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Woodturning Projects.

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Experimental Seasoning of Green Turnings.

Most of the wood I obtain for turning is green (freshly felled) either from tree surgeons or from friends who have had trees felled or branches removed. The drying/seasoning of turning wood is a fascinating topic, and this page explores one experimental approach to get a better understanding of the process and apply the knowledge gained to the drying of the obtained wood.

Green Wood; The Technical Stuff.

I will be using some basic technical terms on this page so lets begin by explaining what they are.

When a living tree is felled, it is in the green state, containing a very large amount of moisture. This moisture exists in two different forms: as free water, contained as liquid in the pores or vessels of the wood, and bound water that is locked within the cell tissues of the wood.

Once a freshly cut piece of timber is exposed to the air, it will immediately begin losing free water. Initially, the wood does not contract or otherwise change in dimension since the fibres are still completely saturated with bound water. Once all the free water has been lost, the wood will reach the fibre saturation point (FSP).

Below the FSP, the wood will begin to lose moisture in the form of bound water, and a corresponding reduction in the volume of the wood tissues will occur. At this point, the wood is no longer considered to be in the green state, and is drying.

How much bound water is lost during drying depends on the temperature and relative humidity (RH) of the surrounding air. At 100% RH, no bound water will be lost. At 0% RH, a steep moisture gradient (MG) will be established and all the bound water in the wood will be progressively lost, to a condition known as oven dry so-called because a kiln or oven is required to completely drive out all the moisture.

The MG depends on the external RH of the surrounding air. As water evaporates from the surface of the wood, more is drawn out from the centre along the gradient thus created. The MG is critically important, as a high rate can create unequal MC's in different parts of the timber, inducing severe stresses. Eventually, with progressive moisture loss, the gradient decreases and stabilizes at a value that is commensurate with the surrounding moisture in the air. At this point, the MC is referred to as the equilibrium moisture content (EMC). The EMC will continue to change (increase or decrease slightly), dependent on the fluctuating temperature and RH of the surrounding air.

The Moisture Content (MC) of wood is calculated by the formula

Where the wet weight is the weight of the original 'wet' sample and the dry weight (oven dry), being the weight of the sample after drying in an oven. Moisture content being expressed as a percentage. Some species of trees, when they are initially felled, may contain more water by weight than actual wood fiber, resulting in a MC of over 100%.

Evaluation of the Wood.

Newly Felled.
When harvesting or obtaining green wood for turning, it is very useful to know when the timber was felled. If it was felled the same day or within a few days of harvesting, then there will certainly be some free water in the wood and it will not start to contract in volume until the FSP has been passed and bound water starts to be lost. This reduces the urgency of preparation and processing.

Felled Several Weeks/Months ago.
If the wood was felled weeks or months previously and just left in the round, it is likely that most of the free water will have been lost and the bound water will be evaporating with likely contraction and splitting of the timber, particulary in the vicinity of the cut extremities, where the rate of drying (Moisture Gradient), will be most intense.

	A Bole of Cherry Showing Intense Stress Cracking Induced by Drying. This bole of cherry was felled just 4 weeks before it was harvested, but already there is considerable stress cracking of the wood at the extremities, where loss of water through evaporation, has been most intense. (Click picture for larger view)
	The same bole after cutting away the cracked extremity wood. Cutting back the extremity with the chainsaw reveals wood that has dried more slowly, with a higher MC and which is consequently undamaged by stress cracking. (Click picture for larger view)

Such timber should either be prepared as soon as possible to prevent further degradation and possible spoiling by fungal infection, or else the endgrain sealed with some form of sealer (I use PVA emulsion). I have seen some harvested green timber wrapped in cling film to conserve moisture until the time can be found for preparation and conversion.

After Rough Turning.

Prevention of Fungal Infection & Insect Infestation
If the wood is not being kiln dried, there is a big risk that, with a high MC, the wood will be infected with organisms (mainly fungi but also woodworm), which cause decay and staining. To avoid this it is useful to treat the wood with a fungicide and insecticide, especially in the early stages of drying. If the completed turning is intended for food use then it is important that the treatment is minimally toxic to humans. Perhaps the most suitable is Borax (sodium tetraborate), which is widely used in household mould treatments and insecticides. This is best obtained as a powder which can be dissolved (5% - 10% dilution), in warm water in a non-metallic (plastic) container. It is most easily liberally applied by brush to the turning, but the wood, if small enough, can also be dipped.

An Example of Monitored Drying.

To obtain consistent results from the monitoring, the drying area should have a reasonably constant RH and temperature. If these vary considerably, then the drying rate and eventual EMC will also show marked fluctuations, making monitoring difficult. If a workshop cannot satisfy these criteria then a possible alternative might be somewhere in a domestic dwelling, where the temperature and RH ranges will in any case, probably mirror that of where the finished turning may end up.

This example is of a rough turned bowl made from freshly felled cherry which, after turning, was simply put on a shelf in the workshop to dry. The bowl was weighed weekly to monitor the drying progress.

	Weighing with Digital scales. These digital scales weigh objects up to 5,000grams (5Kg), and are ideal for monitoring the weight of green turnings. For large heavy turnings in excess of 5Kg, I use digital bathroom scales! (Click picture for larger view)
	Weight Recording. Recording the weight of a rough-turned bowl made of Walnut. (Click picture for larger view)
	The hand-written record. The hand-written sheet detailing the drying record from the day the bowl was rough-turned, on the 30th April 2010, until the final reading some 6 months later on the 17th October. Click HERE to view a PDF blank version of the form which can be printed and used for recording. (Click picture for larger view)

Interpreting the Results.

For interpreting the results, I use a Word-Processor and Spreadsheet. Initially I enter the readings into a table within a document created with either Microsoft Office Word or created with the free Open Office Text word processor.^* Just the dates and the weights (in grams) are entered into the table which is set to automatically calculate significant analytical data in the adjacent columns. These analytical columns are very useful if the data is entered as the wood is still drying.

^* Note that the Open Office software can read and create Microsoft Word documents & Microsoft Excel Spreadsheets.

Weight Recording. Recording the weight of a rough-turned bowl. The table shows the recorded weights of the bowl taken on the date it was rough turned and on a weekly basis thereafter (Dates and Weights in the blue columns in the image). This is a good way of monitoring the seasoning (or drying) progress of the bowl. When there is no change in the weekly readings of weight, then the wood has effectively dried out and has reached it's Equilibrium Moisture Content (which will be controlled by the temperature and humidity of the storage area). Computed data categories are Elapsed days (since the first weighing), the Cumulative loss (grams) (since the first weighing), % loss (in weight compared to the first reading), and Amount lost (in grams) from (the) previous (weighing). There is an optional column for any relevant comments, such as when the EMC has been judged to have been reached. Note the red "Danger Period" where the readings in the Amount lost from previous column are very high. In some instances this might necessitate taking action to slow the rate of drying down to avoid stress cracking of the wood. See discussion of this later. (Click picture for larger view)

Making it Clearer With a Graph.

A much more meaningful picture of the drying process can be obtained by entering the data into either a Microsoft Office Excel or Open Office Calc spreadsheet (or other software which can generate graphs), to create a line graph of the results.

Drying Graph for the Chery Bowl data (in the above picture). The curve of this graph is exactly what would be expected. A low RH in the workshop with a high Moisture Content (MC) in the wood of the bowl creates a very steep Moisture Gradient (MG) with a very rapid loss of water by evaporation from the surface of the wood. There is therefore an initially steep slope to the line, marking a rapid decrease in the MC of the wood. Progressively there is a corresponding decrease in the MG as the MC goes down, with the slope of the line flattening out in a gentle curve as the rate of evaporation decreases and the Equilibrium Moisture Content (EMC) of the wood with the RH of the air in the workshop is attained. (Click picture to view graph)

The "Danger Period".

The "Danger Period", where the turning is most likely to crack, is in the initial period of drying when the Moisture Gradient is at its highest level and the consequent shrinkage stresses in the timber are at a maximum. This period is highlighted an annotated version of our example graph, shown below.

Graph Showing the Danger Period when Drying a Turning. The "Danger Period" is shaded red and lies in the initial stages of drying, after the Fibre Saturation Point has been reached and there is no more Free Water, but the Moisture Gradient is still high, and there is a consequent rapid loss of the Bound Water in the wood of the turning instead. The shrinkage stresses are at a maximum in the timber during this time which, in the example here, extended over a period of about 6 weeks. (Click picture for larger view)

Dealing with the "Danger Period".

The obvious way to lessen the drying stresses in the initial drying period is to slow down the drying process by reducing the MG. This can be done in several ways, as follows.

The "Bury in Shavings" Method.
This is another common method where the turning is placed in a box of shavings and left for a long period of time (years?), allowing the moisture to dissipate slowly from the turning and be absorbed in the dry shavings, which will initially have a lower MC than the wood of the turning.

My "Experimental Box" Method.

This method allows for the turning to be repeatedly, firstly isolated from, and then exposed to the low RH of the drying area for short set periods, which effectively retards the drying process. The drying process can be effectively monitored by simply weighing the wood after each successive drying period.

Typically the newly turned wood is weighed and then immediately placed in a lidded polythene box. Water evaporating from the surface of the wood has nowhere to go, so the RH in the box rapidly increases to the point of equilibrium moisture content. Drying of the wood is thus completely halted. The turning(s) remain(s) in the lidded box for seven days to allow any moisture gradient (and associated stresses), to equalise. On the seventh day, the lid is removed from the box and the wood is allowed to dry for a set period. The weight at the end of this period is recorded before the lid is replaced on the box and it is left for a further seven days to again allow equalization of the MG and associated stresses. Using this cyclical method, the duration of the drying periods, and therefore the rate of drying, can be controlled minutely.

	"Experimental Drying Boxes" for Green Turnings. The turnings are isolated in these lidded polythene boxes which totally stifles the drying process. Once a week, the lids of the boxes are removed for a short period to allow the wet turnings to lose water. At the end of these drying periods, the turnings are weighed, and the lids are replaced on the boxes which halts the drying process and allows the moisture gradient and any associated drying stresses, to equalise. The 7 day cyclical process is repeated until it is judged safe to remove the lids permanently, to allow the turnings to dry out completely to reach the equilibrium moisture constant of the storage area. (Click pictures for larger views)
	"Data from a Cherry Bowl, dried in an "Experimental Drying Box" (as shown above). The bowl was kept in the lidded box, but after every seven days (or thereabouts), the lid was removed for 6 hours to allow water loss from the bowl by evaporation. The recorded weights for the initial Danger Period show a much more controlled and gradual loss of moisture compared to the first example drying data described above, where there was no control of the drying process. A graphical view of the data is shown below. (Click pictures for larger views)
	Drying Graph for the Chery Bowl data (in the above picture). The initial period of the drying process, where drying was controlled, shows a much gentler graph line slope, (compared to the first graph), until the point where the turning was removed from the box to allow it to dry out completely to the EMC determined by the RH of the storage area. (Click picture to view graph)
	Interpretation of the Drying Graph (shown above). Limited exposure of the bowl reduces the rate of drying during the initial "Danger Period" with a gentle slope of the graph line where normally the MG and rate of moisture loss would be expected to be at a peak, and the line at it's steepest. The point at which it was judged to be safe to remove the box lid permanently (after 85 days on 13th June), shows a dramatic increase in the slope of the line reflecting an increased moisture loss, only flattening out, as in the first graph, as the EMC becomes commensurate with the storage area RH. (Click pictures for larger views)

More experimental data will be added here in due course.

Some Conclusions & Observations.

Experiments and their interpretation are usually followed by some conclusions.

• The time required to dry a green turning depends on size but can usually be completed within a year (e.g. A Walnut bowl of 395mm diameter and a Pear bowl of 235mm diameter both took 8 months to reach EMC).
• Weighing is an excellent method of accurately monitoring the drying progress of a green turning.
• Weighing data enables accurate monitoring and precise control of the drying process.
• Long term stability has been demonstrated with some Cherry bowls turned in 2010 & which had been dried using this method, which are still in pristine condition with no warping or cracking.

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