Varnish making - questions for the chemists

[Jacob]

I feel the need to solicit scientific input on (for me at least) vexing questions regarding the preparation of oil varnish - specifically when the resin is some kind of pine resin, and the oil is linseed oil.

 

1. The first concerns the "firm pill/stringing" test - is this to determine viscosity, or does it indicate when the resin has bonded properly with the oil? In other words, 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?

 

2. The addition of fresh gum turpentine at about 150deg C during the cooling down after cooking: does this serve any purpose other than thinning the varnish? Does it, for instance, enhance the bonding of the resin and oil? I realize fresh gum turpentine has drying properties, but what else apart from that?

 

 

[Melvin Goldsmith]

Hi Jacob.....First apologies because I am no chemist but here's some observations from my own experience.

 

Generally I found from reading artist's manuals where permanence etc is very valued that most varnish/glaze mediums  although generally using other resins than colophony do involve turps, linseed oil and a resin and are prepared cold. If I am making a varnish that has turps  white spirit etc in as a solvent I generally make it cold using raw oil. It seems to make a decent enough varnish for my purposes which involves varnishing fiddles & not boats. If I am making a solvent free varnish I only heat the oil enough to combine it with the resin The resin might have been pre cooked. I've never bothered about stringing or pilling etc....so far no problems from doing it that way over 30 years or so....

[DarylG]

Hi Melvin, do you wash the oil?

[Guest]

I remember Melvin stating this on MN and it has stuck in my mind ever since which is why I asked Roger that same question. From his response it seems that he too only heats the oil hot enough to blend.

Thanks Jacob and Melvin.

[robertdo]

the Marciana recipe for example doesn't ask for any pilling stage. But I was under the impression that cooking long enough to get this 10cm long string was making drying time shorter. 250C seems quite a high temperature.

As for the chemist question, I can't help but it seems to me that unless one knows the exact composition of a resin, it's difficult to know how the oil reacts with it. On the other hand we know that oxidation of oil molecules goes on long after the varnish has been brushed and that the peroxides created react with other oil molecules in order to polymerise. I am pretty sure that these peroxides can also react with some resin molecules if they are close enough.

[Urban Luthier]

Hi Jacob.....First apologies because I am no chemist but here's some observations from my own experience.

 

Generally I found from reading artist's manuals where permanence etc is very valued that most varnish/glaze mediums  although generally using other resins than colophony do involve turps, linseed oil and a resin and are prepared cold. If I am making a varnish that has turps  white spirit etc in as a solvent I generally make it cold using raw oil. It seems to make a decent enough varnish for my purposes which involves varnishing fiddles & not boats. If I am making a solvent free varnish I only heat the oil enough to combine it with the resin The resin might have been pre cooked. I've never bothered about stringing or pilling etc....so far no problems from doing it that way over 30 years or so....

 

i'd love to learn more about the making cold varnishes -- is it possible to mix pure turps, colophony and linseed oil together cold and get a decent varnish? I tried the mastic recipe quoted in Michelman as a class 2 varnish (sometimes referred to as Darnton Mastic varnish) but the dried varnish imprints too easily.

 

Cheers

 

Chris 

[FredN]

Using a pine resin does not require incorporating oil and resin because the resin would be destryoyed in the process. The oil has to heated around 600oF for it to obtain a varnishing condition. In making a rosin/oil varnish litharge or lead oxides are needed to harden the oil. A good staring process is to add the litharge into melted rosin, add no more than a third of the volume with linseed oil. If you get a white ring around the surface of the mix ( indicating a condensation reaction between the lead resinate and oil) the varnish will be a clear hard coat in a few hours. I'm sorry I can't state specific amounts cause I can't find my notes made about 10+ years ago. This I think is the simplest durable varnish one can make. I do remember you must get the white condensation ring, otherwise it takes forever for the coat to dry. fred

[joerobson]

I feel the need to solicit scientific input on (for me at least) vexing questions regarding the preparation of oil varnish - specifically when the resin is some kind of pine resin, and the oil is linseed oil.

 

1. The first concerns the "firm pill/stringing" test - is this to determine viscosity, or does it indicate when the resin has bonded properly with the oil? In other words, 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?

Pill is the sign that the resin and oil are fully bonded.  It is more critical in the harder resins, but definitely necessary with rosin and the raw pine resins.  Pill has a range from  "stickiness between the fingers" to long fine crystaline strands.  The time and temperature needed to achieve pill has a lot to do with how "raw" or "prepared" the linseed oil and resin are.  For example: Given the same resin raw linseed will take longer to achieve pill than a prepared [heated, oxidized, washed, etc....] linseed oil.  Different pill stages will transfer differing characteristics to the varnish, like dry to tack time, film drying time, drying pattern [top down, through and through....].  Pill length is not standard.  These characteristics [in relation to pill quality] will change from resin to resin. 

 

2. The addition of fresh gum turpentine at about 150deg C during the cooling down after cooking: does this serve any purpose other than thinning the varnish? Does it, for instance, enhance the bonding of the resin and oil? I realize fresh gum turpentine has drying properties, but what else apart from that?

Turpentine cooked into the bonded resin and oil creates mutual solubility.  This allows the varnish to have characteristics which are beyond [and not predicted by] the properties of the raw materials: hardness, polish character, IR, abrasion resistance, film solubility......

 

Joe

[Jacob]

Thanks for the responses!

 

Melvin, perhaps I should have said "chemical types" rather than "chemists"    - what I meant was that I am looking for a scientific explanation of what happens. Certainlly results based on long experience such as yours are very helpful.

 

Joe, thanks for the explanation regarding gum spirits creating "mutual solubility" between the oil and resin.

 

With reference to the two links dealing with washed linseed oil in the other varnish thread, I get the impression that all the other things you mention regarding the pill stage ("Different pill stages will transfer differing characteristics to the varnish, like dry to tack time, film drying time, drying pattern") this is influenced more by the way the oil has been processed than cooking time or temperature. For instance, it is stated that with thorough washing of cold-pressed oil using water, salt and sand, the oil will dry completely and quickly by itself without the addition of either driers or even resin, that it will not dry from the top down, that it won't darken, and that less resin can be used. Here are a few quotes from http://www.tadspurgeon.com/pdf/Refining_Linseed_Oil.pdf :

 

"While any drying oil will have a stronger paint film if the non-polymerizing fatty acids are removed, this
process is especially important to modify the behavior of linseed oil, whose high proportion of linolenic acid
(Omega 3) can cause yellowing, wrinkling and drying from the top.

 

"Painter-refined linseed oil is significantly different than its commercial counterpart. No commercial
oil, regardless of pedigree or price, has shown qualities comparable to HRO linseed oil. Working with the
first batch of this oil was a revelation. It was finally possible to understand both the origin of certain more
bravura techniques, and why linseed oil could be preferred. It is important to note that no testing has ever
been done on this oil for the simple reason that it is not commercially available. The “linseed oil” of almost
all research is a different product entirely, not necessarily cold-pressed, and either unrefined, or commercially
refined. These constitute substantial differences in practice. These tests, unless done within the context of
technical art history, also ignore factors related to the way the oil has been processed which, to painters, have
long been known to play a significant role in the ultimate behavior of the oil. HRO linseed oil offers a stable
foundation for further manipulations in terms of rheology and working qualities. In terms of technique it is
important to note that refining the oil makes it significantly less fat, thus less prone to all historical issues
associated with “fatty oils.” The materials that have evolved using this oil as a basis help explain why the use
of resin in older painting has turned out to be more tangential than was once generally believed. Using
permutations of HRO (hand-refined oil) linseed oil for the paint and medium, resins – and solvents – become virtually
unnecessary.

 

"Oil refined by the washing procedures that follow exhibits none of the negative characteristics
associated with lower quality commercial linseed oil. These oils do not skin or wrinkle, dry hard without
gumminess, and, once aged in the light, preheated, or used with a calcium carbonate, do not yellow
perceptibly."

 

I have just started using hand-refined cold-pressed oil in varnish preparation, so I don't have any personal observations to offer as yet. Nevertheless, if even half of what appears in the mentioned document is true, I expect a very different result from the varnishes I've made over the past decade using all kinds of commercially-refined and -processed oil.

[Guest]

I'm also going to prepare and put up one gallon of Allback cold processed raw oil using the same method. I found what the article had to say on the "edible oils" very interesting too. I have some experience with HRO linseed oil made by Donald Fels and his amber varnish and it is wonderful. I'm going to buy his book Lost Secrets of Flemish Painting. I invited Mr Fels to join MN and the discussions here and have talked further with him about his amber varnishes. He had some interesting things to say.

I'm also eager to make another batch of varnish using this HRO oil. I'm also going to try using some of the diamondgforestproducts resin and precooking it long and slow to see what kind of color I can get with it.

My plan is to make the next batch of varnish solvent free for long term storage.

[DarylG]

What does HRO mean?

[Jacob]

HRO: hand-refined oil (cold-pressed).

[CarloBartolini]

Oh my, I knew I should have read the pdf before running linseed oil through my HRO - Horizon Reverse Osmosis watermaker....  
 

[Jacob]

I wonder if anybody else had noticed that Roger Hargrave's description of his varnish "recipe" is basically the same as the so-called "Marciana" recipe.

 

Similarities - no particular emphasis on a high cooking temperature, same basic ingredients (colophony, mastic, oil), no particular mention of firm pill/stringing, etc.

 

I've tried all that - without satisfactory results - EXCEPT FOR THE OIL. That seems to be the devil in the details.

 

Without yet another (Roger's) seemingly innocuous mention of cold-pressed, washed oil, I would still be stumbling around in the dark.

[Michael_Molnar]

I thought they were the same - essentially.

[robertdo]

that's right. Except for the alum (but there was no real explanation for adding the alum in the Marciana original recipe. And maybe also the way to make to cook the varnish)

[FredN]

Hi Jacob-  In your response to posts you mention "drying of oil from the top"-  this condition occurs if the normal process of preparing a drying oil is not followed which is to first "heat body" the oil, and then have it "air body" when spread out in a film. fred

[Melvin Goldsmith]

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.

 

 

[DarylG]

Looks great Melvin!

[DarylG]

Has anyone tried pressing their own linseed oil? If you're going to wash it anyways perhaps it could be good.

 

http://www.piteba.com/index.html

[DonLeister]

I was under the impression that Tad's washing process used diatomaceous earth, someone here mentioned using sand?

[Guest]

Don did you read the article?

[Guest]

Jacob, Looks like it might be getting clearer despite the dissimilar and conflicting info...It is a great thread!

[joerobson]

Thanks for the responses!

 

Melvin, perhaps I should have said "chemical types" rather than "chemists"    - what I meant was that I am looking for a scientific explanation of what happens. Certainlly results based on long experience such as yours are very helpful.

 

Joe, thanks for the explanation regarding gum spirits creating "mutual solubility" between the oil and resin.

 

With reference to the two links dealing with washed linseed oil in the other varnish thread, I get the impression that all the other things you mention regarding the pill stage ("Different pill stages will transfer differing characteristics to the varnish, like dry to tack time, film drying time, drying pattern") this is influenced more by the way the oil has been processed than cooking time or temperature. For instance, it is stated that with thorough washing of cold-pressed oil using water, salt and sand, the oil will dry completely and quickly by itself without the addition of either driers or even resin, that it will not dry from the top down, that it won't darken, and that less resin can be used. Here are a few quotes from http://www.tadspurgeon.com/pdf/Refining_Linseed_Oil.pdf :

 

"While any drying oil will have a stronger paint film if the non-polymerizing fatty acids are removed, this

process is especially important to modify the behavior of linseed oil, whose high proportion of linolenic acid

(Omega 3) can cause yellowing, wrinkling and drying from the top.

 

"Painter-refined linseed oil is significantly different than its commercial counterpart. No commercial

oil, regardless of pedigree or price, has shown qualities comparable to HRO linseed oil. Working with the

first batch of this oil was a revelation. It was finally possible to understand both the origin of certain more

bravura techniques, and why linseed oil could be preferred. It is important to note that no testing has ever

been done on this oil for the simple reason that it is not commercially available. The “linseed oil” of almost

all research is a different product entirely, not necessarily cold-pressed, and either unrefined, or commercially

refined. These constitute substantial differences in practice. These tests, unless done within the context of

technical art history, also ignore factors related to the way the oil has been processed which, to painters, have

long been known to play a significant role in the ultimate behavior of the oil. HRO linseed oil offers a stable

foundation for further manipulations in terms of rheology and working qualities. In terms of technique it is

important to note that refining the oil makes it significantly less fat, thus less prone to all historical issues

associated with “fatty oils.” The materials that have evolved using this oil as a basis help explain why the use

of resin in older painting has turned out to be more tangential than was once generally believed. Using

permutations of HRO (hand-refined oil) linseed oil for the paint and medium, resins – and solvents – become virtually

unnecessary.

 

"Oil refined by the washing procedures that follow exhibits none of the negative characteristics

associated with lower quality commercial linseed oil. These oils do not skin or wrinkle, dry hard without

gumminess, and, once aged in the light, preheated, or used with a calcium carbonate, do not yellow

perceptibly."

 

I have just started using hand-refined cold-pressed oil in varnish preparation, so I don't have any personal observations to offer as yet. Nevertheless, if even half of what appears in the mentioned document is true, I expect a very different result from the varnishes I've made over the past decade using all kinds of commercially-refined and -processed oil.

 

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

[Guest]

At face value in this thread...there is Melvin and Roger who are creating varnishes which seem similar cooked at low temperature...Then there is Joe who is doing something different with regard to the dangerously high temperature cooks involving the addition of turpentine...