Making furniture from scratch has two phases: building and finishing. Building involves cutting stuff up and putting it back together in more useful shapes. Finishing is done by applying a special kind of liquid to the wood surfaces, which then dries into a protective layer. This coating transforms the appearance of the wood, making it look rich and elegant by highlighting its color and figure. Finishes also protect the wood surface from dirt and, in particular, moisture.
Wood is very sensitive to water. The process that transforms a tree into usable lumber consists of two steps: cutting the tree into manageable pieces, and then drying it to remove the water found in living wood. Dried lumber is harder and tougher than green wood, and is also more dimensionally stable. During drying, the wood is placed in a kiln, which is a heated chamber, and baked to remove almost all of the moisture. While this is happening, the wood shrinks significantly. It can be very tricky to dry the wood without causing it to crack from the stress of the shrinkage. Kiln operation is a skilled trade in its own right, not for amateurs. Furniture makers buy lumber from special lumber yards that do both the initial sawing and the kiln drying.
If dried lumber is exposed to water, it swells up again. This is troublesome in lots of ways. It can cause joints to crack, and closely fit parts, such as doors and drawers, to stop functioning. Surprisingly, the amount of moisture required to make the wood swell is not large – the ordinary changes in relative humidity encountered as the weather changes, which happens every day, will cause wood to swell and contract. Hot, humid days cause the wood to expand. Cold, dry days cause the wood to shrink. The kiln drying process conditions the wood so that the amount of movement is within usable parameters, and the application of a moisture resistant finish coat slows the moisture exchange even more. So applying a good finish is an important part of making wooden furniture durable.
Finish is also important aesthetically, in two ways. It changes how wood looks and how it feels. Unfinished wood looks dry and dull – not the appearance that we associate with fine work. A quality finish coat makes the surface look shiny and reveals depth to the wood. The finishing steps also make the surface smooth, and pleasant to touch.
There are many, many ways to finish wood, but they are all conceptually similar: some type of liquid that will be poured, wiped, brushed, sprayed, or rubbed onto the wood. That liquid consists of two components: solvent and solids. The solvent part of the finish will evaporate into the air and disappear. The solids will stay on the surface of the wood and, hopefully, transform the wood’s nature from dry, delicate, and ugly to rich, durable, and, beautiful. The solids consist of some kind of resin. Resins are molecules that, when not floating around in solvents, cling tightly to each other and to the surface they are applied to. They are, in a word, sticky. For a while, anyway. Ideally, resins will be completely liquid when suspended in a solvent, and then sticky when they are applied to a surface and the solvent is disappearing, and then suddenly transform themselves into a very hard coating that is not sticky at all (because a soft and sticky surface would be disgusting to touch and easy to damage.) This hard coating should be smooth and shiny, and also transparent. But the resin still has to cling to whatever it is touching, even though it is now hard. Also, it should not be easy to transform it back to a liquid state. We don’t want it to dissolve when someone spills water, or nail polish remover, or hot coffee, or any of the other liquids that we commonly use in daily life. And we don’t want it to soften if the surface gets hot, or cold. And, as long as we are making a wish list, we also want it to be sufficiently hard so that it can’t be scratched, even if someone drags a something hard and sharp across it. But if it does get scratched, we want to be able to repair it easily – I.e., walk it back and forth through the steps of liquid, sticky, and hard so that the repair is indistinguishable from its surroundings.
If you think about it, that’s a contradictory set of requirements. It’s amazing that there are any materials which will perform even a couple of the tricks on this list. As it turns out, there is no single substance which does every one of these things perfectly. There is always a trade-off between ease of application, dry time, appearance, smoothness, ability to resist moisture and scratches, and repairability. You can choose any two characteristics off that list of six, and the rest are compromised.
This explains the wide variety of types of finishes. There are different kinds of solvents, and different kinds of resins. When we hear about waterbased, or latex, or, shellacs or varnish, or oil finish, or urethane, or acrylic resins, or conversion varnish, or UV cured, or catalyzed polyurethanes, what we are actually talking about is a particular combination of solvents and resins, with some particular method of application, a particular chemical transformation during drying, and a particular resultant hardness, moisture resistance, and repairability. Finishes are also formulated for different types of underlying material. There are different mixtures optimized for the walls of a house, or car exteriors, or to line the inside of food cans, or to cling to road surfaces, or to protect the hull of an aircraft carrier – the list is endless.
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