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 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 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.

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

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).

Other Finishes

The following is a list of finishes I have found, most are historical recipes or redacted from historical sources, though no references seem to be had for these, they passed through my note-taking system so if I had them, they are lost. I have not experimented with any of them (yet).

Furniture Cream Polish

Ingredients:

• 32 oz Pure Turpentine (Not Turpentine Substitute or White Spirit)
• 16 oz Boiled Linseed Oil
• 8 oz Beeswax
• 1 oz Carnauba Wax Flakes

Process:

• Grate the Beeswax into a double boiler.
• Add linseed oil, stir well.
• Add Pure Turpentine, stir well.
• Allow to cool.
• If you did it right, it will be a cream. If you had too much liquid, it's soupy, reheat, and add a little more wax, conversely, if it's hard, then and more liquid and reheat.

Beeswax Paste Recipe

Ingredients:

• 8 oz 50% by weight Beeswax
• 8 oz 50% by volume Pure Turpentine (Not Turpentine Substitute or White Spirit)
• 1 oz Carnauba Wax (optional, this makes a much harder wax)

Process:

• Grate the Beeswax into a double boiler.
• Add Carnauba Wax, stir well.
• Add Pure Turpentine, stir well.
• Leave it for 24hrs to set.

Liming Wax

Ingredients:

• 8 oz 46% by weight Beeswax
• 8 oz 46% by volume Pure Turpentine (Not Turpentine Substitute or White Spirit)
• 5 oz 8% by weight White Pigment (tiO2)
• 1 oz Carnauba Wax (optional, this makes a much harder wax)

Process:

• Grate the Beeswax into a double boiler.
• Add Carnauba Wax, stir well.
• Add Pure Turpentine, stir well.
• Add white Pigment stir well.
• Leave it for 24hrs to set

How to use Liming Wax: To prepare an item of furniture for liming, sand the wood smooth and remove any traces of grease or dirt by wiping the surface with a cloth dipped in white spirits (mineral spirits/paint thinner).

Then, and usually, most woods and substrates will benefit from opening the grain with a wire brush or coarse paper. Using a clean wire brush, or coarse paper, scrub and travel on the wood or substrate in the direction of the grain, to clean out and to open the pores. Be careful not to be too aggressive. Check the progress quite regularly by glancing across the wood into the light, ensuring that there is a fairly even distribution of open pores. If required, apply the wood dye and seal the surface with a coat of thin transparent shellac.

Applying Liming Wax: Apply sparingly with a cotton cloth or pad, rubbing well into the grain, rubbing across the grain, forcing the liming wax into the open grain and pores, and removing any excess. Dip the pad into the liming wax and then rub into the grain, with circular overlapping strokes, until the surface is evenly covered. Again wipe across the grain with a clean pad, leaving the wax in the pores. This process is to ensure the liming is worked into the open pores and grain.

After about ten minutes, remove surplus wax from the surface by gently burnishing along the grain with a dry cotton cloth. The next day, apply an overcoat of clear Classic Wax for a more durable finish.

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 it 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.