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Varnish making - questions for the chemists


Jacob

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Jacob,

The time and temperature necessary to reach the pill stage are a function of the preparation of the materials.  The length of the pill affects the other factors.

Joe

 

Thanks Joe, that makes a lot of things a lot clearer for me.

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This is a great thread,

I am currently washing some Kremer raw Swedish oil  re the links Jacob provided. I never had problems with Kremer Swedish cold pressed... but for what it's worth I think that modern optimised linseed  oil is very more aggressive and optimized compared to what the old guys used....ie to varnish a boat rather than a fiddle... Personally I get more interesting results with Walnut  and feel that the linseed that the classics used was much less aggressive than what has been optimized for industrial use these days.

 

Melvin, my take on things is that modern refined/processed oils don't have fatty acids removed to the same extent that the hand-washed method does - in other words, they are not "clean" enough for making good varnish. The articles state that therefore modern processed linseed oil is essentially a different product to what would have been used by painters (and instrument varnish makers?) before 1800. Linseed oil contains much more linolenic acid than walnut oil (more that three times as much percentage-wise) and from what I can gather this acid is responsible for all the "faults" which can appear in a varnish. The articles also claim that linseed oil was the most popular drying oil because it was the best for using in (painting) varnish if processed the way it would have been done then.

 

Your comments about modern linseed oil are in line with what the two articles state, with the exception that whereas you postulate that modern linseed oil is processed too much, in fact it is too little - too much of the fatty acids remain in the oil.

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So what is the final answer to the original first question?...

 

" if one does not cook the oil and resin at a temperature higher than, say, 240deg C, and does not continue up to the firm pill stage, will the varnish fail due to the resin and oil not bonding properly?"

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So what is the final answer to the original first question?...

 

" if one does not cook the oil and resin at a temperature higher than, say, 240deg C, and does not continue up to the firm pill stage, will the varnish fail due to the resin and oil not bonding properly?"

No, IMHO.

In my experience the only "trick" to getting the Marciana/Geary Baese-type varnish to work properly is to pre-cook the oil.

Others may have different experience/viewpoints of course, but this has always worked for me.

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If I can expand on Ernie's question: how would one know if the oil and resin did NOT bond properly? Would that manifest itself in the fluid varnish (resin precipation upon cooling), or in the dried varnish film (for instance, crackling to any degree after some time)?

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Here's a summary of the scattered information at my disposal so far - please add to, correct, or comment upon the various points:

 

1. The assumption is that "we" are talking about a pine resin/linseed oil varnish. For instance, when I made a pine resin/walnut (raw, unwashed) varnish exactly as I do with linseed oil (firm pill and all that), I found that the varnish remains tacky just about for ever. Perhaps the results would have been better if I had washed the walnut oil.

 

2. The qualities of the varnish depends primarily upon the quallity and treatment of the oil. I still can't comment on this, but according to Spurgeon (the links provided in this thread) a hand-washed oil will dry quicker and will need less resin. The oil doesn't seem to mind what the resin is or how it was prepared: just melted, or slow-cooked since Noah was in junior school.

 

3. I have had crackling in varnish when the "firm pill" was TOO firm ("over-cooked"), and there was a noticable precipitation after gum spirits was added while the varnish was still hot.

 

4. In my experience, ALL commercially-refined and -processed linseed oils produce the dreaded "worms" or "seeding".

 

5. Washing the oil removes fatty acids as well as mucilage (Spurgeon), which imparts superior drying qualities to the oil. Commercially-refined oils may still contain too much fatty acids as well as other undesirable added elements.

 

6. Pre-heating the oil involves oxidation (and perhaps polymerization?), which should facilitate drying, BUT DOES IT AFFECT THE BONDING WITH THE RESIN? Therefore, a hand-washed oil, which apparently dries very quickly, will dry even quicker. Applying varnish onto an instrument requires some "open time". So, to me that sounds like pre-heating a hand-washed oil is not only superfluous but counter-productive.

 

7. Insufficient bonding of the resin and oil is not apparent in the finished product, but MAY (??) have noticable effects over a period of time. On the other hand, the addition of gum spirits to an over-cooked varnish may cause precipitation of the resin - ???

 

8. We have available testimony from Roger Hargrave, Melvin and Neil Ertz (amongst others) who don't seem too hung up on cooking temperature and the "firm pill" issue, nor do they add gum spirits to the varnish while still hot. There does not seem to be a need for multiple coats or the use of a thinner in several instances. This testimony is based on roughly three decades of experience.

 

Over.

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Your comments about modern linseed oil are in line with what the two articles state, with the exception that whereas you postulate that modern linseed oil is processed too much, in fact it is too little - too much of the fatty acids remain in the oil.

 

Hi Jacob, I don't disagree with anything you have said so far. I did not express myself very well when I talked about modern oil being optimised. What I meant was not the processing of the oil itself but the breeding and cultivation of varieties of linseed plants to give higher yields in ways desirable for modern industrial uses.....so just as some modern wheat varieties or strawberry varieties are quite different from what was available 3/400 years ago so it might be with linseed? (That is not my idea and I might well have read it on a link to Spurgeon or other painterly scholars posted on MN but it does make some sense to me)

 

In reply to point 1 of your post Number 32 I remember Koen Padding saying that because Walnut oil is a slower dryer it is more suited to being applied in thin layers as a varnish. Where I have been hasty in applying Varnish made with Walnut on test strips it does wrinkle and crack in some quite pleasing ways. I'm not having problems with the unwashed Kremer walnut I used so far not drying though.

 

Thanks for bringing up this whole subject. What I am seeing while washing the oil is most intriguing and reading about it in the art hand books I have at hand is interesting too. I don't have any experience with pressing linseed but I do have some vicarious experience of traditional olive cold pressing processes in Italy versus modern cold press ones using a centrifuge. The modern one gives a much cleaner sample and the more traditional method does give an oil sample that a painter might feel requires washing.

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6. Pre-heating the oil involves oxidation (and perhaps polymerization?), which should facilitate drying, BUT DOES IT AFFECT THE BONDING WITH THE RESIN? Therefore, a hand-washed oil, which apparently dries very quickly, will dry even quicker. Applying varnish onto an instrument requires some "open time". So, to me that sounds like pre-heating a hand-washed oil is not only superfluous but counter-productive.

 

But heat bodied oil isn't any stickier than raw oil (I just compared them). Perhaps using heat bodied oil may result in a thicker varnish that requires more solvents to thin to brushing consistency, and solvent choice certainly has an effect on application ease. Whether heat bodied oil is necessary for a good varnish I don't know. I've always heat bodied my oil but have wondered if it was really necessary because the oil gets heated later while cooking the actual varnish.

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 What I meant was not the processing of the oil itself but the breeding and cultivation of varieties of linseed plants to give higher yields in ways desirable for modern industrial uses.....so just as some modern wheat varieties or strawberry varieties are quite different from what was available 3/400 years ago so it might be with linseed? (That is not my idea and I might well have read it on a link to Spurgeon or other painterly scholars posted on MN but it does make some sense to me)

 

It's optimized to be used for animal feed, i.e. less linolenic acid. I've got somewhere a paper on this and the fix.

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But heat bodied oil isn't any stickier than raw oil (I just compared them). Perhaps using heat bodied oil may result in a thicker varnish that requires more solvents to thin to brushing consistency, and solvent choice certainly has an effect on application ease. Whether heat bodied oil is necessary for a good varnish I don't know. I've always heat bodied my oil but have wondered if it was really necessary because the oil gets heated later while cooking the actual varnish.

 

True.

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It's optimized to be used for animal feed, i.e. less linolenic acid. I've got somewhere a paper on this and the fix.

 

According to Sturgeon the removal of fatty acids facilitates its use as a varnishing medium. Linoleic acid seems to be the one causing the undesirable side-effects (darkening, wrinkling, etc), so linseed oil "optimized" to have less of that should be good - according to Sturgeon.

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Hi Jacob, I don't disagree with anything you have said so far. I did not express myself very well when I talked about modern oil being optimised. What I meant was not the processing of the oil itself but the breeding and cultivation of varieties of linseed plants to give higher yields in ways desirable for modern industrial uses.....so just as some modern wheat varieties or strawberry varieties are quite different from what was available 3/400 years ago so it might be with linseed? (That is not my idea and I might well have read it on a link to Spurgeon or other painterly scholars posted on MN but it does make some sense to me)

 

 

Melvin, for whatever it's worth - I obviously can't vouch for it -  Sturgeon states that "Organic, cold-pressed, unrefined linseed oil is now widely available for the first time ever because of

its positive role in human nutrition."

 

He goes on to say what kinds of cold-pressed "straight", unadulterated oils are available. I imagine that what Kremer sells for the purpose of varnish making could very well be from crops not grown primarily as cattle feed, and may well approximate the kind of "natural" product available in earlier times.

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The Kremer linseed oil I have is 'low acid', at least that's what it says on the bottle. Is that what they mean less linolenic acid?

 

The one you refer to is commercially-refined oil, and it gave me a lot of grief. Perhaps others are more lucky. But the more I re-re-re-read recipes and instructions, the more I'm struck by the injunction to use "cold-pressed" oil, and next to that, "washed".

 

The oil from Kremer's which I have targetted for experimentation is  #73020 (cold-pressed from Sweden, filtered). That one is specifically recommended by Sturgeon as organic and unprocessed.

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I imagine that what Kremer sells for the purpose of varnish making could very well be from crops not grown primarily as cattle feed, and may well approximate the kind of "natural" product available in earlier times.

 

I don't think flax is grown specifically for feed. My understanding is that it's either grown for oil or for fiber with the by-product of the oil pressings being used for feed.

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This doesn't answer any questions, but there are two basic types of flax. The original, derived from wild flax, was probably what we know as linen flax, fairly tall and grown mainly for fiber. The seeds were a byproduct used for food and art products until the paint industry became important. Then seed flax was developed with much shorter stems and adapted to mechanical harvesting. The fiber of seed flax is a byproduct, usually not used but sometimes for papermaking. Modern linseed oil mostly comes from seed flax, but it is likely that in Strad's time it came from linen flax. There could very well be differences.

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 I think one should think of the resin and oil as independent parts of the varnish. If you are using a pine resin where I think it starts to decompose at around 470oF, the oil should be heat bodied before adding to the resin. If you are using a fossil copal that can withstand hi temps the oil can be added after the copal has been cracked (depolymerized) and run up to temps of 600-625 to body the oil, which is getting the internal structures of the oil molecule to form a film. Linseed oil starts to react to form a film at around 525oF, and is a lot quicker, like 10-15 minutes with copal when kept at 600 or higher. These temp's are recognizable when making a copal varnish by the amount of foam on the surface, and its disappearance, indicating you better hurry and get some turp in there before it gels on you. I have to leave and just adding these comments in case this thread is removed.  fred

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Yes walnut oil is nice to use and I use it for Montagnana copies. Not for any scientific reasons; I mainly do it because the Venetian painters used walnut oil and I like the effect. However, it takes a little longer to dry. Now, drying! You asked about pressing it yourself. This is possible and I have tried it, BUT it is a bit of a messy job and not really worth it. The big problem is finding a suitable press. If you use a press that has been used for pressing none drying oils such as olive oil your drying oils will never dry. This is a well know and well documented problem. It is also the reason why you should never use oil meant for human consumption. These are often pressed in presses used for other non drying oils. 

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I just want to mention why the need to use heat of 600oF mentioned in my earlier post, and also apologize for not mentioning the caution needed when heating inflammables to hi temp's. I've made fossil copal varnish hundreds of times and that is probably the reason for indifferently mentioning such a high cook. I never had a fire; failures, plenty when learning. I should also mention a varnish run consists of only about 10gms of resin and a little more linseed oil by weight, fluid contents are added with a medicine dropper. The container with resin and oil sits in a short can with sand to control heat entrance, and also in case there is leakage. Covering this setup is another upside down can closely fitted with a hole on top for a temp probe, adding stuff and use of a stirrer. This outer can heats the walls of the resin container and keeps the resin from crawling up the sides, leaving the bottom of the container, a problem that anyone using fossil copal knows this problem and the difficulty in cracking this resin when this occurs. It is necessary to perform these steps at hi temp's to shorten cooking time because of resin loss while cracking the resin. The typical loss is around 25%, and significantly higher if heating is prolonged. After the oil is added, cooking it to what is referred to as the "String Stage" where a drop on glass is touched and pulled to get a string.In this method it is the reduction in foam that tells you it is time to add turpentine. Turp is added to about 2-3 times the volume of resin/oil, and this can be a cautious phase. What I believe happens when adding turp (with a medicine dropper), the resin/oil is the SOLVENT and the turp is "dissolved" in it, and it is the SOLUTE at this point. (think of sugar and water- water is the solvent and sugar is the solute) As more turp is added it reverses and becomes the solvent and the res/oil the solute, and with its large amount of latent heat added to the turp it causes a sudden boil for a short while. The rest of the turp is added with no problems. It will get your attention pretty quick the first few times this happens. Waiting before adding turp solves this, but I try to keep the process time from start to filtering at around an hour. If you ever make this varnish, high temperatures are needed. fred

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Hi Fred,

 

That is interesting, and scary.

 

However, I don't want to go there. My particular interest is: pine resin and oil. From that, what temperatures, and turp or no turp?

 

I more or less take it as a given that the classical Italian makers did not have "gum spirits of turpentine" at their disposal, so inclusion of this substance in a recipe to me is a modern variant. Is this good or bad?

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I think it was said several times that spirit of turpentine is very old and that Cremonese varnishers or makers of 17th and 18th century could have access to it. Whether they used it or not of course is another matter.

Oil of Rosemary which is quite a good solvent of resins but evaporates more slowly than turps was quite popular at the time in making colored auripetrum type varnishes. It can be substituted for turpentine and lengthens the brushing time but it does require proper summer sunlight to dry.

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the resin/oil is the SOLVENT and the turp is "dissolved" in it, and it is the SOLUTE at this point. (think of sugar and water- water is the solvent and sugar is the solute) As more turp is added it reverses and becomes the solvent and the res/oil the solute,

Could this be because before the turps is added,  there is a high correlation of molecules over a large distance ?    It is not a true solution.  As turps is added,  the long-range forces diminish and resin/oil complexes don't  "see"  nearest neighbors so well.  One is dealing with colloids here,  it seems.

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