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Cooking Colophony-surface not melting.


Marcus Bretto
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Hello all. So, I’ve started cooking down some dark colophony from Kremer to get color for varnish. The internal temperature of the melt is reading right at 200C but the surface that is exposed to air keeps cooling/hardening over and I’m afraid it is making the cook take considerably longer to darken. It’s been on for over 100 hours with not much color change and very little volume reduction. Is this something that I am right to worry about, and if so, any suggestions as for how to combat it? Would adding turpentine and cooking at a lower (safer) temperature be an option? Or perhaps a vented lid to trap some heat and still let air in is better?

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100 hours is a long time. I'm not sure of your set up (heat source, thermometer, cooking vessel).

If it were me I would heat it to 250-275C and see what happens. At those temps I get a very dark resin in under 8 hours. It should look black in the pot and a drop of it when cold will look black. When crushed it will show some red and when thinned with a solvent it will show more of its true color. 

Try to stir it often at those temps. I The crusty stuff will redissolve, or at least mine did. Likely it will smoke more, darken more, crust over more. I would cover it loosely with a lid. My rosin I could barely get it out of the pot before it was too thick to scoop out hot. When it cooled it would scrape out pretty easily it was so brittle.

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49 minutes ago, Michael_Molnar said:

 stir often. 

Molten rosin is too thick for convection to work, and a lousy heat conductor so conduction doesn't work... therefore the top surface gets cool and solidifies.  Gotta stir, which is a problem if you  want to cook for a long time and sleep at night too.  Turning it off at night brings on the startup problem the next day, where you are in danger of smoking the bottom before the top gets hot enough to stir.

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I've never had the slightest problem with rosin burning, even with no stirring and letting it solidify between cooking sessions. Maybe I just got lucky with my pan/hotplate combination. 

Re the OP's question: I've never cooked the dark colophony, only the pale stuff. The pale stuff starts to go dark quite quickly.  Maybe the dark stuff is already as dark as its ever going to get no matter how much you cook it. Or maybe the change is quite subtle. The dark rosin has an undesirable (IMO) greenish tinge in its raw state. Maybe this turns to a reddish hue on cooking?

 

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I too have to let mine solidify, then will cook it further another day. Depending on how much colophony I'm cooking, sometimes I will break it up a bit before reheating, if it's a thick layer.

These days I tend to cook smaller amounts, and just reheat it as is.

I know what you mean about the green tinge, previously I wondered if I had caused this myself with too high a heat. The resulting varnish had a cold look about it, although was fine with some pigments mulled in. Since that time I've tried to avoid it.

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Thank you guys for the response. I am coming to the realization that the Kremer dark colophony is probably not a good product as far as cooking color into the resin. Over the long time of cooking, some of the brown/green hue was mitigated by a gold/red shift, and I say some, purposefully. But I am worried that, without the use of excessive heat and some carbonization, which I understand should be avoided, this resin simply won’t take on enough color to be used on its own. A few days ago I came across an article demonstrating how rosin cooked in an oxygen free environment can actually be bleached of its color (I should find the article again and post it) and I’m wondering if rosin that has been collected from turpentine distillation has, in effect, been stripped of some of its color carrying qualities from its long exposure in this sort of environment. I had heard that different resins respond quite differently in the process, but I’d also be curious to hear people’s experience with distillate byproduct resin vs oleoresins directly from the tree. Maybe partial confirmation of this can be realized. 

Edited by Marcus Bretto
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On 8/27/2018 at 5:48 PM, Marcus Bretto said:

Hello all. So, I’ve started cooking down some dark colophony from Kremer to get color for varnish. The internal temperature of the melt is reading right at 200C but the surface that is exposed to air keeps cooling/hardening over and I’m afraid it is making the cook take considerably longer to darken. It’s been on for over 100 hours with not much color change and very little volume reduction. Is this something that I am right to worry about, and if so, any suggestions as for how to combat it? Would adding turpentine and cooking at a lower (safer) temperature be an option? Or perhaps a vented lid to trap some heat and still let air in is better?

it's all about the volume of resin and shape of your pot and the distance of the pot contents at the top from the heat source...

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