quickly. The stress is minimal and just a few, small surface checks may result.

• The much larger 8x8 (8-inches x 8-inches) timber takes a long time to dry, which could be as much as six months. The inside of the timber will stay wet for months while the outside is dried to well below 20% moisture content. The stress that is set up within the 8x8 will eventually result in a ½ inch crack or larger on one face of the timber. This crack will go all the way to the center of the timber and usually be on one face only.

I have seen people cutting a round, “lily pad” from the end of a log so that they can use it as a cutting board in the kitchen. It looks nice in the green, unseasoned state but after a few weeks it develops a large pie crack. It is eventually discarded as unsightly. After throwing it away they wonder what happened to the wood to allow this to happen.  
 
This is just another reason why wood materials should be properly dried before using them for anything other than exterior purposes, such as fence posts, landscape timbers, or a rough fence. If they are being used where their final moisture content will be 15% or lower, they should be dried prior to milling and installation. This way the stress cracks can be aligned so that they are hidden or discarded when installation occurs.

Preventing Cracks – Proper Drying Techniques and PEG

What can be done to prevent this seasoning degradation in wood? With large quantities of lumber, boards to timbers, one can only resort to proper drying following the many details that make up the complete process. This can be done with either a kiln or by air drying, but many details have to be followed to achieve the desired results. It is not the purpose of this short essay to outline what must be done to properly dry wood. This will be covered in another article.

For small, fairly expensive items such as carvings, another method can be used to prevent degradation due to seasoning checks and cracks. 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 will dissolve in water. The correct method for 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 warm and can be kept this way with a fish tank heater. After the carving is thoroughly saturated with PEG, it must be dried slowly in a cool environment and out of the sunlight. The result is a wood carving that will not shrink when it dries and thus will not crack.

The physical explanation for the above process is this: PEG can only be transmitted through the wood if the wood is thoroughly saturated with water. When the PEG enters the cells, it fills the cell walls and the cell lumen located in the center of the cell. When the wood is dried, it cannot shrink because the cell walls are now filled with a solid (PEG) and cannot shrink. If the wood cannot shrink, it cannot crack or split!

As a final note, I used this method on a piece of sycamore that was buried in a clay pit for nearly 6,000 years. I had it made into a gun stock and after 30 years of use the gun stock looks like new without the slightest hair line crack in any surface of it.