What can be done to stop the wooden roundels from cracking

The wooden rounds are obtained by cutting the log perpendicular to the central axis. This is not the usual way of cutting wood because, when cut in this way, the roundwood is almost certain to crack during drying. It is almost impossible to obtain a whole roundwood without certain treatments and subterfuges. And yet there are all sorts of objects made from roundwood, even table tops. Why wood cut in this way cracks so much more, and how the roundwood can still remain whole after drying, you'll find out below.

Large and different contraction in tangential and radial direction causes stresses leading to cracking

It all starts from the different behaviour of the wood in the three directions - radial, tangential and along the grain - during contracting caused by drying (water coming out of the wood). In normal sawn wood, the most important dimension is along the grain. In this direction shrinkage is smallest. Although shrinkage is important in the radial and tangential directions, the effect is negligible because the dimensions of the wood in these directions are small. Some trace can only be seen at the ends of the timbers and cabinets where shrinkage stresses are discharged.

In the case of roundels, the size along the fibre is small, the important ones being in the tangential and radial direction. These cause the roundel to shrink at the exit, but in different ways. Tangential shrinkage causes shrinkage of the rondelle along the circumference and can reach 10%. Radial shrinkage causes shrinkage in the radius direction and can be 2 times smaller than tangential shrinkage.

Let's imagine a annual ring from which the water is flowing out so that in the tangential direction 10% contracts and in the radial direction only 5%. There are tensions and imbalances in the wood mass and this difference must be compensated for somehow. This creates the crack that rebalances the system. The smaller the difference between the two counterbalances, the lower the stresses. This is a species characteristic. This is why, under the same conditions, some species crack more and others less. The elasticity of the wood is also important because it helps to compensate for stresses. At small differences between the two shrinkages, elastic wood is able to compensate for the tension and remains sound, whereas hard wood cracks.

One factor that exacerbates these tensions is the large number of fibre ends. Through the split fibres, water escapes quickly, which leads to rapid and different contraction in the two directions. In order to reduce the tensions, the outflow of water from the wood must be reduced and/or controlled.

Methods for limiting the cracking of log slices

There are no miracle methods that guarantee total elimination of cracking. Working with roundwood will always have a higher risk of splitting than wood cut along the grain. But there are ways to limit cracking by reducing tension and the speed of water outflow. The species chosen is also important, as roundwoods made of elastic wood with a small difference between radial and tangential shrinkage are much more likely to dry out without cracking.

Reducing tensions. As we have seen, tensions are related to circumference, radius and contraction in these directions. We can't intervene on the circumference dimensions, as the aim is to keep the roundness intact. But a irregularly shaped rounds will crack less than perfectly round ones. If the log or branch has small bumps, ripples and is not perfectly cylindrical, the cut rounds are less likely to crack. They need to be treated to control water outflow, but the results will be better.

Tensions also occur on the beam. To reduce them the radius can be intervened by making a hole in the center of the roundel immediately after cutting. This will reduce the radius and with it the tensions. The larger the hole, the lower the stresses. On this idea, the smaller the radius (smaller slice) the less it will crack. The advantage of gouging is that it also removes the pith, which behaves differently.

Shrinkage can be reduced if the round cut is not perfectly perpendicular to the tree axis. Oblique cutting of roundels significantly reduces shrinkage and automatically reduces the risk of cracking. The problem is that the design of the wood will also be altered, and this can be a problem for those who want perfectly round rounds.

Methods of controlling the speed and mode of water output from wood

It is the control of the water outflow from the wood that limits the cracking of the wood. To be successful, stress reduction methods must be combined with water outflow control. Success is even greater if the wood is free of knots and other defects. Before undergoing the procedures, the wood is sanded on both sides with max.220 sandpaper and sanded smooth. The rounds should be treated immediately after cutting and sanding to reduce the risk of splitting.

Blocking the fibre ends or replacing the water in the wood with other substances are methods of controlling the drying and shrinkage of the wood. There are many such methods. The ones below seemed feasible and interesting to me.

Applying a salt paste to the surface of the wood. A paste made of salt, water, cornstarch and egg white is applied to the entire surface of the wood. The proportions are quite different from craftsman to craftsman. The important thing is to start with a saturated salt solution. For example, to 4 litres of water add 1.5 kg of table salt. Mix well and leave for 3-4 hours for complete dissolution. Then add starch until it becomes a fluid paste similar to oil paints. Finally add 3 egg whites, one at a time, stirring continuously. Do not add the next egg white until the first has been fully incorporated into the paste. This paste is applied to the whole wood (front, back, edge) and left to dry. Place the rounds against a wall so that there is a draught for drying, protected from the sun and rain. The salt will draw water out of the wood and the paste will prevent the wood from cracking. Drying can take 2-3 weeks, even longer, depending on the thickness of the roundwood. The process is finished when the pulp in the centre of the roundel becomes dull, faded and the roundel is much lighter. This is when the paste, which has become very hard, is removed (a rather difficult process).

Dipping the rounds in industrial ethyl alcohol (denatured alcohol). The rounds are placed in a dish in which they are completely immersed in ethyl alcohol. Put weights on them to keep them immersed. They are kept like this for 24-48 hours, depending on their thickness. Cover the dish with plastic so that the alcohol does not evaporate. After the required time, the rounds are taken out and dried under cover, out of the sun and out of the rain, but in a draught (leaning against the wall). The water in the wood is thus replaced by the alcohol which then escapes, the tensions created by its escape being much lower.

Dipping the roundels in polyethylene glycol (PEG). It is done in the same way as dipping in alcohol, except that the rounds are left for 2-3 months in the solution. During this time the water is replaced by PEG which at the end does not evaporate but remains in the wood, stabilising it. The rounds will be heavier at the end because PEG is heavier than water.

Wood can also be immersed in linseed oil, frozen, coated with wax, paraffin or epoxy resin. Here find several methods of treating and stabilising green wood. And in this case, water is also the problem. And here find methods from carpenters for reducing end cracking in normal sawn timber. Some of them can also be adapted for roundwood.

I hope you find the information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.