WMDKdoc020322
6-1
6
SECTION 6 GLOSSARY
DEPRESSION
- Temperature Depression is a term used to describe the Relative
Humidity of air in the kiln chamber. Air that is 100% humid cannot absorb any more
moisture. For example on a misty day washing will not dry outside. The amount of water
that air can absorb depends on its temperature. In the WMDK system the drying process
is controlled by monitoring the humidity and temperature of the air in the kiln. This is done
using a wet/dry bulb thermometer. This is an ordinary thermometer that has an absorbent
gauze on the sensor end. When the gauze is dry the thermometer will show the “dry bulb”
reading. Once the gauze is wetted the moisture on the gauze will begin to evaporate- this
evaporation causes cooling and the thermometer will therefore show a lower reading –
the “wet bulb” reading. The rate of evaporation on the gauze, and therefore the amount of
cooling of the thermometer is determined by the humidity and temperature of the air in the
kiln. The drier the air is in the kiln, the larger the difference between the wet and dry bulb
readings will be. The difference between the wet and dry bulb readings is called the
“Depression”.
MOISTURE GRADIENT
- The difference in moisture content that exists between the
centre of a board and the outside of the board. When the difference in moisture content
between the inside and outside is too great, surface checking and casehardening will
result. Therefore, the moisture gradient is one of the things to watch when drying timber.
The lower the gradient is at the finish, the better the timber will work.
SURFACE CHECKS
- Caused by the moisture gradient becoming too great. This usually
occurs when the timber is drying from the green stage down to 20% moisture content.
However, it cannot always be detected in air-dried timber until the timber is placed in the
kiln and the drying begins. Almost all air-dried oak, especially that stock cut in hot, dry
weather, will contain surface checks. It also frequently occurs in the heartwood of other
species.
CASEHARDENING
- Caused by a steep moisture gradient, but frequently cannot be
detected until the centre of the board begins to dry. The outside of the board becomes dry
in an expanded or stretched condition - due to being dried too rapidly - while the centre of
the board is of much higher moisture content. It can be relieved by a higher humidity
treatment which softens the surface and allows it to take its normal shrinkage.
HONEYCOMB OR HOLLOWHORNING
- Follows severe casehardening which has not
been relieved before the centre of the board becomes fully dry.
WARP AND TWIST
- Sometimes caused by improper sticking and in some cases by
drying the timber at too low of a humidity.
