Tuesday, September 20, 2022

Interview with Joe Robson, Part 2

Joe Robson's logo--an alchemical formula for varnish.


In part 1 of my interview with Joe, we discussed general issues of varnish and other finishes. Here, we focus in a more granular way on the three components of traditional varnish: Oil, resin, and solvent.


SV: Linseed oil, whether raw or boiled, has gotten a bad rap, especially from the finishing gurus who write for magazines like Fine Woodworking and Popular Woodworking. I recall reading some less than flattering reviews of Tried and True.

JR: That's when I stopped buying Fine Woodworking!

SV: Obviously you disagree, and I'll note that for 19th century writers, linseed oil was the most desirable of the natural drying oils. In your opinion, what did the modern gurus get wrong? Are their testing methods flawed? Are they using inappropriate criteria? Or are they perhaps unwitting shills for the petrochemical finishing industry?

JR: Experience has a lot to do with it. A lot of the people who wrote for those magazines–we won't name names–are former engineers for 3M. I think that they tend to believe what's written on the can. So what's boiled linseed oil is boiled linseed oil, what's raw linseed oil is raw linseed oil, neither of which are particularly good products. The same goes for the tung oil products. I think the prejudice comes from being too scientific, and not studied in the historical aspects of the work. And I think that it comes also from not really being good woodworkers. Because we've all been at the stage where putting a coat of oil on it and getting it out the door was the way to make a living. And it still may be true in a lot of cases, but that's not good finishing. And good finishing comes from understanding the materials and how to apply them and using the best materials you can find. And when you're buying things off the shelf, you're generally not getting the best materials you can get your hands on. So linseed oil, since at least the 12th century, has absolutely been the finish of choice. It wasn't like everybody was happy with a poor finish. It works, but it only works if you're using the right materials in the right way. And again, if you're not a passionate woodworker, you're probably not going to go through what it takes to find out what's good and what you like and what your customers like and the criteria that are beyond the science guys.

SV: Does all oil need to be washed for varnish making?

JR: Yes.

SV: Does it matter what type of water is used?

JR: H2O works.

SV: So does one need to use spring water or distilled water?

JR: I don’t think so. Do you know the easiest way to wash oil?

SV: I probably don't!

JR: Take an empty plastic bottle. Pour your oil in it halfway up, fill the other half with water, shake the hell out of it. Put it in the freezer. When the water is frozen and all the crap is in the water, cut the bottle in half. Throw that ice away. Take the oil, do it again. At a certain point you will have water that doesn't have a lot of little things floating in it and it looks clear. There are other ways to do it but that's the goal. In my experimental days, that's what I was doing. Just having washed the oil doesn't make it good for varnish making but it’s a start.

SV: Is washing with salt or sand ever necessary?

JR: I've read the recipes. I've never used them.

SV: Let’s talk about the “break.” In my own experiments, I took cold pressed oil that had been washed several times, and heated it to around 225° C, but I never observed the “flocculent cloud” that is supposed to appear and then dissipate, indicating that the break has been removed.

JR: You know how fast break happens?

SV: I don't.

JR: (claps his hands together)

SV: Okay, so I probably just wasn't paying attention.

JR: Yeah, you just didn't get to know what to look for. The break is just removing protein that's bound to the oil molecules. There's a point in the cook, where you’ll start to get very large bubbles. And they're kind of hazy when they come off. And they come off fast. And they come off all at once. And then the break is done. What temperature that happens at varies significantly, according to how much was done to the oil previously, how well it's washed, where it came from. So there's no textbook on break that I've ever seen. But yeah, the oil has to reach break to make it usable for varnish. And it's fast. I mean, even if you’re watching it, you'll miss it most of the time.

SV: Is it safe to assume that if you heat the oil to 225° C, you've eliminated the break?

JR: My thresholds are 250° to 275°, depending on what the oil is. When I prep oil after it's washed, I know that I've got to hold it at 250° for half an hour to ensure the break. That works for my oil for the kind of varnishes I'm making.

SV: Can you say anything about how the linseed oil in Tried and True is treated?

JR: Yes, it's a cooked oil, but it's also an oxidized oil. So it's a process of both cooking and introducing oxygen, bubbling it at particular temperatures for particular periods of time to make the oil polymerize correctly.

SV: I see. So it’s a blown oil?

JR: It’s a semi-blown oil because it's a balance between how much it's heated and how long it's been oxygenated.

SV: Is it heated and oxidized at the same time?

JR: For part of the process. There’s also some heat without oxidization, and there's some oxidation without heat.

SV: Where did you learn this particular combination of oxidizing and heat treating? Was it in old books?

JR: Some of it was in old books, but I would say 80% of it was experiments. For years I had rows of glass plates all over the shop with XYZ formulas. This one didn't work, throw it away. Basically you throw away everything that doesn't work, until finally one of them works.

SV: This is very interesting to me, because I've been experimenting with blowing oil. I got a mini-aquarium pump and I’m heating the oil to between 130° and 150 ° C and bubbling it at the same time. Can such an oil be used for varnish?

JR: Oh, sure. Absolutely.

SV: So there aren't any negative consequences?

JR: Well, I assume it was washed before it was blown?

SV: Yeah.

JR: Yeah, then you're fine. It’s a good way to vary the viscosity in the varnish; you can maintain the same oil to resin ratio, and change the viscosity of the product by the degree to which you've blown the oil. Raw linseed oil runs around seven pounds to the gallon. if you run it up to about 7.5, it's kind of like maple syrup in thickness. I've brought it up to about eight and a quarter, and it's like dark honey, like clover honey. All of which again, changes both the viscosity and the drying profile of the varnish. Because the more you blow the oil, the faster it'll dry.

SV: Right, that’s the main reason I’ve been experimenting with it, to speed up the drying time. I blew some for about 16 hours, And my impression is that I need to blow it for a lot longer.

JR: You just have to check the weight when it's going in, and then you'll know where you are.

SV: Should the oil be blown first and then heated to break, or vice versa?

JR: Yes, blow it first, then heat it to break.


SV: Like linseed oil, colophony has gotten a bad rap; nearly all the old texts I’ve read consider it to be an inferior resin. But as you mentioned, it was the resin for the Golden Age violins of Stradivari, which lots of people have devoted their whole lives to duplicating. And obviously, you and many others use colophony and raw pine resin. What account counts for this discrepancy? Is it an inferior resin?

JR: Colophony has a very low melting point and a very low softening point. So for varnish making in the sense of durability, it's not a good resin. You can get good durable varnishes out of it, but they take a lot more work. The industrial standard had to do with durability, not with whether it made a good varnish or not. I think that there was a prejudice because of price. It was cheap and easy to use, right? It didn’t challenge the varnish makers at the time. So varnish makers could make cheap and easy varnish out of it. They could sell it cheap and easy. And they did. So I think it developed a bad reputation because of that. Not because the resin itself was bad, but because they were making cheap and easy varnishes with it.

SV: It also has a reputation for darkening very quickly.

JR: That's not the fault of the resin. It's the fault of the varnish maker.

SV: How so?

JR: Overcooked rosin, overcooked oil, use of iron in the process, just poor skills and poor craft. How much preparation was done on the resin? How much preparation was done on the oil? It's not the fault of the resin. I have samples of varnish that I made 30 years ago that are as clear and crisp as the day they were made.

SV: Can you talk a little bit about how you cook colophony?

JR: Colophony has a certain percentage of water in it. And you have to get rid of the water before you can do anything predictable with it. So the first cook is always to get it up to the boiling point of water and hold it there until all the big bubbles go away. And when the big bubbles go away, the water is gone.

SV: So there are multiple cooks.

JR: Yeah, depending on what I'm doing with the resin. I cook it anywhere from 10 to 500 hours.

SV: So you’re talking about pre-cooking for color.

JR: For color or other properties. Color being a big one in the violin world but yeah, I mean, it's just there's nothing there's nothing fast or, or predictable about the learning curve for making varnish. I cook the raw resin until it gets to a certain point. And then I can pump the heat up or not pump the heat up, depending on what I want to do with the resin. And violin varnishes I always cook in cast iron, because that's the way it was done. And I cook at temperatures that are predictable within the trade of the time. So, the business of scientific analysis of varnish making is interesting, but it really doesn't apply as far as I'm concerned. You know, the fact that somebody makes a varnish that they took the resin to a certain point and held it for a certain number of hours, in certain atmospheric conditions is interesting, but that's not the way it was done. And so, I try to put myself in the mindset of the people who were doing it and say what's possible, what's probable and somewhere between the possible and the probable, you can come up with a methodology, temperature wise, duration wise, and fit into what you know.

SV: I gather there is some controversy amongst violin makers about the appropriate temperature for cooking rosin for color. Can you say a little more about the temperatures you cook at?

JR: My approach to rosin cooking is low and slow. As I mentioned, the first cook is at 100°C to eliminate water. Then I raise the temperature gradually to 150°C and cook there until the desired color shows.

SV: Is liming (adding calcium hydroxide) necessary?

JR: Depends on what you want. You lime resin because the softening point of the colophony or pine resin is too low. And if you lime it, you lower the pH, which raises the hardness and the melting and softening points. If the resin you're using melts on the table when it's in direct sunlight, your varnish is going to melt when it's in direct sunlight. So yeah, I would think for durability purposes, it's absolutely necessary. I have long ago stopped arguing the point to my colleagues, because obviously, there are other opinions, and it's not universally accepted.

SV: You mentioned that you cook colophony for up to 500 hours for color. Do you lime it first, or lime it after cooking for color?

JR: Yes (chuckles). For some reasons you might lime it first, for other reasons you lime it after. If you know your desired outcome, you can adjust your material towards that outcome.

SV: Is there any particular reason you would lime first or line second?

JR: Yeah, it has to do with the textural qualities of the varnish, how much you want the varnish to self level or not self level. Okay, that's not the only factor. I mean, once you've made that decision, you make decisions about the oil, you make decisions about the turpentine. But that is one factor in that process.

SV: Still on the subject of liming, some varnish makers claim that using calcium hydroxide is inferior, and they recommend zinc oxide. Do you agree?

JR: You have differing results with different materials. I like lime because it keeps the varnish closer to the color that you get from the oil and the resin than anything else I've tried. And it was the tradition. I'm not trying to reinvent something; I'm trying to reproduce something. So all of those things, all of the alternatives work. Doesn't make them bad or good, it's what you want.

SV: In the 18th century, there was a gradual move away from ancient resins such as pine, spruce, and sandarac, and instead copal became the most popular resin. Do you have any thoughts on how and why this shift happened?

JR: Yeah, it made a beautiful varnish. The copals that were coming out of Central Africa in those days were amazing. They were predictable in the cook. The outcomes were controllable. You could make varnish for patent leather shoes, or horse carriages, or hats, or what have you. And people liked them, people paid for them. There was a period of time in the early 1700s that it was like the cocaine of the day. If you had those materials, you were the man. The Vernis Martin is great varnish. It's not particularly difficult to make. But it's probably more mythologized than even the Stradivari varnish.

SV: So you’re a big fan of copal.

JR: For durability, and beauty, there's nothing better. When the copals came into the varnish making world, they were so well received by varnish makers, because they didn't have to cook amber anymore. They could get similar results, not the same, but they could get good results with copal and with a lot less fuss than cooking amber.

SV: You brought up something interesting when you mentioned hats and other things. I think most people think of varnish as something that you put on wood. But in the 18th and 19th centuries, varnish was used on everything.

JR: Yes it was, because it introduced durability to a lot of common objects that were not durable before then, and people loved it. The better you were at making that durability factor, the more successful you were as a varnish maker.

SV: Sandarac is one of the most ancient resins, and recipes are given in a number of old manuscripts. Is a pure Sandarac oil varnish desirable for any application or should it always be blended with other resins?

JR: As long as there's sufficient oil in the varnish, it's good. Sandarac tends to be a bit chippy. So maybe you add a little more oil to that varnish than you would some other varnish.

SV: So greater than a 1:1 oil to resin ratio.

JR: Yes. Koen Padding made violin varnishes for a number of years and his basic varnish was always sandarac. And he was very good at it; he made it very clear. It wasn't a color product at all. It was a very nicely made varnish. But he knew Sandarac; I mean, he had his tricks.

SV: Livache gives a recipe for “furniture varnish” that combines colophony, sandarac, and mastic, but he doesn't give any proportions at all. The version I made was five  parts limed rosin, four parts sandarac, and one part mastic. And I was pretty happy with it, but I don't have a lot to compare it to. Can you suggest any proportions?

JR: What I made was, if I remember correctly,  three parts rosin, 2 parts sandarac, and one half of one part mastic. And the mastic wasn't cooked in. Mastic was added at the end, just as a plasticizer. It's a balancer for the Sandarac, to add a little bit more elasticity to the resin. But that was a very transitional varnish for me. I was learning to make the old varnishes. And that was like a first successful step into the old varnish.

SV: I’ve read that sandarac can be difficult to run, but I found it to be pretty easy to work with.

JR: Well, if you're not cooking sandarac for color, then you can get it to run and obviously you can cook colophony quite easily. So it's a great way to get your feet wet.

SV: When you cook sandarac, what kind of weight loss are we talking about?

JR: I'm cooking to somewhere between 75 and 90% weight loss. Depending on the batch of resin, it usually runs around 80%. It's a beautiful red.

SV: That’s a lot of weight loss–you’re ending up with only a quarter of the resin you started with!

JR: Well, sandarac doesn't reach true oil solubility until you hit that 75 or 80% weight loss. Otherwise you will have stuff in the varnish that does not combine with the oil. And in terms of transparency, that can be a problem. Sometimes that material will filter out and sell to the bottom, but sometimes it'll just remain suspended.

SV: Do you run the sandarac in more than one cook?

JR: Three cooks.

SV: Why three?

JR: That's what works.

SV: How do you decide when to turn it off, cool it, and then cook it again?

JR: I think the resin will tell you when it's done.

SV: How does it tell you?

JR: It won't change any more in the pot. You cook it, you cool it, it looks like this. Okay. Then you put it back in the pot and you cook it and cool it, and it looks like that. And finally you cook it and cool it and it looks like what you want it to look like. But if you go through that whole process, and try to do it all at once, you'll never get to what you want.

SV: Can you say anything about temperature?

JR: Hot, but not amber hot. Copal hot. At lower temperatures in your initial cook, you will see a lot more stuff coming off. You have to raise the temperature higher in the second cook. You're gonna raise the temperature again until you get stuff coming off. And if it's coming off too fast, you're gonna burn it. On the third cook you're not going to see much coming off.

SV: Any thoughts on using amber?

JR: Nothing compares with Amber. It's the standard. It takes a much higher temperature though. Much higher, and there’s a longer learning curve. Once you get it, it's simple like everything else. But it is a long learning curve. And you don't cook amber in small quantities, because you'll burn it.

SV: What does pill, or string length, tell us? You've mentioned that pill can vary from a little bit of stickiness to long strings. How do these differences translate into qualities in the finished varnish? (Note: this question refers to putting a drop of hot oil/resin on a hard surface, letting it cool a bit, then dipping a finger into the mix and pulling it away. If the mixture produces fine filaments, or “strings,” it indicates the resin and oil have properly combined.)

JR: The hardness of the film is a factor, and the application technique is a factor. The shorter the pill, The harder the varnish, and the longer the open time. That's very important in violin varnishing, because you're dealing with this weird surface that has holes in it. If you're varnishing a coffee table, it might not be so important to you. So you might want to cook to a longer pill because it will be a more durable varnish. Given the same preparation of the resin and oil, the durability will go up and the open time will go down.

SV: Is maximum string length correlated to the oil resin ratio?

JR: It's not.

SV: So can a two to one (oil to resin) varnish produce long strings?

JR: Yes.

SV: If a batch isn't producing strings, what are some of the possible reasons?

JR: Probably the oil was not processed correctly, or it wasn't ready to accept the resin or you didn't reach the temperature threshold necessary for that particular resin. If you're using a very raw oil and a very raw resin, they're going to take a long time to get together and produce a pill. If you're using a highly processed resin and a highly processed oil, they are going to take a very short time to come together. It can also be the surface you drop the pill on. Get a piece of marble or Corian.

SV: When making varnish, do you always preheat the oil separately and then add it slowly into the liquefied resin?

JR: Yes and no. Some varnishes want to be cooked together, some varnishes don't want to be cooked together. The varnishes I make are almost always cooked together. Process the materials and heat them up.


Note: it's not mentioned below, but Joe is adamant that Diamond G turpentine is the only brand worth using. See my article in Mortise and Tenon Magazine for more info.

SV: Let’s talk about solvents. Turpentine is the traditional solvent–

JR: The only solvent.

SV: The only solvent?

JR: For varnish it’s the only solvent.

SV: Well okay! So, what's your opinion on when varnish makers started to use turpentine?

JR: The earliest records we have are from the 12th century. But the use of turpentine and the presence of turpentine goes back to the medicinal records of Pliny the Elder. So much older. If you have a varnish that dried to a film, it had turpentine cooked into it. If you make a varnish that didn’t dry to a film, it didn't have turpentine cooked into it.

SV: But I've looked at some of these Renaissance manuscripts and they don't mention turpentine. It's usually just oil and resin.

JR: I think that much has been lost in translation. If you look at the varnishes from those times, they have the qualities of a material that's been made in a certain way, so they exist in a certain way. If you have qualities that are made in another way, they will exist in that way. The materials haven't changed. The way they want to be cooked  together hasn't changed. So, if you have a film from the 12th century, 99 times out of 100, it'll have turpentine cooked into it. If you have a film that doesn't build to a surface, to a polishable surface, it's likely it didn't have turpentine cooked into it.

SV: So you're saying that turpentine is necessary to create a film forming varnish? Why?

JR: Because it's the difference between a mixture and a compound. Oil and resin cooked together make a beautiful mixture, and it will give you a certain type of surface. Can you build a film out of it? If you can, will it have the same characteristics as the film of the same materials with turpentine cooked in? No, it won't. And the catalytic properties of turpentine cooked into the varnish, make it a compound. That’s just that's the way it is; A plus B equals C, A plus B plus C equals something else equals varnish. And we can call it whatever we want. But without turpentine, it doesn't go through the chemical changes.

SV: You've written that “the addition (very carefully!) of turpentine during the varnish cook does more than just dropping the viscosity; the reaction of turpentine with the oil and resin promotes a mutual solubility that cannot happen without the turpentine. You've already said a fair amount about this; anything else to add?

JR: In the violin world there's this subculture of solvent-free varnishes. Where that comes from I don't really know. Violin makers can take a detail and make a religion out of it faster than any other group I've ever encountered. And that's one of those things.
Some say the old varnishes were solvent free. Show me the way the varnish on the old instruments looked, the way it wore, the way it looks on those few instruments that have not been beat up and refinished by shops. I see varnish: resin, oil and turpentine. Oil and resin and turpentine make varnish; oil and resin together don't–they make oil and resin. And excuse me for being a purist. But I studied this for a long time. And it's not my opinion, it's the opinion of decades, centuries of varnish makers. Just because they didn't put it in the recipe, well, they don't tell you to wash your hands when you're done either.
But the proof is always in the observation. The materials have been here forever. And the materials haven't changed a bit, but our ability to manipulate them may have changed a great deal, because we have forgotten the lessons of the past. So I just think it's people believing a story and repeating it until it becomes the truth.

SV: I get the sense that how you cook with turpentine is a pretty closely guarded part of your methodology.

JR: Yes, for three reasons. One, it's dangerous. And two, it's dangerous. Three is that the amount of variability is huge. So, yes, I believe turpentine has to be part of the process. If I told you what I do, and when I do it, you would probably get hurt. And I have been hurt. So I know where the dangers lie. The other thing is that the amount of turpentine, and the temperatures used, vary the quality of the varnish tremendously.

SV: Yeah, I understand. I'm chuckling here because the question I wrote down was, can you suggest some guidelines that will allow me to experiment without blowing myself up? Specifically, do you cook the turpentine above the boiling point of 310°F/160°C?

JR: A little higher, a little lower, depending on what I'm looking for. But right in that neighborhood, right in the danger zone.

SV: I assume you turn the heat off before you add the turpentine?

JR: I take it off the heat.

Final Thoughts

SV: Like you, I work in a craft (planemaking) that nearly disappeared in the 20th century, and I spent a lot of time trying to figure out old methods that were lost to history. So I can relate to varnish making and what you do. It seems to me that we're in the midst of a resurgence of interest in hand tools, and more generally pre-industrial crafts and ways of working and living. Do you agree? Are you optimistic about these things? Do you see a resurgence of interest in real varnish made from trees and plants?

JR: Certainly, in the violin world it’s huge. I don't think the information has really filtered out much beyond the violin world. Primarily because most woodworkers are just working wood, and finishing is really an afterthought for unless you're a finisher. I think the one of the keys to this is that I'm seeing a lot more use of shellac. And I think as people understand shellac, they will begin to understand that there are organic alternatives. And once you get past that, and you begin to understand what you can do with a good finish, then your interest is piqued.

SV: Is there ever going to be a Joe Robson finishing or varnish making book?

JR: I’ve thought about it, been encouraged and discouraged. I would probably sell three copies and then they'd be photocopied on the internet.

SV: You'd sell a lot more than three copies!

JR: Well, yeah. My brother is a historian with connections in the publishing world. So that path is done. Whether I ever actually get around to doing it or not, I have no idea. But my notes, my observations are written down, and there are people who know where it is. So the craft of it won't be lost whether I put it together or not. I've come too far to think that I should just die with this information.

SV: Joe, thanks so much for talking varnish with me, and being so generous with your time.


  1. This is awesome, awesome information. As a chemist and woodworker, these are the kinds of things I want to know and appreciate someone has taken the time to do the experiments.
    Thank you so much for sharing. I think it is worthy of a book (even if it ended up being one of those downloadable Amazon books). If it isn't documented well the knowledge will disappear again though writing it here really helps. Any chance we could get similar discussions on shellac and then tung oil?

    1. Thanks, glad someone found it useful. There will be a book, and not on Amazon…can't say anymore about that yet though.
      There is already a ton of information out there on shellac. I talked a bit about tung oil in my M&T article…my basic take is that tung is somewhat overrated, and became popular only after the quality of commonly available linseed oil had declined. I will probably write more about it at some point; right now I need a break!

    2. Thanks. The quick take on tung oil is helpful.