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 nor 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.

This article is a survey of period finishes. In this case, period is pre-20^th Century. During this time finishes were one of five basic types:

wax
oil
varnish
shellac
paint

Modern chemistry has added polyurethane and catalyzed lacquer to the list of possible finishes though these could be classified as a type of varnish (more on that below). Both of these are fast drying and much harder than previous finishes. But they lack the warm and tactile feedback of earlier finishes and I rarely use them even on my modern projects (exception: table-tops).

Here we're looking at a brief overview of each finish, why, and how it works. Then we'll get into how to prepare it and use it in the workshop.

N.B: Like many of the things I write, this got completely out of hand. If you start reading and get to feeling tl;dr but want to know stuff about finishing, there is no better single source than Bob Flexner's Understanding Wood Finishing (2010). If you can wait for it, a revised 3rd edition is due in March 2021. I recommend the print version (versus the eBook); high-resolution photos are exactly what you want here.

Waxes

Waxes are among the most ancient of coating materials, predating written history. Traditionally, waxes came from either animal or vegetable sources. What's left you ask? Parafin, a petroleum by-product. In the British Isles wax is almost always beeswax.

Source and Preparation

Beeswax is secreted by bees for use in the construction of the honeycomb and is removed from the comb by melting it with heat. Beeswax tends to be soft with a low melting point of 60-65°C/140-149°F. Its properties can be modified somewhat with processing, though likely the extent of that was filtering and purification. It can be lightened with bleach and colored with pigments.

Once the New World was opened, Carnauba wax (also known as Brazil wax) became available. This was made from the Carnauba Palm tree, a native of Central and South America. It's one of the hardest natural waxes and by itself is too hard and brittle to make a good finish. However, add 15% or 20% Carnauba wax to Beeswax and you'll have a wax finish that is a lot more durable.

Application

The most basic application of wax is to rub solid wax over the surface to be finished and then burnish it to the desired sheen. The mechanical and thermal action partially melts the wax and drives it into the pores of the wood.

A less labor-intensive approach is to prepare a liquid(ish) version which can be spread more easily and then buffed out. Liquifying wax involves dissolving wax shavings in a solvent like turpentine or alcohol. To speed the process along, you can heat the mixture. However, I hope it's obvious that this can get pretty exciting given the low flash point of the solvents. I would not use an open flame.

The drawback to a wax finish is that it never cures into anything harder than the wax you put down. While it's water-resistant, it can easily be removed by mechanical action (wear) or solvents (alcohol) and must be frequently renewed.

The Score on Wax as a Finish

Advantages

Mildly water resistant
Moderately resistant to acids and alkali
Does not color the finish of the wood
Easy and quick to apply (rub on, rub off)
Very forgiving during application
Easy to refresh the original finish if it becomes worn or damaged
Non-toxic and food-safe (Once the solvent has evaporated!)

Disadvantages

Damaged by alcohol
Must wait for the underlying finish to cure completely, or solvent in wax may damage it

Recipes and Usage Notes

Go to Wax Recipes for several different recipes, how they can be used, and my experience with them (if any).

Oils

The second oldest approach to finishing wood products is the use of oils. You can use any oil, even animal fat, though that has some obvious (and odious) drawbacks. For wood finishes, especially furniture, we prefer what are called drying oils. These are plant-based oils that polymerize (harden) when their fatty acids are exposed to oxygen. Linseed (flax), walnut, and poppy were the most popular sources of drying oils. Other types of oil (e.g., Canola oil, Mineral Oil) don't dry. They will soak into the wood and be dry to the touch, but they don't polymerize and so are easily washed off.

Linseed Oil is the most common historical oil for finishes since it is made from the seeds of flax plants (Linum usitatissimum) which were widely cultivated for linen. Although hard references are lacking, it is believed that it was being used in England as a wood finish by the Middle Ages. It is still a common wood finish today and can be purchased in most hardware and big-box stores.

While you're there, in addition to cans of "Linseed Oil", you will see cans labeled "Tung Oil" and "Danish Oil". Unfortunately, these aren't made from tongues or Danes.

Tung Oil comes to us from China where it's been used for centuries as a wood preservative for ships. It is made from the seeds of the Tung tree (Aleurites fordii) and has properties very similar to linseed oil (water-resistant, dries slowly, non-toxic in its pure form). Pure tung oil doesn't penetrate wood well and the initial coat must be thinned for a good base.

Pure Tung Oil is less stable than Linseed Oil and must be protected from sunlight before use in order for it to cure properly. It also may have a mildly disagreeable smell while it dries. It is a darker color than Linseed Oil so it's a good choice for that first coat if you are looking for that bold wood grain look. It is also less susceptible to mold that Linseed Oil

However, the Tung Oil you are likely to encounter in Home Depot is almost certainly not pure Tung Oil. It's usually a wiping varnish that's trading on the exotic-sounding name for marketing purposes. These formulations are usually vaguely equal parts Tung Oil, varnish, and a solvent with a splash of metallic driers tossed in to speed the curing process.

The Danish Oil you'll find is a wiping varnish. No pure Danes ever. It's just a combination of oil (either Tung Oil or Linseed Oil), varnish, and a solvent base with some driers. The popular WATCO brand makes their Danish Oil in several tints which is handy for repair work or coloring the wood without resorting to stain.

Neither these are bad finishes, you are just paying too much for a wiping varnish you can make yourself. We'll focus on Linseed Oil as our drying oil of choice.