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-20th Century. During this time finishes were one of four basic types:
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.
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.
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 18th 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 obtained but pressing the seeds of flax plants (Linum usitatissimum). Since we already have a ton of flax plants around to make linen, it's nice to be able to do something with the seeds. The oil is edible and has been used for centuries in Europe as a food additive and a nutritional supplement though it's usually called flaxseed oil in this context.
Linseed oil is a triglyceride that contains an unusually large amount of α-linolenic acid, which makes linseed oil is particularly susceptible to polymerization reactions upon exposure to oxygen in the air. This polymerization (aka "drying"), results in the rigidification of the material. And while rigid, it's not brittle, it retains some flexibility. And that's why it makes a good finish (among other uses). In fact, linseed oil has about a billion other uses outside of a wood finish.
NOTE: the drying process is exothermic therefore rags soaked with linseed oil and dumped in a pile are a fire hazard because they provide a large surface area for rapid oxidation of the oil. The oxidation will accelerate as the temperature of the rags increases. The spiral continues until all the oil has oxidized or you exceed the temperature dissipation rate of said rags, in which case you get a cheery blaze. In your shop.
Spread out your rags to dry (on a non-flammable surface), or place them in an air-tight container, so soak in water. Make sure whatever container you store linseed oil in is airtight. The surface area of the oil in a jar say is way too small to be a fire hazard, but the oil will polymerize and all you'll be left with is a chunk of rubbery...stuff. Or at least a layer of that on top of your remaining oil (that remaining oil is still good, BTW).
Linseed oil is notorious for taking a long time to "dry", days sometimes depending on the conditions. So, how do we fix that? Read on...
Boiled Linseed Oil
Linseed Oil comes in two types: Raw and Boiled.
You might think that "Raw Linseed Oil" is what we get straight from the plant. But, no. What you get when you buy "Raw Linseed Oil" is linseed oil that's been boiled. Wait! What? Then what is BOILED linseed oil? I'll get to that.
Raw Linseed Oil is made by heating linseed oil to near 300 °C for a few days in an anaerobic atmosphere. Under these conditions, the polyunsaturated fatty esters convert to conjugated dienes leading to crosslinking. Basically you are getting the oil as close to drying as you can and still be liquid, just add oxygen and you are good to go. The resulting oil will dry faster, be more flexible and less prone to yellowing than pure linseed oil.
And BOILED Linseed Oil?
Modern "Boiled Linseed Oil" is a combination of "Raw Linseed Oil" and metallic dryers to accelerate drying. I have been unable to learn exactly what these dryers are. However, Boiled Linseed Oil is considered mildly toxic as a result, so gloves are recommended.
In the Middle Ages, boiled linseed oil was created by boiling linseed oil with lead oxide (litharge). The lead oxide forms lead "soaps" (lead oxide is alkaline) which promotes polymerization of the linseed oil and the heating further reduces its drying time.
Due to its more rapid polymerization, Boiled Linseed Oil should be considered more of a fire hazard than the Raw or Pure varieties.
How I use it
OK, the chemistry lesson is over. Boiled Linseed Oil (BLO from here on out) is pretty viscous and as noted above, by itself, not a good long term finish. So I use a blend of 1/3 BLO, 1/3 Varnish, 1/3 Mineral Spirits. This flows more easily and more quickly builds a nice sheen that's also more durable than BLO alone.
I generally make it a quart at a time. That first coat is rather generous as much of it will be absorbed into the wood's pores. Subsequent coats are ragged on and will dry in 6-8 hours making building a sufficient finish more tolerable to the impatient woodworker (me). Stuff I use around the shop gets 2 coats (allows you to easily scrape off glue and differentiates that piece of wood from scrap wood). Furniture that leaves the shop gets 4 coats, sometimes more if it will see high use. Even this isn't sufficient for something like a tabletop, that gets a top coat of polyurethane or lacquer.
 Merrifield, Mary P. (2012). Medieval and Renaissance Treatises on the Arts of Painting: Original Texts. Dover Publications, Inc. ISBN 978-0486142241.
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.
Cennino Cennini mentions varnish in his Il Libro dell'Arte and Theophilus Presbyter gives two recipes for making it in his early 12th 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).