Varnish making... and Lake making

[DenisR]

Great, thanks a lot, I'll give it a try this weekend.

[Steve Voigt]

3 hours ago, MikeC said:

 The article says to cook for a week for the color of the varnish.  For an uncolored varnish you don't have to cook that long.   I cooked the rosin for about 3 hours at 350f.

A public service announcement. Cooking rosin for 3 hrs/350 F will get you a soft, acidic, colorless varnish. It will dry poorly. On a fiddle, it will forever take imprints from fingers or a case. In all likelihood it will grow brittle and crack, flake off, or otherwise break  down. 

I am not trying to be harsh—just realistic.

The point of cooking for a week, as discussed in the Ruschil article that Mike cited, is not just color. Long or hot cooks will reduce acidity and partially polymerize the rosin, making it harder, more durable, and faster drying. 

I get that some people are concerned about leaving the cooking pot unattended. I am not giving safety advice and I don’t recommend you do anything that feels risky. With that said, I just park the hot plate outside, away from any structures, and let it stew for a week or whatever. I throw a piece of tin over it if it rains. 

What to do if you’re unwilling to cook continuously?

1. You can pour the resin out and recook at the start of each day, as Mike suggested.

2. I have had no problems with leaving the rosin in the pot and restarting it each morning. I use a thin aluminum disc between the hot plate and the cooking pot, to smooth out temperature spikes. And I use a pan with a thick bottom. Start at low temp and gradually increase the heat.

3. You can cook hot and fast. A range of 250-300 C (480-540 F) will work. At 300, you might get it done in one long day. I’d probably do it in two days. Cook until the resin is black in mass tone. Search for MN posts by Jim Bress—he’s given some good descriptions of how to cook this way. While this is not my preferred method, the results will be far, far better than cooking for a few hours at low temp—that won’t get you anywhere.

Good luck and be safe. 

[MikeC]

Steve, thanks for the comments.   

I was trying to follow the directions in the article.

 It says for a clear varnish cook the resins for two hours before mixing with the oil.   The color varnish will cook for 7 days. 

So that's why I thought a 3 hour cook at 350f would be ok based on the article.    

The article also says to use 1% to 2% siccative.   I didn't do that since I don't have any, but have some on order now.  

I still have plenty rosin so I'll be cooking some for a week,  maybe not all at once,  just when I'm around to keep an eye on it.  

[Davide Sora]

32 minutes ago, Steve Voigt said:

A public service announcement. Cooking rosin for 3 hrs/350 F will get you a soft, acidic, colorless varnish. It will dry poorly. On a fiddle, it will forever take imprints from fingers or a case. In all likelihood it will grow brittle and crack, flake off, or otherwise break  down. 

I am not trying to be harsh—just realistic.

The point of cooking for a week, as discussed in the Ruschil article that Mike cited, is not just color. Long or hot cooks will reduce acidity and partially polymerize the rosin, making it harder, more durable, and faster drying. 

I get that some people are concerned about leaving the cooking pot unattended. I am not giving safety advice and I don’t recommend you do anything that feels risky. With that said, I just park the hot plate outside, away from any structures, and let it stew for a week or whatever. I throw a piece of tin over it if it rains. 

What to do if you’re unwilling to cook continuously?

1. You can pour the resin out and recook at the start of each day, as Mike suggested.

2. I have had no problems with leaving the rosin in the pot and restarting it each morning. I use a thin aluminum disc between the hot plate and the cooking pot, to smooth out temperature spikes. And I use a pan with a thick bottom. Start at low temp and gradually increase the heat.

3. You can cook hot and fast. A range of 250-300 C (480-540 F) will work. At 300, you might get it done in one long day. I’d probably do it in two days. Cook until the resin is black in mass tone. Search for MN posts by Jim Bress—he’s given some good descriptions of how to cook this way. While this is not my preferred method, the results will be far, far better than cooking for a few hours at low temp—that won’t get you anywhere.

Good luck and be safe. 

An addition to the public service announcement, this one in particular :

2. I have had no problems with leaving the rosin in the pot and restarting it each morning. I use a thin aluminum disc between the hot plate and the cooking pot, to smooth out temperature spikes. And I use a pan with a thick bottom. Start at low temp and gradually increase the heat.

Don't try this with a glass pan or jar, because it will break, even if it's Pyrex or other heat-resistant glass (learned the hard way). Steel is better.

[MikeC]

2 hours ago, Davide Sora said:

An addition to the public service announcement, this one in particular :

2. I have had no problems with leaving the rosin in the pot and restarting it each morning. I use a thin aluminum disc between the hot plate and the cooking pot, to smooth out temperature spikes. And I use a pan with a thick bottom. Start at low temp and gradually increase the heat.

Don't try this with a glass pan or jar, because it will break, even if it's Pyrex or other heat-resistant glass (learned the hard way). Steel is better.

I have stainless steel pots, but I still won't leave it unattended.     

[Davide Sora]

20 minutes ago, MikeC said:

I have stainless steel pots, but I still won't leave it unattended.     

Me too. I have had two beakers break while letting the resin cool inside the jar. The shrinkage caused by the cooling and the adhesion force of the rosin to the glass cracked the beaker.

[Sean Couch]

Hey Mike, don’t end the rosin cook based on time alone. I did the 350F for 168 hours and only had about a 25% reduction in the weight of the rosin. The low reduction also meant it didn't develop much color. I emailed the author of the article and he said to aim for at least a 50% reduction, and 60% would be better.  

[Steve Voigt]

6 hours ago, Davide Sora said:

An addition to the public service announcement, this one in particular :

2. I have had no problems with leaving the rosin in the pot and restarting it each morning. I use a thin aluminum disc between the hot plate and the cooking pot, to smooth out temperature spikes. And I use a pan with a thick bottom. Start at low temp and gradually increase the heat.

Don't try this with a glass pan or jar, because it will break, even if it's Pyrex or other heat-resistant glass (learned the hard way). Steel is better.

Good point, Davide. As an added bonus, steel/stainless/cast iron are all better at conducting heat, compared to glass.

[MikeC]

So since I've already made this varnish, can I rescue it?  Could I just put it on the heat and cook it more?  

[jefcostello]

There is no way to change the soft characteristics. My experience...

It is better to trust yourself than to trust others. Take a fingerprint test...

[uguntde]

On 4/6/2025 at 1:24 PM, Davide Sora said:

Interesting, so some questions arise:

How long does it take for rosin to dissolve in oil in a cold mix? 

Once dissolved, is it a homogeneous and crystal clear solution? 

Are you talking about untreated (and therefore more acidic) rosin, cooked rosin, or limed rosin?
Or raw resin (tree exudate), which also contains turpentine?

Do you think cold dissolution could be enough to make a good oil varnish? It would be great to avoid all those annoying and dangerous cooking.

 

I am trying this in the lab at the moment.

 

[LCF]

1 hour ago, jefcostello said:

There is no way to change the soft characteristics. My experience...

It is better to trust yourself than to trust others. Take a fingerprint test...

Added security with that too. You will know that it's your varnish and not some other maker's 

9 hours ago, Steve Voigt said:

Good point, Davide. As an added bonus, steel/stainless/cast iron are all better at conducting heat, compared to glass.

If you use an iron pot ( or tin below 232c/450f ) it can give the oil an extra kick along the peroxidation cascade pathway. This happens when cooking polyunsaturated cooking oil in iron frypans, takes you well on the way to vascular disease.

 

 

Edit, I was talking about the oil not the resin. But a little bit of iron or some iron salt ( eg sulphate) can redden the resin so long as you don't go overboard. 

[LCF]

Btw, you can add a bit of tung oil in there to give it an extra kick. 

[Davide Sora]

3 hours ago, uguntde said:

 

I am trying this in the lab at the moment.

 

Good! Keep us updated.

[Bodacious Cowboy]

Gosh. I've been making colophony/mastic/linseed oil varnish for over 20 years and according to the above posts I've been doing everything wrong.....

Cook colophony for 40 hours or so at just above the melting point - relatively low weight loss, huge colour change.

Liberally thinned with mineral spirits - no problem.

No siccative

"String test"? I don't even know what that means,

 

And yet, my varnish is a very nice colour, brushes on nicely, dries in about 8 hours in the light box, is acceptably wear resistant, doesn't print and folks tell me it looks lovely. 

[Dr. Mark]

This could get interesting...

[MikeC]

Yes interesting,  There are some who seem to get good results without much cooking.   

Well,  I made a short video showing a couple of varnish samples.  I have one more sample that will get some sun tomorrow and that will be the un-thinned original varnish.   It's viscous enough that it's barely brush-able. 

But this in the baby food jar has been thinned down with turpentine. 

 

[Brian in Texas]

Cool progress, Mike. Can anyone weigh in on how best to add pigment to the mix?

[MikeC]

That's what I'd like to know.   Do you have to mull in the lake pigment or just dump some in the jar and stir it up?  

[jefcostello]

3 hours ago, Brian in Texas said:

Cool progress, Mike. Can anyone weigh in on how best to add pigment to the mix?

[HoGo]

I only cooked oil varnish twice but never for more than hour total time and my samples (now nearly 30 years old) seem to hold well. It's pale colorless but no traces of other ill behavior. I varnished small maple slab with the varnish that I've been using for trimming bindings since the day the varnish dried. the varnish on undamaged parts of the slab is still holding fine.

I used ordinary artist linseed oil from art supply, didn't even wash it as back then all the info available online was not common. I prepared my own lead acetate (or oxide?) by letting piece of lead "rust" in a jar with vinegar in bottom and repeatedly scraped the whitish layer. I cooked the oil with this lead substance and and it seemed to dry nicely on it's own.

I cooked varnish on hotplate simply by adding pieces of rosin into hot oil and cooking till it all combined maybe 15 minutes or so. I believe I added small amount of copal varnish (from artist store, contained just copal and turpentine) while cooking.

According to some info above, that shouldn't work but it worked fine. I guess there is more than just cooking time of rosin in the equation, the drying quality of oil being quite high, IMO.

[MikeC]

4 hours ago, jefcostello said:

Can you explain please?  It looks like you are using a muller,  are you adding in lake pigment there?   Do you mull in some pigment and then add that to a jar of varnish to color the whole jar of varnish?  Or do you just add color to what you are about to use on the instrument?  

[MikeC]

2 hours ago, HoGo said:

I only cooked oil varnish twice but never for more than hour total time and my samples (now nearly 30 years old) seem to hold well. It's pale colorless but no traces of other ill behavior. I varnished small maple slab with the varnish that I've been using for trimming bindings since the day the varnish dried. the varnish on undamaged parts of the slab is still holding fine.

I used ordinary artist linseed oil from art supply, didn't even wash it as back then all the info available online was not common. I prepared my own lead acetate (or oxide?) by letting piece of lead "rust" in a jar with vinegar in bottom and repeatedly scraped the whitish layer. I cooked the oil with this lead substance and and it seemed to dry nicely on it's own.

I cooked varnish on hotplate simply by adding pieces of rosin into hot oil and cooking till it all combined maybe 15 minutes or so. I believe I added small amount of copal varnish (from artist store, contained just copal and turpentine) while cooking.

According to some info above, that shouldn't work but it worked fine. I guess there is more than just cooking time of rosin in the equation, the drying quality of oil being quite high, IMO.

Thanks for the info.  It seems the drying quality of the oil is most important and the lead acts a siccative.   I have some japan drier on order and will add that to my varnish and test some more samples.   

[LCF]

9 hours ago, Brian in Texas said:

... Can anyone weigh in on how best to add pigment to the mix?

What type of pigment?

There's a lot of discussion about lakes on M'Net. Madder, cochineal, weld, mulberry and many more besides.

 

[jefcostello]

1 hour ago, MikeC said:

Can you explain please?  It looks like you are using a muller,  are you adding in lake pigment there?   Do you mull in some pigment and then add that to a jar of varnish to color the whole jar of varnish?  Or do you just add color to what you are about to use on the instrument?  

 

1 hour ago, MikeC said:

 

I don't recommend adding color to an entire can of varnish because we are not the manufacturer and it is very likely that we will add too little or too much. Simply put, we don't have that many tests.

I suggest you take out a little varnish, add the pigment, and transfer it to another small jar.