Turning Topics
Pentacryl: The Taming of Green Wood

Woodworker West, July-August, 1999

By Alan Hollar

Turning green wood allows easier cutting, but the turned form is subject to warping and cracking as the wood shrinks. Pentacryl can be a useful solution for green wood enthusiasts. There are many ways to create an object from a chunk of tree, and my favorite is by turning. For each tested and true orientation of the work that will yield an interesting and stable vessel, however, there is another if it would only stay together! Unstable woods and objects with pith included are the two I deal with often; so I am always curious when a new stabilizing product or method comes along. I never cared much for PEG (polyethylene glycol), since it requires a lot of time and then limits the choice of finishes so severely. Stabilizing with glue or wood hardening solutions reach into the surface so slightly that they are almost more aggravation than assistance. A recent product seemed to offer more flexibility and usability, so I decided to give Pentacryl a test run.

Pentacryl is a blend of 11 solvents, polymers, and emulsified silicone, and a data sheet is available from the manufacturer with most of what you need to know, if desired. It is non-toxic and can be applied by brush, spray, or immersion. My first concern was the presence of silicone. As an experienced finisher, I am too aware of the problems with film finishes can display from silicone contamination. To my surprise and delight, I found that lacquer laid down nicely, with no hint of fish-eye. According to the manufacture, emulsifying the silicone oil prevents the problems usually associated with silicone contamination. I have not run into any problems with any finishes that I have tried with it.

With the finishing question settled, the matter of performance could be addressed. In my first test, I used two small conical bowls of cherry, pith centered and vertical, with the bottoms left a bit thick. One bowl was left untreated, the other was painted with the Pentacryl - over the course of an hour - until the bowl was no longer absorbing the product. Both were left on an open shelf in the shop; by the next morning, the untreated bowl had cracked from foot to rim. The treated bowl remained intact and dried with negligible warping. A later version of this test with two small Birch bowls, one untreated and the other dipped in Pentacryl for an hour, yielded more startling results, as the untreated bowl cracked and split completely apart (as shown in the photos below).

Next, I rough turned two Cherry bowls, and three Birch hollow forms with pith included - horizontal and centered through the sides. One bowl and one hollow form, along with a 1/2" thick disc six inches in diameter, were immersed in a container of Pentacryl overnight. The other bowl, one of the hollow forms, and another disc were left untreated. The third hollow form was immersed in the product for one hour and removed. The hollow forms were measured several times over the next month, and observed for any changes. The Pentacryl saturated form showed 10% less warping and had no cracking at all. The form soaked for an hour showed a bit more warping, but no cracking. The untreated form warped more and faster, with minor cracking through the pith. I feel, since the treated forms took several days to completely warp, and the untreated piece took over- night, that the slowing of the process may be the important issue for preventing cracking. The plain-sawn Cherry disc left untreated behaved as expected, cupped and checking slightly. The treated disc also cupped, nearly to the same amount, but with no checking. It did come out of the immersion with a weathered gray color, but this was easily sanded away with fine grit abrasive. Both the Birch and Cherry pieces showed some color change, about as much as if they had been oiled. A piece of fairly soft spalted Birch was also darkened, apparently permanently, and again about as much as if oiled.

The Cherry bowls were returned to the lathe and trued and finished, then carved for texture. I enjoyed the difference in the working property of the treated piece immensely. It cut smoothly, with less dust and larger shavings than the untreated piece. Again, there was about as much change in color as with an oil finish. Pentacryl seemed to have penetrated deeply into the wood, as the color was consistent. I found very deep penetration into spalted Birch after only an hour of wetting with Pentacryl. There are some things to keep in mind, If you expect to do a lot of sanding after treating a piece, wipe the surface with a solvent - lacquer thinner works well. I found that pouring the Pentacryl into a covered container and dunking the work into the product was much simpler than trying to repeatedly brush it on, and much less messy. The product has a distinct and pleasant odor and an oily feel, so some ventilation, and hand washing afterwards would be prudent for comfort.

Pentacryl seems to control surface checking, and to slow the warping process, quite well. The Birch test samples that had been treated took two days longer to reach the maximum distortion. This may be the key to minimizing the radial checking in pith-included objects, and I find it worthwhile for this reason most of all. There is also some benefit from treating spalted woods; partially turning, wetting with Pentacryl, and completing some hours later. Smoother cutting with less tear-out was the result for me, though sharp tools and careful cutting are still necessary. For problem woods or high- risk orientations, it can make the difference between a finished object, and a handful of kindling.
Pentacryl was tested on two identical bowls turned from green Birch wood. One was dipped in Pentacryl for an hour, while the other was left untreated-and both were left on an open shelf in the shop overnight. In the morning, the untreated vessel (front) was found cracked in two, while the treated vessel (back) remained intact and dried with negligible warping. The photo on the right shows that the cracking of the untreated bowl occurred around the pith, while the bowl treated with Pentacryl had no cracking at all.

Alan Hollar is a woodturner who backed into the craft during 15 years as a furniture restorer and finisher. A member of the Southern Highlands Craft Guild and the AAW, he makes big piles of shavings at the foot of Grandfather Mountain, in the northwestern hills of North Carolina.