Buying Balsa by Ted Horne

Until about 1930 balsa was unknown in the modern civilised world, and as far as I know only existed in Ecuador where it was considered to be a weed. Wind blown seeds of the balsa tree lie on the forest floor for many years until the right conditions for germination occur. The right conditions in Ecuador occur when the natives clear a patch of forest for cultivation allowing sunlight and hence heat to fall on the dormant seeds on the forest floor thus starting the process of germination.

Young balsa seedlings grow faster than any of the other seedlings which are found on the rain forest floor, and they develop enormous leaves, which in an adult tree can be as much as eighteen inches across. These leaves in turn block out the sunlight and warmth from all the other plants, including weaker balsa seedlings, which just shrivel and die though lack of light. It is for this reason that the forest dwelling Ecuadorians consider the balsa tree a weed, as it is of no use to them as food, too soft for construction purposes, and to harvest it commercially requires a certain amount of skill and careful husbandry.

Young balsa trees grow very fast and can reach a height of sixty feet in less than eight years, and yet have a diameter of only ten inches. If the tree is left to grow to a maximum size, it could reach a height of one hundred feet, and be as much as eight feet in diameter. The density, however, would be in the order of fifty or sixty pounds per cubic foot; this is of no use as a modelling material. A tree of about eight years, providing it has had the right treatment, nutrients, sunshine etc., will have a density of between four and eighteen pounds per cubic foot, and it is at this age that the tree will be harvested for the purpose of supplying the modelling industry. Should the main trunk be rubbed, bumped, or bruised at any time during these eight years, hard areas will develop which will destroy its value.

When the raw log is taken to the sawmill it is cut up in such a way so as to get as many boards as possible out of a single trunk. This ignores the way in which the growth rings lie relative to the way the board is cut. The board that passes right through the centre of the tree will be quarter or ‘C’ grain, and be very stiff when it is bent across its width as the growth rings will be closest together and lie at right angles, relative to the width of the board. As the boards move further away from this central board, so the growth rings tend to lie in line with the width of the board and as a result the boards become more flexible across their width. The board closest to the outside comprises just two or three growth rings instead of ten or so. This board is known as ‘A’ grain and is very flexible across the width of the board. Boards between types ‘A’ and ‘C’ will contain what is generally known as ‘B’ grain which is a mixture of ‘A’ and ‘C’ with some ‘A’ at the centre and some ‘C’ at the extremities. The exact ratios will depend upon the position relative to the central board.

From the above it can be seen that very little ‘C’ grain wood is produced from a single tree, maybe less than 10%. To cut a tree up in such a way as to produce a reasonable quantity of ‘C’ grain wood, would create such an unacceptable quantity of waste, that the price from the sawmill, and consequently the model shop, would have to double, or even treble.

When I go into my local model shop to buy wood I often see somebody buying 20 or 30 sheets of say 1/8’’ wood, and just taking the first 20 or 30 sheets out of the rack, without thinking about the grain or the density. For me, if I want 1/16’’ ‘C’ grain sheet I pull a whole rack of sheets out and examine each sheet individually and maybe select one sheet in ten or less, I then weigh them and invariably reject at least 50% as being too heavy. I am most grateful to those people who just take the first 30 sheets because it means that there is a good chance that there will be some sheets that fulfil my requirements, unless of course someone like me gets to the rack before I do.

Most of the small models that I build contain about 70% 6 pound per cubic foot or less ‘C’ grain wood, and the power models perhaps 40%. If I cant get what I want from the local shop I go to someone like SAMS, Mike Woodhouse or John Tipper where I know that I can specify the weight and grain and get just what I want. The price that I have to pay is higher than the local shop, but as the quantity of wood I use during the year is relatively small the cost is not a real factor.

Knowing the density of the wood that is needed for a particular component, there is only one set of figures that need to be remembered before buying a sheet of wood, and these are:- 1 sheet of 1/8’’x 4’’x 36’’ six pound per cubic foot wood weighs 28 grams, or to be exact 28.35 grams. Using these figures it is not very difficult to work out that if the selected sheet weighs 35 grams, then the density is 35/28 x 6,which is 7 .5lb wood. Similarly for 3’’ wide 6lb density wood, it would weigh ¾ x 28 = 21 grams. From this single set of figures the density of any single piece of wood may be established, by using a set of scales calibrated in gram increments, and a simple calculator.

Identifying the different types of grain can sometimes be difficult, bending them across the grain is a good test with thin sheets, but with thicker sheets it is a bit more difficult. ‘A’ grain wood generally has long straight grain lines and be uniform in colour, whereas, C’ grain tends to have short grain lines, as well as very short lines which go across the sheet, and almost appear like circular saw marks. If a thin sheet is held up to the light, these marks become very apparent. To me, ‘C’ grain has an almost satiny sheen to its surface and appears blotchy whilst ‘A’ grain is smooth, has straight grain lines, and is very uniform.

You may think by now that you know all there is to know about buying wood of the right quality for the job in hand, but nothing is quite that easy. During the life of our tree it may have had a year of poor sunshine which resulted in one of the growth rings not developing properly allowing the individual cells to grow closer together, thus creating a hard area on one side of the sheet. Your scales may say the sheet is eight pound wood, but the reality is that one side may five, and the other eleven pound wood. Holding it up to the light will quickly tell you which is which, if you have not already done the finger nail test. There are two more areas that can give you incorrect densities on thin sheets of wood, and these are:- 1) 1/16’’ sheet (.0625’’) can vary by as much as 15% in thickness, and 2) The thickness across the width of the sheet can vary, again by as much as 15%. I have come across both of these faults, but mainly in kits that I have been asked to build and review.

With large radio models the density of the wood used in the construction is not the critical factor that it is with indoor models. The first Lacey M10 that I built, albeit from a kit, weighed 15 grams, the last one, less what I considered some unnecessary structure, came in at 8 ½ grams which in ultimate performance gave a significant increase. Don’t be put off by all of the foregoing when you go to your local model shop to buy wood, but be aware that with a little bit of careful selection, your finished model could perform well instead of having the characteristics of a brick.