Seasoning And Moisture Content Of Wood

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Seasoning And Moisture Content With Wood

A growing tree that is full of sap can contain anything from 50% free water up to 200%. Wood seasoning is used to accelerate the evaporation of surplus water to make the wood lighter, stronger and less susceptible to any mould attack or discolouration.

Any water within a tree is held in the fibres, pores and vessels in the form of water and moisture. When the tree has been cut these vessels and pores will begin to dry. In logs this can be an extremely slow process, but by cutting the logs in to planks the process is speeded up a bit. By cutting in to planks the water levels will fall steadily until a 30% moisture level is reached. After this the drying out slows down until an equilibrium moisture content is reached.

Relative Humidity

Even on the driest of days the atmosphere will contain water vapour which can vary from hour to hour. Water vapour in the atmosphere can be measured using a hygrometer and is termed as relative humidity. It is measured in a percentage rate. Thankfully wood is a slow conductor, so any big fluctuations in air moisture content away from the average will not have too much impact.

Equilibrium Moisture Content

Wood is an organic and hydroscopic material. It is susceptible to moisture changes in the atmosphere and will always try to put itself in a position of equilibrium with its surrounding atmosphere. Equilibrium moisture content is a measurement of how much water content the wood has in a particular humidity level.

Seasoning Methods

Seasoning is carried out to reduce wood moisture levels down to an acceptable level where the wood can be used to best effect. Seasoning wants to be done quickly, yet gently. Once the pores of wood are emptied of water, the fibres are dried by surface evaporation and capillary action. If evaporation is too quick the result will be vacuums being created and the cell walls collapsing. This will cause splitting and distortion. If the drying rate is controlled any water loss in the wood will be replaced by air and any strains on the wood from moisture loss will be accommodated by the natural elasticity and strength of the wood.

The 2 methods of seasoning used these days are air drying and kiln drying. Air drying is slower yet cheaper to perform. Kiln drying is quicker and simpler but much more expensive to perform. Air drying can never reduce the moisture content of wood to much below 15%. For lower levels than that, kiln drying is the only option.

Air Drying

When hardwoods are sawn they should be piled for drying when the humidity levels are high. With green wood they should never be piled without air spaces between them or mold and fungi will intrude. When you stack wood the stack should be built up on concrete piers and then weighted down at the top with wood slabbing. A lean too roof over the stack will protect it from any rain and sunshine. Rain will not be too harmful to your wood stack as long as it has the ability to dry out but sun could be an issue, especially with the ends of wood. You have to expect some splitting on the ends of wood as the moisture is driven out. A special paint can be used to limit this splitting.The drying time of wood with air drying to reduce moisture content from 25% to 15% is about a year for every inch of thickness in the wood.

Kiln Drying

Using hot air for seasoning wood does require a lot of knowledge. For kiln drying the wood is stacked the same way as with air drying, but in enclosed compartments. Hot air is then passed through these compartments. Because hot air would cause rapid evaporation of moisture and cause severe distortion and shrinkage, steam is also put in to the compartment to keep the evaporation at a suitable level. The job requires constant attention as the amount of heat and level of humidity in the compartment has to be adjusted to create a progressive fall in the relative humidity percentage. This has to be continually monitored until the required moisture content level in the wood has be obtained. The advantages of kiln use are that wood can be dried down in just a couple of weeks and lower moisture content levels can be achieved. A disadvantage is that there is an increased risk of the wood warping or buckling.

Suitable Moisture Level Contents

In order to avoid excessive shrinkage, the maximum moisture levels below should be adhered to.

22 - 15% The usual moisture content limits of air dried wood.

20% The dry rot safety line

16% For outdoor furniture

15% For new joiners work

12 - 14% Bedroom furniture that has infrequent heating

11 - 13% Living room furniture that has central heating

9 - 11% Furniture for public buildings and offices which have continuous heat

8% For radiator shelves and such like items.

Measuring Moisture

The most accurate way of measuring moisture content is the oven method. Here wood is weighed and heated in a drying chamber and then re-weighed again until all the moisture has been driven off.

The moisture meter is the second way of measuring moisture content and give instant and accurate measurements. These meters work on the principle that, as wood itself is a bad conductor of electricity, any moisture present will facilitate the passage of an electrical current passing between two electrodes spaced apart and inserted into the wood, and the measure of resistance to the passage of the current can be expressed in terms of moisture present as a percentage of the total bulk.

The range of measurement varies according to the type of meter, with a normal coverage of from 6 to 30 per cent, and a margin of error of from 1 to 2 per cent. It should be borne in mind. however, that the presence of mineral salts in the wood may affect the readings, while some woods give large errors, especially in the upper moisture range; but most manufacturers give a table of adjustments covering many of the commonly used woods.

In taking any measurement, whether by oven method or moisture-meter, the readings should not be taken from the end of the plank, but from the thickness of a cut section at least 9 in 228 mm) in from the end and immediately the cut has been made, otherwise surface evaporation may falsify the test. If an approximate moisture content only is required, then the electrodes should be pushed into the surface at several points throughout the length of the plank not less than 3/16 in (4.5 mm) deep, and then the average of the reading used.

Moisture Meter Guide 2010 Contact details:garnett65@hotmail.com

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