Woodworker's Journal 2011-35-Winter, страница 37
Tannin reacts differently to ammonia gas than to liquid ammonium hydroxide. Fumed wood must be darkened by evaporating ammonia gas. devoid of tannin, such as birch, maple, basswood or pine will show almost no change. However, there's a way to fume non-tannin wood: add tannin to the wood before fuming. An easy method is to apply a wash of strong black tea to the wood. It will raise the grain and increase the tannin content, and thereby darken the color. Denser sticks of wood tend to contain more tannin than lighter ones. When choosing wood for a fumed project, try not to mix heavy and light wood in the same piece. Getting Started John Brock, a friend who's done a lot of fuming over the years, shared some of his experiences with me, offering a lot of good advice on how to go about fuming safely. It all starts with a fuming tent — anything from a sealed room for large items, to a Rubbermaid® storage container for small ones. John usually makes custom tents that are just the right size and easy to heat. "My tents," he explains, "consist of a wood or plastic pipe frame covered in clear plastic sheeting and sealed airtight with duct tape. I usually leave the bottom open so I can place it over the piece I'm fuming. I also leave some extra plastic rolled and stapled on the bottom edge of the frame to act as a seal. Next, I make the base of the tent from a sheet of plywood covered with black plastic." The clear and black plastic combination is important, as you will soon see. John loads the tent Tannin reacts differently to ammonia gas than to liquid ammonium hydroxide. Fumed wood must be darkened by evaporating ammonia gas.
with pieces that have been finish sanded, removing all hardware, as ammonia can react with metals. He wears an organic respirator, gloves and goggles, and starts a fan behind himself to blow the vapors away. You'll find 28% ammonia solution at reprographics supply houses for about three dollars a gallon. Pour some into one or more small, shallow bowls, slide them under the tent, and quickly cover it. Avoid any contact with the ammonia vapors. Ammonia gas is quite hazardous, but fortunately, it elicits such a strong reaction when you smell even a tiny whiff of it, that you will be unable to remain in the same room with the fumes. If you forget to wear a respirator and goggles, it will quickly remind you. Controlling the Color How fast your fuming occurs will affect the resulting color. Higher concentrations of ammonia and warmer temps speed up the process. Slow fuming results in a greenish-gray brown color. Quicker fuming tends to minimize the greenish tint and yields a warmer, more grayish brown. "The real breakthrough to excellent results," John told me, "was the addition of a heat lamp." After loading the tent, he sets the heat lamp to shine through the clear plastic onto the black base. This raises the temperature in the tent to 80° F and lowers relative humid ity. A drop in humidity makes the wood "thirsty," so it more quickly absorbs the ammonia vapors. John likes a dark, warm color on the wood, and 36 hours at 80° gets him there. 'Without the heat lamp," he notes, "I was unable to get the depth of color I wanted." Unload and Finish When it's time to unload, turn off the lamp, suit up with safety gear and remove the ammonia. An exhaust fan clears the smell, but the fumed wood will hold a slight odor for a few hours. A very light finish sanding may be needed, and the fumed wood should sit a couple of days to let the moisture content drop. "After fuming," says John, "the wood will look brownish gray and not very pleasant. Don't let that worry you. To see what it will look like under a finish, wipe a little naphtha on the wood and look at it in a strong light. It is beautiful and very complex. I finish with shellac, or oil followed by shellac, to maximize the chatoyance and 'pop' the wood grain." However, any finish will go over fumed wood. And, of course, you can always use fuming as a first step toward getting the exact color you want. Gustav Stickley used a variety of dyes to adjust color after fuming. You can do the same, with dyes or pigment stains, especially on sapwood areas. & __/ WINTER 2011 37 |
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