When wood is first cut and shipped, it is still green-- the cells inside are still filled with water and have not truly died yet. This water can take a long time to reach the surface of the wood and then evaporate, even in normal boards. If wood is still green when it is used in construction, there is a danger that as it slowly dries out over the months it will shrink and warp, destroying the level lines made by the mill and carpenter and threatening the stability of the construction project.
The easiest solution is to wait until the wood is fully dry and then use. This can take months or longer, however, and rather than wait, different chemical methods are used to help avoid splitting. One of the first and most common methods was using melted wax to coat the ends of the wood. Since most water evaporates through the ends of the timber where it has been cut, the wax slows down the process, perhaps enough, so the wood does not warp as it dries.
Other methods use special sealants that coat the entire timber in a protective shield that traps the remaining water inside. These sealants penetrate down several layers into the wood and effectively seal the cells, keeping the water from escaping. These are known as wood stabilizers or wood sealants and can be applied by both individuals and mills.
Pressure-treated wood uses solvents to completely replace the water inside the wood with petroleum-based chemicals. This requires putting the wood into vats of the chemicals and using pressure to force the water out and the chemicals in, so they harden and preserve the wood cells. This method is the least environmentally friendly and because of its hazards is usually only done in large mills, but it is also the best way to avoid any kind of splitting.
Because of the environmental hazards of pressure treating, more eco-friendly measures have been developed using natural substances such as minerals and plant resins to seal the wood and replace water, instead of the petroleum-based methods. These techniques also work, but the wood is left more susceptible to cracking and wear from outside weather.