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.

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.

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 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. It's 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 finish and then burnish it to teh 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 disolving 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 solvent has evaporated!)

Disadvantages

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

Recipes and Usage Notes

Go to Wax Recipes for several different recipes, how they can be used and my expericence 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. Historically the preference is for what is 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.

Linseed oil is still a quite common wood finish and can be purchased in most hardware and big-box stores. While you're there, in addition to cans of "Boiled Linseed Oil", you will 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.

Linseed oil dries pretty slowly. Depending on conditions, it can take from days to weeks to fully cure. Clearly, that's a problem for the furnituremaker trying to earn a living. Luckily you can shorten the curing time by heating the oil, saturating it with oxygen, or by adding metal salts (called "driers") to the oil. Often all three are employed. The resulting Boiled Linseed Oil will dry in a few hours.

I ended up doing a deep dive on Linseed Oil and you can read All About Linseed Oil.

I have a page for Linseed Oil Recipes and use.

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 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.

It's important to note that the optimal heating point for linseed oil is very close to its flashpoint. If you feel like you want to try this, I'd recommend doing it outside and always use a controlled heat source (double boiler, lab heater, etc.).

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.

Another advantage to the introduction of these toxic driers is that you don't have to raise the temperature of the oil as high, making it much safer to produce.

Due to its more rapid polymerization, Boiled Linseed Oil should be considered more of a fire hazard than the Raw or Pure varieties. This specifically applies to the safe handling of rags which could spontaneously combust if balled up and tossed in a trashcan of, say, sawdust.

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 ⅓ BLO, ⅓ Varnish, ⅓ 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.

Another Technique

I got this from makesomething.com, it's another technique for doing the same thing I outlined above. Captured here because the original link is already dead. His blend is exactly the same as mine except he uses polyurethane instead of varnish.

Prep

1. Sand down to 220 grit and remove all machine marks.
2. Using the maroon, grey, and white abrasive pads, finish sanding the project.

Apply

1. Flood the surface with the oil/poly blend with a foam brush and immediately rub off any excess finish with a rag.
2. Let the project sit for 10 minutes and come back with a fresh paper towel to wipe down all surfaces.
3. After 24 hours repeat all the steps from day 1.
4. Once again wait 24 hours and repeat all the steps from the previous day.
5. After waiting another 24 hours apply the final coat with a rag and this time do not wipe it off.
6. After waiting 24-48 hours for the project to dry, apply a coat of Briwax wax by rubbing it generously onto the surface and then buffing it in with a fresh paper towel or rag.

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

Varnish was available in the Middle Ages although its first documented uses were on paintings as a final protective layer.

Theophilus Presbyter gives two recipes for making and using varnish in his early 12^th Century work De diversis artibus. His varnish recipes are basically the same (allowing for translation) as we have from post-Rennasiance authors like Andre Roubo (L'Art du Menuisier, 1769).

Cennino Cennini mentions varnish in his Il Libro dell'Arte (~1400) in the same context as Theophilus. Interestingly, he includes a recipe for making paintings look like they have been varnished. Was it an expensive issue or an availability issue?

It is not clear when it started being used on furniture. We have evidence of it being used on furniture by the sixteenth century based on images in paintings. But, we can't say if it was common, only on the highest status items, etc.

More research to come.

Other Finishes

The following are some other finishes I have found, put here for future reference. I have not experimented with any of them (yet).

Danish Soap Finish

Yeah, really, soap. Source: Caleb James. There is also some information on Chris Schwarz's Lost Art Press Blog.

One of the most mysterious things about Danish modern furniture to me when I started making it was this strange soap finish that is talked about so often. I wondered, "What is this all about"?

Well, the easy answer is that it is a soap that is simply washed onto the wood surface. You may wonder how that protects the surface, though. Basically, soaps traditionally were made of oils of some sort or another. Your grandma or great-grandma would have used "Ivory" soap flakes that were made from vegetable-based sources, palm and coconut oils usually. Everything from clothes to who knows what was washed with it.

Why use soap to protect wood surfaces? Wood has pores and those pores will get clogged with dirt and oils from use if not protected. Soap clogs those pores and keeps the dirt and oils out. Also being soap, it also releases dirt and oils easily thus keeping them off of the surface.

Wood also benefits by having a finish that slows the exchange of moisture from the wood to the surrounding atmosphere, thus keeping it more stable. Soap finishes aid in doing this like other finishes though to a lesser degree.

The biggest advantage, I believe, is that it leaves woods like white oak and ash looking as natural as the raw wood itself. As a woodworker I love the organic feel this gives a piece. The soap finish is also incredibly smooth to the touch and ages so beautifully.

Some suggest that soap finishes are a lot of maintenance. I have found that since they age nicely I really don't do anything to maintain their appearance. If it does get soiled, simply wiping the surface with a damp cloth will usually remove whatever is unwanted. Wiping the surface with more of the same soap will freshen it up as well. In Denmark, this is often used to finish floors even. I think that attests to its durability and effectiveness as a finish. I purchase my soap flakes from a U.K. based producer. www.msodistributing.com currently can supply this in the US.

Here is how I prepare my soap finish. I mix boiling water and soap flakes in equal parts, 1/1 ratio. If I am making a large batch I will reduce the water a bit. I only want enough water to dissolve the flakes and it should create a whipped cream-like consistency when mixed together but with a thicker body to it, not so airy.

First, measure out your flakes and then pour the boiling water in equal parts over the flakes. Let that sit long enough for the flakes to absorb the water and it will get a translucent appearance and will become sort of jelly-like. Now mix in a bowl until you get the consistency I mentioned above. See the photo below.

You can now place a small amount inside a lint-free white cotton cloth that is folded over itself. Squeeze the soap through the fibers and to the outer surface. This will thoroughly saturate the cloth with soap. Now wipe it on the surface of your wood. Put on enough to fill the pores. Don't let it cake on the surface, though. Once it is dry you can knock down any raised grain with 220-320 grit sandpaper. Apply one more coat and that is usually sufficient. Finish by buffing with a soft cloth if desired.

To freshen up your finish follow the same steps. If you have a set in stain you can try pouring some boiling water on the spot. The soap will often force the stain loose. Let it dry and reapply a finish.

A note of caution; Do not wet end-grain surfaces too heavily. These areas absorb more water than the face grain and can crack if continually saturated. Thus it is best to make your soap with less water and more like a paste. Also coat both sides of a panel, such as a tabletop, evenly so that it will reduce the potential to cup or warp.