Popular Woodworking 2006-10 № 157, страница 73
Flexner on Finishing Some Reflections on Sheen Understanding how sheen works allows you to control the gloss on your finishes. One of the most important qualities of a finish when it comes to appearance is sheen. A finish can vary from a gloss so high it reflects the sharp outline of an image to a sheen so dull that nothing is reflected. In determining how you want your project to look, you need to take its sheen into account. In the last issue of Popular Woodworking (issue #156) I explained how you can control sheen by rubbing it with abrasives of various grits. The coarser the grit the lower or flatter the sheen. The finer the grit the higher or glossier the sheen. The problem with rubbing is that it is a lot of work. An easier method of controlling sheen, and the method most commonly used, is to choose a finish with the sheen already built in. Then all you have to do is brush or spray the finish onto the wood and the sheen will come about automatically. A hundred years ago, before electric lighting, people preferred a gloss sheen because gloss reflects a lot of light and makes rooms appear brighter. Today, most people prefer a flatter sheen. Manufacturers offer a variety of choices. Unfortunately, the words they use to describe these choices are vague: gloss, semi-gloss, satin, eggshell, rubbed effect, matte, flat and dead flat. There are no fixed definitions for these terms; so one manufacturer's satin may be another's flat. To be successful in getting the sheen you want on your projects, you need to understand how the sheen-creating elements in a finish work and how you can manipulate them. Flatting Agent The stuff in a finish that creates all the sheens lower than gloss is called "flatting agent." It is a very fine silica product that settles to the bottom of the can and has to be stirred into suspension before using. Gloss has no flatting agent added and therefore nothing to stir. Though silica is a more complex material than fine sand, it's often helpful to think of it as fine sand for the impact it creates at the surface of a finish. Here's the way silica works. The manufacturer adds the amount of by Bob Flexner Bob is the author of "Understanding Wood Finishing" and a contributing editor to Popular Woodworking. A finish with flatting agent added appears dull because of light being scattered off the m icro-roughness at the surface of the film. This m icro-roughness is created as the solvent evaporates by the finish "shrink-wrapping" around the particles of silica that lie at the surface. silica necessary to create a given sheen to a polyurethane varnish, standard varnish, lacquer, catalyzed finish or water-based finish. All film-building finishes except, unfortunately, shellac are available with silica added. (It's not true, as you sometimes hear, that lacquer is always glossy.) Bob is the author of "Understanding Wood Finishing" and a contributing editor to Popular Woodworking.
A finish can have an infinite number of sheens depending on how much flatting agent (silica) is added. Above are examples of how the sheen, and resulting reflection, changes when flatting agent is added to a gloss finish. 90 Popular Woodworking October 2006 |
