Turning Archive 2004

Drying a Bowl

>The question was asked if there was any reason for coating a rough turned bowl with Anchor Seal. The short answer is YES, and for the same reasons that we would put the bowl in a paper bag.

Now for the long answer.....
The mechanics of drying is not a simple topic, but here goes with an attempt at a simple answer.

It doesn't matter whether it is lumber in a kiln, a sheet of thin veneer, or a bowl in our hand; the two factors for drying are the RATE of evaporation of the moisture from the surface of the wood, and the MOISTURE GRADIENT through the thickness of the wood. The slower the moisture evaporates from the surface the more likely that the moisture will be uniform through the thickness, and the less likely there will be cracking on the surface. With a fast rate of evaporation, the surface will be dryer than the internal wood; therefore the surface wood will shrink faster, and it will crack whenever that shrinkage exceeds the tensile strength of the wood.

The rate of evaporation from the surface is a function of the relative humidity in the air around it. When the relative humidity is high, the surface evaporation is slow. When there is a low relative humidity around the wood, the evaporation rate is fast, and the resulting moisture gradient will induce surface cracking.

The reason that "thin" always dries more easily is because there is no moisture gradient through the wood as the moisture evaporates. And that is why we always have a problem in those thicker areas of a bowl around the foot and spigot, if we have left one of them on the bottom.

All of this has nothing to do with temperature, only the how much moisture can be absorbed by the air next to the wood. We can control the rate of evaporation with the environment and air circulation around the bowl, or we can provide a barrier between the wood and the surrounding air.

A thick paper bag is a permeable barrier and the moisture has to pass through the paper where it evaporates from the surface of the paper and not the wood.

A plastic bag provides an artificial environment in the air around the wood. The air inside the bag will become saturated and moisture will start to condense on the inside of the bag. We have to empty the water out of the bag when this happens, or we can turn the bag inside-out and put the bowl back in.

Coating the wood with wax or AnchorSeal (a wax) does the same thing for the bowl as it does for end-grain wood in log or bowl blank form. It seals the grain, and provides a barrier to moisture evaporating from the surface. It does the same thing as the bag, it can be easier to use, and we can vary the thickness of the barrier with subsequent applications of wax.

We could also provide an enclosure and control the humidity inside to control the evaporation rate from the surface of the wood. And, we would call this a drying kiln.

Or we can throw the rough bowl in the corner of the shop and hope for the best. Sometimes we can get away with doing this, but it is better to do something to reduce the rate of evaopioration of moisture from the surface of the wood. AnchorSeal is just another way to accomplish this.

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