Today Dave is back with more on how furniture ages, and a little bit more on what you can do to achieve the look you want with finish selection.
Our last post talked about the tendency of wood to change as it ages, based on it’s interaction with the atmosphere: air and water. Today we’re going to look at the biggest impact you can make on that change through control of those two elements in your home, and finish selection.
As we mentioned last week: Wood is made from a vast array or organic molecules. These molecules in wood react to heat, light, humidity, air, and all the things that touch the wood’s surface over the decades. A major purpose of finish is to provide a barrier to these things – it’s the “stainless” equivalent for wood. The type of finish and its stability in the face of those assaults change how quickly the wood reacts to its environment.
Let’s consider the environment first. Heat as a factor by itself is only significant at extreme variations, and otherwise mostly impacts humidity.
Humidity variations make the wood swell or shrink, and this occurs naturally as the seasons change the air from warm and humid to cool and dry. One of the biggest differences between high end furniture and mass market furniture is in care and understanding of wood as it handles humidity: properly designed furniture accounts for the fact that wood expands and contracts with humidity changes. So as long as your piece is built properly, furniture can handle changes in humidity, but cupping (curling of flat boards across their width) is a frequent issue. Humidity control in your house is the best present that you can give to your furniture: a humidifier for the dry winter months, and a dehumidifier or air conditioner if summer humidity is high. My shop is humidity controlled at 40 – 50% year-round to ensure stability in my pieces as they’re being constructed.
There’s not much that can be done about air, other than encasing the piece in thick plastic. That’s just ugly or means you live in a museum, so let’s not worry about it.
Light we touched on briefly in our last post on this topic, but to reiterate: light accelerates the rate of change for wood tone because it contains UV rays. Because of the exponential exposure to light that exterior woods face, exterior finishes often contain a UV inhibitor – a photoreactive molecule that preferentially absorbs the UV so that the wood underneath doesn’t. That increases the longevity of exterior wood but at the cost of a thick, very yellow, surface coating. That’s fine on your ipe deck, but not so much on a cherry desk with inlays.
For interior finished: some oils are also photoreactive – they darken with sunlight. Linseed oil is very photoreactive, boiled linseed oil less so. Tung oil is a little reactive also. Before 1960, varnishes were made with linseed oil, so guess what color they turn? That is one of the reasons old furniture is so dark. It’s not actually the wood, it’s in the finish. Mahogany is red when freshly worked. “Mahogany” colored modern furniture is stained to be the color of old varnish on dark red wood, because that is now the color people associate with mahogany.
So how does this inform my work? Since I make mostly decorated furniture, I want finishes that are very pale: with as little color shift as possible. Also, I prefer to use finishes that do not darken with time. I don’t use boiled linseed oil for both reasons. Oil based varnish is almost unavailable anyways these days due to air quality regulations that prohibit some of the key solvents. That is a sad thing for custom furniture, but a good for society.
That leaves us with my preferred traditional finish: shellac. You may have heard of shellac in the context of a nail salon, but it is an excellent finish with many desirable properties. Shellac ages pretty well, and also can be bought “super-blonde” – a very pale yellow. Unfortunately, water repellency isn’t one of shellac’s features, so I use it alone only when protection from water is not a requirement. When it is, I topcoat the super blonde shellac with water based polyurethane. I know that purists don’t approve. It looks a little plastic, because it is plastic. However, a thin coating protects the wood from water while minimally building thickness. The latest products are also “water white”: they add no noticeable hue at all*. They are quite an amazing improvement from their 10 year old forbearers.
In addition to how the wood itself changes color over time, it acquires additional character called “patina.” Patina is the result of the dirt and oil and dust and such that gets on a piece of furniture, then gets caught in the paste wax that is the traditional protectant for furniture in the house. Traditional paste wax is a pretty good means of polishing your furniture – much better than aerosol based furniture polishes that have silicone in them so that they level better. Finishes, by the way, do not need to be fed. That is pure marketing gibberish. Clean your furniture when it needs it, and apply wax when it looks dull, or every twenty years, whichever comes first.
Proper care of your wood furniture, and understanding finish selection from the start, will mean that your piece will mellow and acquire character as it ages, ready to grace your family’s home for generations.
* A note: in hardware stores you may notice finishes advertised as “clear.” It should be noted, that this is NOT the same as “colourless.”