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Doug Ayen

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vacuum chamber [Mar. 1st, 2004|02:32 pm]
Doug Ayen
Busy weekend. Saturday, with the much appreciated help of Carl H., I put together the next generation in vacuum chambers for vacuum stabilization of wood. This one is larger, has a modern vacuum source, and hopefully will do the job of stabilizing the wood for the current generation of knives.

Stabilization, in this case, refers to two things: first, the removal of residual moisture in the wood, and secondly the addition of some substance to keep the wood from reabsorbing moisture. When wood absorbs water from the air or other source, it of course swells, only to shrink again, and possibly crack, when it dries out. By putting the wood into a vacuum for a period of time, the moisture is evaporated out rapidly, usually on the order of a few hours, depending of course on temperature and amount of vacuum produced. By replacing the vacuum with some sort of stabilizer, such as PEG (PolyEthyleneGlycol), Nelsonite 30B02, or my favorite due to cost and availability, polyurethane thinned down with paint thinner at a 2-1 ratio. (That's 2 parts thinner to 1 part polyurethane.) It may not be quite as effective as some of the commercial preparations, but it does seem to do the trick when I've used it, and unlike PEG it doesn't leave the wood waxy or discolored, and unlike Nelsonite 30B02 it doesn't cost $35/gallon.

The vacuum chamber is made of 3 feet of 4" schedule 40 PVC, good to 200 PSI, capped at one end with a standard pipe end, and with a flange and screw-in plug at the other. There are two valve-controlled fittings, a vacuum outlet and a stabilizer inlet, both sealed in place. The vacuum source is a venturi-style vacuum pump, powered off the air compressor.

Once our brains had recovered from the fumes of the PVC cement, I did a test run. After loading it with some test scraps, I put a vacuum on it for half an hour, then sealed the valves. Sadly, half an hour later the vacuum was gone, so I knew I had a leak. I did some bubble testing at about 20 psi, and fixed a couple of leaks with Shoe Goo, a tenacious rubbery compound. With a now-sealed container, I left a vacuum on it overnight. I did notice that it still lost vacuum over the course of an hour or so, but decided to just keep the pump going overnight rather than try to troubleshoot the leak.

The next morning, I shut off the vacuum pump and mixed up the stabilizer and poured it into a bucket. I put the inlet hose into the bucket, then turned the vacuum pump back on and cracked both valves until the hose feeding into the chamber was full. I then closed the vacuum source valve and fully opened the stabilizer valve. Without any additional vacuum, the chamber sucked in over 2 gallons of stabilizer, then stopped. I toyed with the vacuum a bit to see how much more was needed to top it off, and it seemed to be a small amount. I then shut off the vacuum and let it sit, having marked off the level in the stabilizer container so I could see if any more went into the chamber. Over 15 minutes, I'd say around a cup of stabilizer went into the chamber, I'm guessing soaked into the wood.

I let the chamber sit undisturbed for a couple of hours, and discovered the location of the pinhole leak -- somewhere around the end-cap, before I use it again I'll re-bubble test to see if I can seal that up. While I wanted to leave it sit longer, I decided to pull out the wood at the three hour mark to see how much penetration I'd had. The wood is currently drying in the workshop, I'll saw it up in a couple of days, see if I can get an idea of how much, if any penetration I'd gotten.

If I didn't get good penetration, I'll go buy a vacuum gauge and see what kind of suction I'm getting, and if it's not impressive I'll see about improving it. If *that* doesn't work, I might try changing thinners to something with a smaller molecule than mineral spirits, or putting on some pressure after the vacuum has done its work sucking in the stabilizer. If *that* doesn't work, I might even try the Nelsonite.

--doug
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