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Michael_Molnar

Cooked Rosin is Fugitive

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On 10/17/2019 at 3:27 AM, Ernest Martel said:

It does look more red than Peter's example. From what I remember about Peter's cooking is that he cooks hot and fast which clearly shows in the cold brown example that he posted. Joe's example looks more red probably form cooking at a lower temp. I'd like to see it smeared out as MM suggested to see what it looks like in a thin layer. I remember too that Joe also attaches color to his oil so that will also affect the varnish color.

So, colophony can be cooked until it turns to carbon and it will still be light colored in a thin layer. And further lighten with UV exposure.  I don't see any point trying to achieve a red varnish from cooking colophony on it's own. 

 

Ruby.JPG

Yeah, cold brown ;)

Varnish color and photos ....

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8 hours ago, Ernest Martel said:

DarkVarnish2.thumb.jpg.6ae120122aec067d8b97dd5bdaa297ca.jpg.0148d6293b7f961815ad3fb33fa07d45.jpg

Quite a difference between the two examples....;)

Ruby.JPG

:)  and the worst is that it is the same varnish, still a believer in images? don't trust mine! I'll trick over an over again

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I would like to say that I (and maybe Joe R) are at least a little bit wrong? I reviewed some of my examples that has been out in sun, rain, snow .. for years (2-5years) that I took images of, now with my now new phone, and the "burned maple" pieces are in fact a little bit lighter than the originals, but maybe some degrees redder?

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4 hours ago, scordatura said:

Just to clarify, the one on the left was exposed to uv and the right no exposure to uv?

On the left side is varnish,  on the right cooked colophony.

 

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How long do you all cook your resin? That is the only part of Roger H's directions left open. He is vague about cooking "as long as you can" or something like that. Following these directions leads to a perfect varnish, much darker than Mike M's example. Over 50 hours is pretty good. More is better. Double or triple that. I think that is kind of what Peter is trying to get at here, and also Joe R, at least one of the things. Even if you are following  Roger H's directions exactly, which Peter in particular doesn't (but I bet, not because he didn't try long and relatively low heat cooks),  there is that one annoying variable.  But then, the results are great if you just do what Roger said and are patient enough for the process. If you don't want a ton of trial and error, or a costly research project, if you just want a perfect varnish, do the process. Sunlight/ uv box doesn't seem to change things too much. I tend to believe cooking for color is very possible, look at that delicious red of Peter's varnish (!) ...but you need a color arsenal too (especially if antiquing).

 

 

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11 minutes ago, not telling said:

How long do you all cook your resin? That is the only part of Roger H's directions left open. He is vague about cooking "as long as you can" or something like that. Following these directions leads to a perfect varnish, much darker than Mike M's example. Over 50 hours is pretty good. More is better. Double or triple that. I think that is kind of what Peter is trying to get at here, and also Joe R, at least one of the things. Even if you are following  Roger H's directions exactly, which Peter in particular doesn't (but I bet, not because he didn't try long and relatively low heat cooks),  there is that one annoying variable.  But then, the results are great if you just do what Roger said and are patient enough for the process. If you don't want a ton of trial and error, or a costly research project, if you just want a perfect varnish, do the process. Sunlight/ uv box doesn't seem to change things too much. I tend to believe cooking for color is very possible, look at that delicious red of Peter's varnish (!) ...but you need a color arsenal too (especially if antiquing).

 

 

I think the important variable is the % colophony reduction by weight.  You will need to reduce the colophony by ~80%.  Basically getting your cooked rosin as dark as possible without burning.  Low and slow is safe, but I think not critical. I cook at 300 C until the rosin starts reducing at a noticeable rate then reduce the heat to 250 C as the volume is decreasing.  I hold at 250 C (ish) until the rosin is dark enough or my courage fails.

1261416013_2019cookedrosinchip.thumb.jpg.eafff303e736ddd63966e4ad2ad26746.jpg.a275e68cd0fc1c40d9f765766eac6ef6.jpg

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Ha, exactly. Looks nice. I haven't done the process enough to plan exactly how much the colophony gets reduced. It was a very significant reduction but I'm not sure it could have been 80% reduction. I can only comment that it is amazing how great this varnish is that is made with a very long cooking time. It's a bit soft on its own, but siccatives have always been used and the OW siccative product is very nice.

It's easier with more than one person, for sure, I definitely don't want to be alone with my thoughts stirring a pot for that long.  

I had some beautiful, really primo dark colophony from Fred N. I don't know where he got it. The rosin cooking time (well over 100 hours) stretched limits of dread and terror all around, because each passing hour is a possible moment that must be repeated with other ingredients, but the result of this is actually a classically pleasing varnish. Actually, a half gallon of magic. We had backup colophony to make me feel better, especially getting into the varnish making steps with the cooked rosin, but nothing was wasted.

I find it kind of wild how easy it is. Takes awhile is all. Eventually the process might become fun and relaxing. I must have undiagnosed anxiety, undiagnosed lots of things. It wasn't fun and relaxing. Anyway, it's a process. I just wanted to say that if one reads the directions 1000 times, intentionally and absolutely following what Roger says to do, you'll probably make a beautiful and easy to control varnish. I say "you" meaning anyone can, even someone like myself.

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Cooking time for dark color varies a bit.

Average for colophony is about 350 hours.  Weight loss about 10%.

Raw resin is 450 - 575 hours.  Weight loss about 15%.

on we go,

Joe

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For testing light-fastness, here's what I suggest:

Apply your varnish equally to two glass slides. Use the thickness and color intensity that you would expect to use on a violin (UV has very low penetration of most varnishes). Also, use the finished product as it is when mixed with oil, because behavior of the resin alone, and after being cooked into oil, seems to behave differently.

Put one piece of glass in a dark drawer, and expose the other to UV.

I like to use glass rather than wood, because on wood, the wood can change color too, confusing the outcome.

One more thing about cooking rosin (and other resins) to darken them: The rate of darkening highly depends on the air circulation rate, along with the amount of surface area exposed. In wind, or with a fan, the resin can reach the same color change in 8 hours, that it will reach in 30 hours in still air, if the temperature of the resin itself is the same under both scenarios.

Oh, one more-more thing: :lol:  Cook and darken a resin too long, and it will no longer melt into and truly combine with hot vegetable oil. Instead, it will be incorporated as more of a pigment suspension (or some parts of it will),  which will change the optical qualities, maybe for better or worse, depending on your perspective.

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4 hours ago, joerobson said:

Cooking time for dark color varies a bit.

Average for colophony is about 350 hours.  Weight loss about 10%.

Raw resin is 450 - 575 hours.  Weight loss about 15%.

on we go,

Joe

http://www.thestradsound.com/varnish/varnish-recipe-16-06-2018

This year 10+ hours, weight loss 120 -> 70 g (~40% loss)

http://www.thestradsound.com/ongoing/varnishcookingtime-1

I use a wide open pan and I choose a windy day, stir a LOT

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1 hour ago, Michael_Molnar said:

Has anyone else noticed the high viscosity of cooked rosin? How did you work this into a fluid varnish. I use kerosene with some success.

Did you mean cooked varnish? 

Yes I have noticed. As i wrote during the cooking of colophony I have to raise the temperature as it is reduced otherwise it would not be fluid enough to fume and reduce more. As for the fluidity of the later varnish, the same amount of linseed oil in weight is added to the reduced colophony. And then add turp to get the desired viscosity of varnish in the jar. Even then it is quite thick. 

 

 

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