Post #218740 by amiotiki on Fri, Mar 3, 2006 6:57 PM

Okey dokey...this is going to be a little long (I think). I'll try not to be too technical or get into too many of the scientific details. I'm going to start with some of the signs of trouble:
Face checking (FC), radial shrinkage (RS), tangential shrinkage (TS) and longitudinal shrinkage (LS). FC is a good sign that the wood has undergone cellular collapse, which is irreversible - the surface of the wood has sort of a checkerboard of cracking or crazing on it. RS and TS are responsible for the big cracks in logs that run outward from the heart of the wood and lengthwise. LS is primarily manifested as shortening - doesn't really cause any cracking.
All of these are caused by the collapse of the wood's cell walls when the water normally contained in them leaves too quickly. It's really important to remember that this is irreversible - the cells can't be plumped back up once they collapse.
So, the way we avoid the cellular collapse depends on what kind of wood we're working with. Some of you are probably working with dry, seasoned wood - while others are working with fairly fresh (green) wood that contains a lot of moisture. Either way, the wood should be bulked - probably before work begins, but certainly after a piece is finished if you want to keep it from cracking or deforming. Essentially, all bulking does is replace the water in the cell walls with something that will strengthen them during the dehydration process.
Equipment needed:
A vessel or tub large enough to hold your 'artifact'- agricultural feed troughs work great
Bubble wrap for insulation
A means to heat the solution in the tub to a top heat of 70 degrees C (158 degrees F) - waterbed heater, restaurant sink heater.
A solution circulating device - electric fish trolling motors (easily powered by a 10 amp battery charger)
Methodology:
There are two products that can be used for bulking - PEG or sucrose (plain old white sugar). Sucrose is the cheapest and easiest to acquire, and gives the best results as long as you follow the procedures that follow.

1. If you're working with hardwood, start with a 10% solution of sugar to water; if softwood (palm is considered a softwood) use 5%. Weigh the wood first - this is important.
2. Put solution and 'artifact'in tub, position the stirring device and heater, place bubble floating on the surface, and fire her up. There is one drawback to using sugar, and that is its tendency to ferment - nice for brewskis and other beverages - not nice for wood. To prevent runaway fermentation, which occurs when the sugar concentration is less than 40%, we apply heat. 50 degrees C generally controls the fermentation, but you can pasteurize the solution by heating it up to 70 degrees C for a few hours at a time, usually without harming the wood. If you can't get the temp up, you can use 100ml of Lysol per 50 liters of solution for several weeks until the 40% mark has been passed.
3. Increasing the strength of the solution: If the 'artifact' weighs less than a few pounds, the percentage of sugar can be increased 10% per week until a concentration of 50% has been achieved. Once the 40% mark has been passed, the solution becomes hypotonic to organisms and they can no longer survive, so you don't need to worry about heat or Lysol anymore.
   If the 'artifact' is large, you may need to wait longer between increases - perhaps up to a month...but I think for most of y'alls purposes two weeks would be sufficient.
   NOTE: The gradual increase of sugar concentration is critical in order to minimize osmotic pressure differentials - in other words, to keep the exchange of water and sugar steady and consistent so cell walls don't collapse.
4. Once you've achieved the 50% solution level, weigh the 'artifact' weekly until you see about a 20% increase in the weight. If the wood was missing a lot of cellulose before it went in the tank it could gain as much as 25-35% of its original weight.
5. At this point the wood is ready for slow dehydration in a humidity chamber (I'll tell you how to make one of these at the end). Remove the 'artifact' from the tank and allow excess solution to drip into the tank. Slow drying in the humidity chamber (HC) reduces the possibility of osmotic collapse of the cells from drying too fast, and the adjustable humidity reduces stress on the different layers of the wood.
6. Load your wood into the HC from the top down on a rack that will allow excess fluid to drip onto the floor of the chamber (for easier clean-up). Close the plastic curtain and turn on your humidifier - this should bring the relative humidity up to 100% pretty quickly. Shut off the humidifier. Check the artifacts every few days, spraying the inside of the HC with Lysol to eliminate the possibility of MOLD growth. After four - five weeks the ambient humidity in the chamber should be about 50%, and you can open it up to expose the wood to the atmosphere.
7. You're going to want to keep the wood in a situation where the relative humidity is at 40-50% (typical of air-conditioned buildings). Otherwise there may be some shrinkage or expansion, but this shouldn't cause any significant damage to the wood (unless you're in like a 6% relative humidity part of the country, hehe).

To build a HC you just need some kind of container (a plastic agricultural tank with an opening in the side cut out and covered by a shower curtain works great), a rack for the inside, a humidifier - preferably that can be turned on from outside the HC, and that also has a gage indicating current humidity in the chamber.

While all of this is relatively cheap to build and easy to use, it does require patience. The whole process can take 3 - 6 months so you would have to plan ahead to prep your wood if you wanted to bulk it before carving, or the same amount of time if you do this after you carve. The payoff is that you'll have a very nice, stable piece of wood to work on that won't crack or split.

All of the above information is courtesy of Bradley Rodgers, PhD, in his book The Archaeologist's Manual for Conservation published 2004.

Best of luck to you all!

amiotiki

P.S. If you want to know how to remove stains from your wooden items, let me know and I'll post info about how to go about that.