Monday, July 31, 2023

Making Copal Varnish


Note: Last year, I wrote a post that included instructions for making natural resin varnishes. The feedback I got from several people was that these varnishes were somewhat frustrating to make. And I've come to realize that they weren't ideal for furniture varnish, which is what most people are likely to be interested in. So, I decided to remove that information, and try again. I don't like the idea of deleting old posts, but I don't want substandard information floating around on my blog that's likely to cause people frustration…so let's have a do-over.

In the 18th and 19th centuries, oil varnishes were extremely popular, and the most popular type, by far, was Congo copal. "Copal" is a somewhat imprecise term that just refers to tree resin used in varnishes. Some copals come from living trees, but Congo copal is a partially fossilized resin (similar to amber, but amber is much older). When cooked correctly, it makes a varnish that is hard, water resistant, and beautiful. 

I got interested in making natural resin varnishes a few years. I liked the idea of making a finish that was made from plants and trees rather than from petrochemicals, and I like having control over the finish and knowing exactly what's in it. I learned to cook these varnishes, and I wrote an article about them for Mortise & Tenon Magazine. 

Learning to make a Congo copal varnish was difficult, because no one else was doing it; I had to figure it out for myself. I had help from a lot of dusty old books from the 19th century, but they dealt with batches of 100 pounds of resin, cooked over coal fires, so the methods described didn't always translate well. But I got there eventually, and I'd like to make it easier for anyone else who wants to try it. If you're into it, the instructions and videos below should help you out.

There are two things that need to be understood before trying to make varnish. First, it’s dangerous. High temperatures are required, the materials are flammable, and the fumes can be hazardous. Never cook inside, and never cook over an open flame. I recommend setting up an electric hotplate on sawhorses, at a safe distance from the house, the shop, the car, or the cat. Wear long sleeves, gloves, safety glasses, and a mask or respirator. Have a fire extinguisher close by. Above all, think about what could go wrong, and plan accordingly.

Second, remember that cooking varnish is like baking bread: Sticking the loaf in the oven is the easy part. Success or failure is determined by all the preparatory steps like feeding the starter, kneading, and rising the dough. Varnish is similar: Preparing the oil and resin is most of the ballgame; if you do those steps correctly, the rest is a piece of cake. Or bread.

If you're interested in trying this, you'll need some equipment:

  • Two hot plates. I use a 1500 watt hot plate to run the copal (you need some power to get the stuff up to 625-650 F), and a smaller, cheaper hot plate to heat the oil. 
  • Cooking pots. My favorite pots are enameled cast iron--they conduct heat well and clean easily. You don't need Le Creuset; cheap used pots can be found on Ebay for $20-$30. Stainless steel also works well. I used to use very sturdy Corning Visionware pots (also found cheaply on Ebay), but they can eventually break, so I reserve them for light duty tasks now.
  • A thermometer or two. The cheapest solution is a $10 analog candy thermometer. Make sure you get one that goes up to 550° F and has room for the needle to go past that, so you can estimate higher temperatures.  IR thermometers are handy, but they fail completely  when there's a lot of foam or smoke (I talk about this in the video). 


Here's my trusty $10 candy thermometer. Ignore the goofy setup--it's for making ester gum, which is a whole other ball of wax.

Then, you'll need some stuff to cook:

  • Copal resin. I recommend you use Congo Copal from Wood Finishing Enterprises. Get at least a pound and don't cook less than half a pound at a time.
  • Linseed oil. See my earlier post on this subject. You can follow the procedure I describe for washing and heating the oil, or you can use alkali refined oil. If you use the latter, apply the same heating procedure as for the washed oil: If you don't, the varnish will not combine properly.
  • Solvent. I recommend good quality turpentine--don't use the stuff from the hardware store. The best turpentine is from Diamond G, but it's expensive. My friend David Weaver recently tried turpentine from DIY Chemicals and liked it. You can also use mineral spirits, but be careful adding it--it is more likely to throw the resin out of solution  than mineral spirits.
  • Driers. Japan drier from the hardware store will work fine. I recommend 1-2 teaspoons.

In the videos below, I'm using half a pound of resin, half a pound of oil, and a pound of solvent. Oil to resin ratios vary from 1:1, like I'm using here, to 3:1, 4:1, or higher. If you want a glossy varnish that can be built up to a thick coat, use a 1:1 ratio. If you want more of a satin finish and a thin coat, use a 2:1 ratio.

A note on adding the solvents and driers, since that part was skipped in the video. I like to mix the driers into my solvent and add them together. The boiling point of turpentine is 310°F, so don't add the solvent when the varnish much above that--you don't want the turpentine boiling. If does, put the cover on the pot to deprive it of oxygen, so that an exothermic reaction doesn't start.

Ready? Watch the videos below before you try making any varnish. Questions? Leave a comment below. Experiment, have fun, and be safe.