Basic Principles and Terminology of Natural-Resin Varnish

I mentioned I’m not going to talk about how to make varnish other than to say that you can go out to youtube and read about it, along with old texts. The texts written around the turn of the century or a little earlier are still accurate for us, sans the quantities where things are in proportion to 100 pounds of resin or 100 gallons of finished product or some other such large numbers. Some of the resins are $6 a pound in quantity, and some are $70 even in quantity if they are uncommon (semi-fossil copal). But I want to define some things because you may want to go figure this out, and I want to talk about varnish and if I use terms without defining them, you’re not going to follow what I’m saying because it’ll seem like jargon. However, some of the terms apply to all finishes and since I want to talk about varnish anyway, this post is a limited intro to terms used with varnish making.

By the way, when we make varnish, it could be a very small amount up to probably some fraction of a gallon. I typically try to make a quart that in better varnishes will be two when it’s properly thinned. A quart of varnish that’s 70% solids is a *lot* of finish. The large quantities in older books would not only be a logistical and budgetary issue, but also probably not legal to make in a lot of jurisdictions because of VOC rules. That sounds like a weird restriction for a hobbyist, but varnish cooking was an industry, so the rules were written for it as well as manufacturing other coatings.

Defining the Basic Process of Making an Oil Varnish

An oil varnish, like what you’d think of as being on a boat, woodwork or a violin is made from a resin (usually from a tree), an oil (usually linseed, but sometimes walnut or tung) and a thinner (usually real turpentine, but many solvents will work and commercial varnishes are typically made with cheap hydrocarbon solvents – good fresh smelling turpentine is $62 a gallon at the very cheapest, and some is much more than that.

The process of making varnish is typically several steps, but it’s not like a recipe for brownies.

Step 1 – prepare the resins and oils. Preparation is focused mostly on cleaning things off of the resin or out of the oil that complicate step 2.

Step 2 – cook (“run”) the resins and cook (to “break”) or cook and oxygenate the oil. This step prepares both parts so that they will bond together in step 3.

Step 3 – introduce the finished products from step 2 to each other, cook them together until they bond with each other, and then perhaps longer if searching for certain properties, and at the tail end of the process, introduce enough turpentine to make the varnish suitable to be poured later.

Steve Voigt is doing an enormous amount of work on this subject, and he’s got lots of information on his blog that’s much more precise about actual cooking and details on the steps, and I’d direct you to him as he’s done a lot more of it than I have.

He also has an excellent recent video trio on making copal varnish, which is probably the most desirable varnish for woodworkers, and would’ve been on instruments, in my opinion, if adherence to tradition were less important than results.

There’s enough there for you to do what he shows, though semi-fossil copal is expensive and it’s not a beginner’s first step. It’s also potentially dangerous to make varnish, and if you introduce open flame like a propane burner or a fire, you risk very serious injury. You may hear rumors of bad burns from candy making with sugar at 350F, and so on. Varnish resins often run around 600F or higher, and the oil can be in the same range for preparation – there is no doubt that in larger quantities, people have died making varnish, and anyone who does a lot of it has some scars. Lucky for me, I’ve had two little half-bb sized spots of hot resin spit out on my arm, but even that was enough for blisters and two little purple scars on my forearm. Just keep danger in mind, skin graft type burns aren’t out of the question.

Terminology for Oil

Washing – literally shaking oil in water, vinegar and water, water and salt or all types of things. Washing oil is a matter of separating things in the oil (like antioxidants) that will prevent bonding of the components in longer polymer chains to themselves, or in bonding resins and oils in a later step to make longer polymer chains.

Breaking – cooking or perhaps some other treatment I’m not aware of (chemical?) to separate parts of the oil from each other and destroy unwanted proteins.

Blown – oil that’s been introduced to oxygen while preparing it – like blowing air through the oil while heating. Obviously, drying oils dry with exposure to air. Treating the oil with air can initiate the drying process, but this isn’t like epoxy or two part finishes – raw linseed oil can be blown or manipulated and then the treatment stopped and the oil will just dry faster later, but it won’t finish drying on its own.

Terminology for Resin

Rosin – this is cooked pine tree resin. Pine rosin quickly cooked into a varnish isn’t particularly good, but violin makers used a lot of pine rosin based stuff. It can be aged or oxidized or treated to make it better. But it’s the same rosin you think of with baseball pitchers or as a block for violin bows.

Fossilized – resins that have been around so long they have fully transitioned to being a fossil. Amber is the only one that I can think of.

Semi-Fossilized – resins that are between sort of “new” and fossilized. Could be hundreds to millions of years old depending on the type. India and Copal resins are of this type. Some Copals are debatable. There is a stark difference between the aged resins and the type that have just fallen on the ground recently. The former makes a harder and better varnish. The latter is far less expensive and I haven’t seen a good reason to use it, but new or unaged resins are popular for incense or “spirit” varnishes (like shellac – just dissolve the resin in a solvent and have an evaporative finish).

Congo/Madagascar – the two semi-fossil Copals that can be purchased and used. Both make a hard varnish that is essentially waterproof and that won’t soften appreciably when exposed to hot temperatures. Madagascar is apparently much younger, but it’s a *very* hard resin.

Run or Ran/Has been Run – resins that have been cooked to get undesirable stuff out of them so that they will bond chemically with oil when cooked together. Running resin can stink so bad, depending on the type, that a rendering plant manager would’ve called the police about the smell. Amber is by far the worst thing I’ve smelled thus far, the fumes are really unhealthy in the first place and shouldn’t be breathed, but the smell will make you wish you had only stuffed your nose with cat poop.

Terms for Cooked Varnish

String – after oil and resin have combined in a cook and have become a “block co-polymer”, the polymer chains will link together allowing the varnish to stretch out in strings if you touch a cooling drop of it and pull your finger away.

Clear – when cooking, if you haven’t gotten oil and resin to bond together and drop a drop of the hot mix on bright metal, it will be cloudy looking as it cools. Once it’s clear, you’re either at string or working toward it.

Pill – a property of varnish such that the oil and resin are thick and not oily, too think to string, and can be rolled into a little pill. Asphaltum in a cooked varnish strings easily and will get to the point that it pills.

Gelling- I’m guessing a little at this, but I’ve had two varnishes gel. I believe this happens when the cook does something to introduce so much air or heat to varnish that it’s partially cured and will no longer flow.

Quick and Light or Cheap Short String– I’m coining this, but it’s described in old texts as an inferior varnish. It’s gotten by mixing components at high heat, getting a string quickly and then removing varnish from the heat rather than cooking it to a better quality. Reasons for this generally have to do with cooking a shorter length of time so that the varnish remains light colored.

Strong String or Long String – it’s not yet clear to me what level of string was typical in the old days. Perhaps a foot or longer. Longer string varnishes mean the varnished produced has longer polymer chains, will be tougher, and probably will be harder. This property also makes the varnishes of the same components feel sticky (very hard to get off of hands without a solvent) and much thicker at the same thinned level.

Long oil or Short Oil – you can vary the amount of oil in a varnish in proportion to the resin. The more oil (long), the more flexible the varnish, but softer and slower drying. The “shorter” or less oil, the faster the varnish will dry, the harder it will be in general, and also, if pushing limits, it will be hard and easy to crack. 1 part resin and 1 part oil would be neither long nor short. Some of the old recipes specify very little oil for cheap furniture varnishes to get a hard bright polish.

I think that should cover enough that I can blog about the varnish that I’ve been making and you can read older sources if you want and there will be overlap so that varnish-related posts don’t just lead to . ………. o O (?????).