Slabs are cut perpendicular to the trunk of large trees, for tables, craft projects and specialty items. They contain no man-made seams, relying solely on the diameter of the tree for size. Slabs vary in thickness from about 3 to 6 inches; they are cut when the tree is still wet. Proper curing and drying prevents the slab from cracking.
Time and Patience
Curing times vary because of species, initial moisture in the tree, thickness, density, temperature and ambient humidity when drying. The general rule of thumb is to allow slabs to cure at least one year for each inch of thickness.
Even though its a yearly rule, it's centered around summertime. It requires patience for a 3-inch thick slab to dry -- about three summers to be specific. The process can be hastened by using a kiln to dry the wood, requiring only about three to four months, but kiln-drying wood is an expensive prospect.
Bugs Don't Follow Rules
Depending on the temperature, humidity and how the slab is dried, different issues are involved when trying to dry a slab in less than one year. Trees may or may not be full of pupae or larva; most of the bugs in trees require a year to hatch.
Cracks and Splits
But insects are not the main concern. Because of the grain orientation in the slab -- grain lines that radiate out from the center -- cracking and splitting is the real issue when curing slabs. Dimensional boards, with their long, parallel patterns, won't crack like round slab lumber, with its short grain lines. When the slab dries, it pulls the grain apart when it dries too fast.
Cupping is a situation that involves the curvature of the slab. Cupping is caused from uneven drying; one side dries faster than the other, shrinks and pulls the outer perimeter upward. Regular monitoring and turning of the slab helps to prevent cupping. Cupping can also be avoided by adding significant weight to the slab to keep it flat -- while at the same time allowing the flow of air around it.
To effectively cure a slab for use in a table or other specialty item, it's critical that you adhere to specific steps.
Choose a spot where the slab can remain for the extended time required. Use the formula of 1-inch of thickness per year for the curing time. Place the slab out of direct sunlight, rain and weather.
Place the slab upright, off the ground, leaning it at a 30-degree angle to provide air flow behind it. Or optionally, lay it flat atop and across two sticks, braces or anything that won't cover the bottom. It should have a minimum of 4 inches of air flow beneath it.
Rotate the slab often -- every three to four months -- so that each side faces away from walls or anything that could impede air flow.
Equilibrium Moisture Content
Wood begins to shrink at 30 percent moisture content. Slabs will shrink considerably, depending on species and existing moisture in the wood. Equilibrium-moisture-content or EMC is the point where the wood equalizes with the moisture in the air, and that can differ depending on the climate where you live.
When the EMC is reached for your area, the slab won't shrink, crack or warp, and is ready to use. Locations such as Florida will have a higher EMC than the Southwest desert. The rule of 1-inch per year is typically safe enough, but it's best to monitor the curing process with a hand-held moisture meter to be sure. The meter is a simple device with two sharp prongs; when you plunge them into the wood, it reads the moisture content on a display.
Relative Humidity and Curing
The relative humidity in most homes averages between 30 and 40 percent, translating into a required 6 to 8 percent moisture content of the slab before its ready to use indoors. Studs in your home, for example, typically have a moisture content of about 9 percent.
Err on The Safe Side
To verify your slab is ready to use, bring it in the house when it reaches about 10 to 15 percent moisture content on the meter's display. Allow it to continue to cure inside the house until it reaches a level of between 6 and 8 percent moisture content.