Chemistry in the Workshop

I am not a chemist and CHEM101 was a long time ago. Some of the things described here are hazardous, but few are actually dangerous unless you're stupid or careless. And all of that is on you. I'm explaining what I do or have done and I've neither poisoned myself or burned my house down. Yet. YMMV.

Chemistry is a lot of fun as the second most practical science (after physics), there is a lot of interesting things you can do with common things around the house.

For instance, wood ash. You could throw it away, most people do. But it's mostly potassium carbonate and if you mix it with water, you get sodium hydroxide (lye). And lye is useful for all kinds of things from making soap to drain cleaners. If you want to try this, you should know this is one of those potentially dangerous things I mentioned. You see potassium carbonate is extremely hydrophilic and it will happily draw that water from your skin. And, as a bonus, as the lye forms, it will start to make soap from the oils in your skin. Cool, add more water and lather up! Actually that's going to hurt. A lot. Wear gloves.

As fun as stuff like that can be, we're here to talk about making and using period finishes. In this case, period is pre-20^th Century. During this time finishes were one of four basic types:

• oil/wax
• paint
• varnish
• shellac

Modern chemistry added polyurethane and catalyzed lacquer. Both are fast drying and much harder than previous finishes. But they lack the warm and tactile feedback of earlier finished and I rarely use them even in my modern project (exception: table-tops).

Here we're looking at a brief overview of each finish, why and how it works and how to prepare it and use it in the workshop. Research on evidence for use can be found elsewhere.

Oil/Wax

Using oils and waxes to protect or enhance the look and feel of wooden items has been going on for a very long time.

The most common oil for wood finishing is linseed oil. In fact, it's hard to find any other kind. Walking through Home Depot or the like, you'll also see things labeled "Tung Oil" or "Danish Oil". These aren't made from tongues or Danes. These products almost always have linseed oil as a base with some other additives to justify the extra cost.

The most common wax for wood finishing is beeswax. The upside of beeswax is that when used as a top coat, it can be buffed to a very high sheen. However, it's not very durable, nor water-resistant. Modern, commercial waxes are carnauba wax or a blend of carnauba and beeswax. Carnauba is much harder and produces a much more durable finish. However, when it comes to hand finishing, its hardness is a real chore to work with. As a result, there are a number of products that are blends of the two (and possibly other things as well). With the goal of having something soft enough to pleasantly work by hand and produce a more durable finish than beeswax alone.

For our purposes, when I say wax, I mean beeswax.

Use

If you don't care about chemistry and just want to understand and use the finish, this part is for you.

Linseed oil isn't a film finish (like varnish or shellac), it soaks into the pores of the wood and drys. This accentuates the figure and grain of the wood but it isn't very protective. However, it's easy to repair, just recoat. To get a shinier finish, add more coats. To keep it shiny, add a coat (or more) of wax.

I usually start with a plain linseed oil coat and apply it liberally with a paintbrush. If by the time I finish coating a project, there are dry areas already, I coat that area again. Come back at least 30 minutes later, but it could be hours and wipe off any excess that remains. Allow it to dry overnight. If it's cool, it might take as many as two or three days to fully dry.

It looks pretty good already, doesn't it? Well, yes, but it doesn't have much of that sheen yet. So, you will want to add additional layers of oil. These additional layers will be thinner. You are building finish now, the wood won't be soaking up much more if any. So for subsequent coats, I apply it with a cloth and wipe on a layer then let it dry overnight (or longer).

There is an old adage that I believe is 18^th Century:

One coat a day for a week, one coat a week for a month, one coat a year forever.

Nice, eh? Well, it won't stay that way. Over time, the wood will dry out and start to look dull like it is covered in a fine layer of dust. To avoid that, you need to either add more layers of oil or some other finish on top to protect the oil layer. That's wax.

Once the oil has fully cured, apply wax. The commercial stuff or something you whip up yourself. Rub it in and be sure you get a nice even coat. Allow to dry and then buff off. Now, you're done. For now. The wax will also dry out over time, just slower. Again, really easy to renew it by adding a new coat.

Don't get too froggy here. Putting on 6 coats of wax is a waste of time. The paste includes solvents, for wax, so you are at best making a slightly thicker layer of wax. You can't really build layers like that.

All About Linseed Oil

Linseed oil is made from flax seeds (flax is grown for its fiber to produce linen). It's going to be common basically everywhere before, during and after the Middle Ages. And cheap. So, we'll take it as a given that it's an appropriate finish for our furniture.

Add Wax

Another very common finish both on its own and in conjunction with linseed oil is beeswax. Rubbed into the surface of wood you get a nice finish, though not very durable. Applied over an oil finish, it looks even better and is the way we see it used later in period and into modern times.

Varnish

Cennino Cennini mentions varnish in his Il Libro dell'Arte and Theophilus Presbyter gives two recipes for making it in his early 12^th Century De diversis artibus. His varnish recipes are basically the same as we have from post-Rennasiance authors like Andre Roubo (L'Art du Menuisier, 1769).