Woodworker's Journal 2011-35-Winter, страница 41
dye, but some non-soluble dyes stay behind in the resin itself. As a result, shellac comes in a variety of colors, depending on the type of tree, the geographical area and even the time of year it is harvested. The most common is orange shellac, but colors range from dark garnet through very light super blonde and platina. Seedlac contains the shellac resin, about 5% wax which the bugs create as "breathing tubes" and the random bits of tree bark, twig wood and bug legs that get caught in the scraping process. It is then either melted or dissolved in alcohol and strained. The cleaned resin can be dripped onto a sheet and cooled as buttons, but more commonly it is dried and stretched or rolled into sheets, which are broken up into flakes. It can also be filtered to remove some of its color, get bleached with chlorine to create "white" shellac or have the wax removed. Uses for Shellac We woodworkers have plenty of reasons to prize shellac. It makes a beautiful non-toxic finish for chairs, cabinets, cradles and cribs, bookshelves, boxes, beds, picture frames, musical instruments, turnings and anything that gets normal but not excessive wear. Shellac has very good resistance to acids, water, stains and scratches, but it won't hold up to high heat or alka-lines, such as ammonia-based window cleaners. I would not choose it for areas that are near heat sources, like fireplace mantles or cabinets above stoves, or for a kitchen counter or table that might be a repository for a hot coffee pot But most anywhere else, shellac would be a beautiful option. Perhaps the most important use for shellac in the woodshop is as a sealer. Technically speaking, the first coat of any finish seals the wood, but shellac does far better than most in that department. Refinishers know that a coat of dewaxed shellac after stripping will seal in old stains, wood resins, knots, wax, grease and even silicone oil, which causes those annoying pockmarks called "fisheye" that show up in later coats of lacquer. Shellac can act as a barrier between incompatible finishes. It can improve adhesion between two dissimilar coatings because it sticks well to most any surface, and most finishes stick well to it. Waterborne coatings present their own problems, and shellac as a sealer makes many of them go away. Water-soluble dye will bleed up into coats of water-borne finish, but a thin coat of shellac will prevent that. It will also act as a "tie" coat to improve adhesion to oil-based stains, and it will even add some depth to darker woods that sometimes look "washed out" under water-bornes. It's a wonderful way to forestall fisheye and other contamination problems that sometimes prevent waterbornes from laying out smoothly. Shellac will even prevent the water from raising the grain of the wood. Applying Shellac I routinely use a wash coat of dewaxed shellac as a sealer under lacquer, waterborne coatings and sometimes even polyurethane. My method of hand applying the sealer coat results in more uniformity. Typically, when you apply the first coat of finish to wood, it sits nicely atop the flat grain areas, but gets completely sucked into the end grain. As a result, the end grain looks raw while the less porous flat grain looks shiny. So even though you applied a uniform coat, you end up with an uneven one due to selective absorption of the wood. To prevent that, I flood the first coat of shellac on, then wipe off the excess while it is still wet Thin the shellac first by adding denatured alcohol until the mixture is about the consistency of skim milk. By flooding it on liberally, the end grain has a chance to absorb as much shellac as it can. When I wipe off all that has not been immediately absorbed on both the flat and end grain areas, the result is a uniformly sealed piece. The first coat of shellac will raise the grain of wood very slightly. I let it dry about an hour and sand very lightly with 320-grit paper — just to smooth it a bit After that, subsequent coats of shellac don't need to be sanded, since the alcohol in each successive coat will redissolve the first one enough to give perfect adhesion. You can apply more shellac with a brush, spray gun or even with a cloth pad, a process called French polishing. winter 2011 41 |
