What I learned making historic varnishes

(At time of writing) the author recreates varnishes on his balcony. Heating linseed oil and resins to high temperatures is not recommended as there are risks of fire and vapour inhalation.

Words and photos: Shane Orion Wiechnik

In my efforts to become a better finisher I found a barrier to improvement that started with just how unapproachable and confusing some finishes can be. Many are industrially produced in ways most of us have never seen, and there are lots of rumours and anecdotes around finishes that are hard to know when to trust.

‘Varnishes’ never appealed to me originally in the way that shellac, oils, and waxes did, partly because of how easy those other materials are to understand. Varnishes seemed an old fashioned and potentially a dated material. This isn’t true however.

1. A portable electric hotplate at an outside location was used for recreating recipes one and two.

What is a varnish?

While the term ‘varnish’ has lost its prominence, the concept is still relevant and useful. Varnish describes any finish containing a resin. Resin is a pretty broad term. It encompasses almost every solid used in varnish making from shellac, to tree sap, to polyurethane, to wax, to epoxy. If it is solid (or semi-solid), has carbon in it, and can stick to stuff, it’s probably a resin.

If the resin is dissolved in a solvent (like shellac in alcohol) then it is called a spirit varnish, and if the resin is mixed with an oil it’s called an oil/ resin varnish. As examples, Danish and Scandinavian oils are actually better thought of as varnishes than oils; they simply have a lot more oil than resin in them. Even the newer hardwax oils can be thought of as varnishes, with the resin being waxes.

2. Heating linseed oil and sandarac, a refined sap from the cypress Thuja articulata.

Learning from the past

While I understand shellac, the oil-resin varnishes have always intimidated me, particularly the modern ones. So, I decided it was time to try to make my own.

Making furniture has definitely helped me understand furniture better, so the same should apply here, right? I’m not suggesting you make your own varnishes because there are real dangers – my aim was to learn more about them.

Using modern cooking appliances I figured my balcony was the safest place to experiment¹. In the process, I felt a lot of the mystery surrounding varnishes fade away.

3. The oil was heated for three hours before the resin was added.

Recipe 1: Theophilus’ Sticky Varnish

• 2 parts linseed oil (by weight)
• 1 part sandarac (by weight)

The medieval ‘sticky varnish recipe’ recorded by the monk Theophilus Presbyter around 1100AD is one of the simplest available². Let’s take a closer look at our ingredients.

5. Sample coating with the linseed oil and sandarac varnish.

Linseed oil

Here we will first notice a familiar friend, linseed oil. There is so much to say about this wonderfully useful material. It is one of four common drying oils in the world (alongside walnut, poppy, and tung), meaning that it will transform from being a pure liquid to being a solid on its own. Most other oils famously don’t do this, which is why they can be used as lubricants. Imagine how unhelpful it would be if your lubricant became solid: zero helpfulness.

Linseed oil is sold as either raw or boiled. In this article boiled linseed oil refers to what is commercially sold rather than what I have cooked.

The fact that linseed oil naturally hardens and crosslinks into an insoluble solid is something humans have taken advantage of for thousands of years. It really is key to note how important and special this is. Without drying oils, we wouldn’t have the Mona Lisa, or easel painting in general. When we talk about oil painting, that oil was most commonly linseed oil. Mix it with powdered pigments and you get paint. Linseed oil can also be used on surfaces that might get regularly soaked with water, including some furniture and anywhere outside. As such, it is the basis for all of these varnishes.

5. Getting linseed oil up to 270C. The oil was kept at that temperature for three hours.

Sandarac, a tree resin

Without sandarac, this is just an oil finish. Sandarac is a refined sap from the cypress tree (Thuja articulata) which grows in north-west Africa. Sandarac has the properties of being very hard, shiny, and not water soluble, perfect for furniture finish. On its own, linseed oil is a matt waterproof finish which is very good at ‘penetrating’ timber, but it isn’t very hard or scratch resistant. Sandarac can make a spirit varnish but can have poor water penetration resistance and heat resistance. It can also be too brittle, creating cracks or flaking effects. These two ingredients can add a lot to one another when cooked together.

Unfortunately, Theophilus was not the most detailed of chroniclers. The instruction for cooking is a single sentence² which reads, ‘When you have placed it over a fire, heat it carefully, without letting it boil, until a third part has evaporated, and be careful of the flame because it is extremely dangerous and difficult to extinguish if it catches fire’.

I decided to use an electric stovetop to avoid the dangerous flame, but this didn’t tell me what temperature to cook it at, or for how long. For that, I took some tips from the next recipe.

6. Combining the linseed oil and colophony produced a rich yellow-amber coloured varnish.

Recipe 2: Violin Varnish – replication attempt

• 1–3 parts linseed oil (by weight)
• 1 part colophony, also known as pine rosin (by weight)
• Some turpentine if desired

Yes, I know we haven’t finished Recipe 1 yet, but we’ll get back to it. This recipe comes from research done recently about 16th to 18th century varnish making³. I wasn’t the only one who noticed a distinct lack of clear instructional information about temperature and time in many of these recipes.

My sources found that heating is essential for getting the two materials to properly mix with one another. In their tests, if the heat was too low, or the cook time too quick, the colophony and linseed oil would separate again after cooling.

Cooking also increased the viscosity of the liquid, or made it thicker. Having a very thick/not runny varnish can be very desirable. When applying the finish to an irregular or vertical surface, you don’t want the finish to run.

Sometimes, you might test whether a finish is done by dripping a drop onto a cold surface and pulling the drip up with your finger to see how long a strand of caramel-like string you can pull from it – 10cm is a recommended length for some varnishes.

Varnish makers of the past were able to make varnishes with different qualities using similar ingredients by adjusting how much they were heated and how long they were cooked. Importantly, 170C is too low for the ingredients to properly mix, and 343.3C is the temperature at which linseed oil could spontaneously combust. With that in mind, I used the following process for both of the first two recipes.

• Heat linseed oil to 270C and let sit at that temperature for 3 hours.
• Cool to 150C
• Add crushed/powdered resin to hot oil over no more than ten minutes.
• Heat to 250C and cook for minimum of 30 minutes or until desired viscosity is reached.
• Turpentine or gum turpentine can be added to thin, if desired.

The process was surprisingly simple. Using an electric burner and thermometer I could control the heat easily without any danger of an open flame. The linseed oil created a lot of black smoke as it cooked so the outside location and breathing protection were essential.

I aimed for the consistency of a brushing varnish and the results of both recipes were beautiful yellow-amber varnishes that smelled very pleasant. They were thick, and could be applied evenly. They both coated a wooden surface well and built up a semi-glossy finish (recipe 1 had a higher sheen). They both took about two weeks minimum to properly harden, and even longer to fully dry.

Because of this slow drying time, there were no remaining brush strokes on the finish, but they had collected some dust in the process. In order to shorten this drying time, I tested a third recipe.

7. Storing the varnish that resulted from recipe two.

Recipe 3: Oil and Copal: Carriage varnish

• 1 part copal (by weight)
• 2 parts boiled linseed oil (by volume)
• 2 parts turpentine (by volume)

Linseed oil is a molecule consisting of three connected unsaturated fatty acid chains (like an octopus that is missing five tentacles). Unlike other natural oils, it also has a high percentage of some specific fatty acids with carbon double bonds in the chains. You don’t need to know what that means, but it is important. The double bonds will interact with oxygen in the air to branch out and connect with chains from other linseed oil molecules. Imagine our poor three tentacled octopus now has a weird growth coming out of the middle of its tentacles that is binding it permanently with a nearby octopus, and this continues to happen until a whole school of octopi is permanently joined into one large horrific net.

This type of polymerisation is called crosslinking, and it’s a naturally occurring process (for linseed oil, not octopuses) that is very slow, but it can be sped up with the addition of metallic driers, or metallic salts, that help encourage this reaction. This is where we get boiled linseed oil (which is not boiled: never boil linseed oil) as well as lead paint. While Theophilus doesn’t mention using any metal driers in his recipes, they have been common additions from the 15th century onwards.

8. The copal used in the third recipe starts to melt.

Copal, another tree resin

This varnish recipe comes from 1867, specified for use on carriages⁴. Around this time, the resin of choice in Europe and North America had become copal. Copal is technically a tree resin, but rather than referring to the resin of any specific tree, it’s a general term for partially fossilised tree resin. Over a long period of time resins undergo their own natural polymerisation process and fossilise into amber. Copals are resins found part way through this process. One of the most popular copals used in the latter half of the 18th century in North America was actually kauri pine resin from New Zealand.

Because copal has already partially undergone a polymerisation process, it is a lot harder and more durable than softer resins. It can be polished to a high shine and is even used in jewellery on its own. This also means it doesn’t melt as easily in the warm oil, and so it needs to be melted separately first, and then added to the hot linseed oil.

For this process. I did as follows:

• Cook copal resin on high temperature until melted and then reduce temperature to stabilise.
• Heat boiled linseed oil to 270°.
• Lower the temperature of the linseed oil and add the copal resin.
• Cook at 240–270° until thicker than desired consistency (3 hours).
• Lower temperature to 170° and add turpentine. When done, the varnish was as thick as honey and a deep red/green colour. Once cooled, I diluted it 1:4 in gum turpentine to make it brushable.

There were a lot of impurities in the copal, so filtering was necessary. The varnish brushed evenly and levelled out nicely on the surface. It was touch dry and ready to sand and recoat in the morning the following day.

After a second diluted coat, it was already building up on the surface and providing a nice durable shine that I couldn’t scratch away with my fingernails. My copal and oil had combined nicely to create a strong glossy finish which added richness to the timber.

9. Adding the molten copal to the hot linseed oil.

Chef’s notes

The making of varnish changed and advanced with a combination of new technologies and wartime limitation of supplies over the 20th century. Tung oil became more available and was favoured in a lot of recipes. Processes were developed to artificially make normally non-drying vegetable oils harden, replacing linseed and tung in making cheaper paints and varnishes. Modified natural resins like alkyds were developed as well as synthetic resins like polyurethane and acrylics.

In addition to drying agents, other additives were included as well. UV inhibitors for instance help protect the product from degrading or discolouring in sunlight. All of these processes became more exact, more tested and more refined, but fundamentally they remain quite similar in spirit to their earlier counterparts. Having made my own, I feel a lot more confident in my understanding of all finishes. I can think of them in terms of their materials and making. Does it have an oil and what kind of oil?

10. The next day the resulting copal varnish was brushed on to a sample board.

What resin is in it and what properties will that give it? Is there a higher ratio of oil to resin (making it a long-oil varnish like Danish oil and hardwax oil) or is it more resinous (making it a short-oil varnish like yacht varnish)? I believe firmly that a better understanding of our materials helps us refine our techniques and improve our results. Not only are the finishes I made going to go to great use on many projects of mine, but making them has undoubtedly made me a better finisher.

Notes

1. Note well: Linseed oil is combustible and heating it is dangerous. The smoke is also hazardous. We do not recommend making your own varnishes.

2. Unfortunately, Theophilus was not the most detailed of chroniclers. The instruction for cooking is a single sentence, see: Theophilis Presbyter, De diversis artibus or Schedula diversarum artium (ca. 1125). The translation used is Dodwell, C. R. The Various Arts. De Diversis Artibus. Oxford: Clarendon Press, 1961.

3. Source 1: Weththimuni, Maduka L.; Canevari, Claudio, et al (2016), Experimental Characterization of Oil-Colophony Varnishes: A Preliminary Study. International Journal of Conservation Science. 2016 SpeciaI Issue 2, p813-826, 14p. Source 2: Tirat, Sophie & Echard, Jean- Philippe & Lattuati-Derieux, Agnès & Le Huerou, Jean-Yves & Serfaty, Stéphane (2017). Reconstructing historical recipes of linseed oil/colophony varnishes: Influence of preparation processes on application properties. Journal of Cultural Heritage. 10.1016/j.culher.2017.08.001.

4. Bottler, M. and Sabin, A.H. (1912) German Varnish-making. New York: J. Wiley & Sons

First published in Australian Wood Review, issue 110, March 2021

Shane Orion Wiechnik @shane_orion_wiechnik is a Sydney based furniture restorer/ conservator. He studied at West Dean College in England and works with International Conservation Services and Renaissance Conservation. He is dedicated to preservation of both historic objects and the skills used to make them.