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Cooking Colophony Down For Color, Low and Slow?


Berl Mendenhall
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Nice red, but I think you need something that looks black with a red edge, much like the proverbial dark cloud with a silver lining. When you dilute your cooked resin with linseed oil to make varnish, it will look orange. I think that's why Strad goosed up his colored varnish with some pigments - not much BTW.

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One thing to consider is not the lime itself, but the watery-ness of the mixture.

 

If the precipitates of calcium rosinate, for example, are very fine, then the rosin will appear like muddy caramel, rather than forming large accretions at the bottom of the pan.

OK...So I'll try it again using a thick mixture of paste instead of a watery mixture like in the video. Will I add it the same way? Is there a certain ratio I should use? How much lime to mow many grams of rosin? And the end product of hard rosin should be as transparent as it was before adding the lime? I don't understand what I'm looking for in the bottom of the pan...

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Nice red, but I think you need something that looks black with a red edge, much like the proverbial dark cloud with a silver lining. When you dilute your cooked resin with linseed oil to make varnish, it will look orange. I think that's why Strad goosed up his colored varnish with some pigments - not much BTW.

I was told by another maker also to cook the resin until black in color. At this rate it would take a month of Sundays to get there I think. I'm also factoring in the high heat darking the color when I do cook it into the oil. I'm sure it will push it more towards brown. Which is fine by me. I have a hunch that is why Roger and Neil don't cook there varnish at high temperature. Rather they just heat the resin and oil enogh to blend them together, thus preserving the red color. I will have enough resin to try different things this summer but to me the secret is in the refined oil...nuff said about that...gotta mow the yard before it gets too hot. Later

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OK...So I'll try it again using a thick mixture of paste instead of a watery mixture like in the video. Will I add it the same way? Is there a certain ratio I should use? How much lime to mow many grams of rosin? And the end product of hard rosin should be as transparent as it was before adding the lime? I don't understand what I'm looking for in the bottom of the pan...

> The varnish I just used on NC 1-5 has 1.8g slaked lime paste to 250 g Kremer's colophony

 

> I did not test the transparency of rosin before adding oil to form the varnish - the latter is transparent (no haze at all)

 

> They look like white crumbs at bottom of pan (several mm) - may just be lime that dehydrated too quickly and formed these accretions.

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> The varnish I just used on NC 1-5 has 1.8g slaked lime paste to 250 g Kremer's colophony

 

> I did not test the transparency of rosin before adding oil to form the varnish - the latter is transparent (no haze at all)

 

> They look like white crumbs at bottom of pan (several mm) - may just be lime that dehydrated too quickly and formed these accretions.

Do you have any limed rosin at the moment in storage? I would like to see what the finished product looks like.

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I don't - it all got converted to varnish.

 

These are a couple of photos:

http://www.maestronet.com/forum/index.php?/topic/328485-janitos-bench/page-2#entry638070

Thanks your varnish looks very nice as always and nice an clear. The reason I need to see the limed rosin before it is cooked is to avoid making mistakes and wasting good refined oil. As I mentioned the first and last rosin I limed looked like caramel. I would like to see what the finished product is suppose to look like.

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Thanks your varnish looks very nice as always and nice an clear. The reason I need to see the limed rosin before it is cooked is to avoid making mistakes and wasting good refined oil. As I mentioned the first and last rosin I limed looked like caramel. I would like to see what the finished product is suppose to look like.

 

Could you be using too much lime?  Once the lime neutralizes the rosin acid, I assume the excess acts like a filler and would make the mixture opaque.

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Could you be using too much lime?  Once the lime neutralizes the rosin acid, I assume the excess acts like a filler and would make the mixture opaque.

I did exactly as described in the video. You tell me.

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If anyone has some limed rosin before converting it to varnish and would post a photo of what it looks like then that would be very helpful. And answer some of these questions and theories.

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I did exactly as described in the video. You tell me.

The amount of lime you use to neutralize the rosin acid would depend on the purity of the lime and acid number of the rosin.  Knowing these, you can calculate the correct amount of lime to use.  Otherwise, it's a matter of trial and error.  The amounts in the video may not apply to your materials.

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If anyone has some limed rosin before converting it to varnish and would post a photo of what it looks like then that would be very helpful. And answer some of these questions and theories.

 I've had a similar experience to yours. But, as Michael Molnar stated in an earlier post, if you make varnish with this "caramel", the excess lime just precipitates out. The resultant varnish is crystal-clear.

 

I felt I had no option in liming the rosin, as the melting point of the varnish is too low for practical purposes in my local climate. It's a real let-down to have an instrument stuck in the case...

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Overall I get the impression that liming colophony is a very inexact procedure. Firstly, unless one actually tests the acid level of the rosin or colophony, there is no way of telling how much lime will be needed to neutralize is. Secondly, from some responses in this thread it also seems that not all lime is the same.

 

I'm pinning my hope on the possibility that excess lime will in fact remain in suspension and eventually precipitate out, instead of turning the varnish into a soap, but of course I don't really know if this is the case. If excess lime just precipitates out, I'm happy. What I know for sure is that the varnish I end up with after slow-cooking colophony has too low a melting point for my climate and purposes. Obviously it is possible that I'm messing up in some detail of the preparation, but Joe Robson and David Burgess have both cautioned about the low melting point problem, so perhaps it's not just me messing up.

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I don't think I have heard it described that way before, Carlo,glad you mentioned it. I'm not questioning your statement by the way just curious. I have usually heard it said that liming makes the resin harder, which to me means tougher.

I haven't cooked with rosin in a while, just straight resin from the tree, and the last batch cooked a long time (I never add lime) was too soft after the cooking, it will flow a bit with time, just like bass rosin. So there may be hope for it if I cook it again with some lime! I'll try it.

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I don't think I have heard it described that way before, Carlo,glad you mentioned it. I'm not questioning your statement by the way just curious. I have usually heard it said that liming makes the resin harder, which to me means tougher.

I haven't cooked with rosin in a while, just straight resin from the tree, and the last batch cooked a long time (I never add lime) was too soft after the cooking, it will flow a bit with time, just like bass rosin. So there may be hope for it if I cook it again with some lime! I'll try it.

I not sure "higher melting point" equals "harder". Perhaps someone can provide clarification on this.

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I not sure "higher melting point" equals "harder". Perhaps someone can provide clarification on this.

 

These are glassy, amorphous materials which strictly speaking don't have a "melt point".  Instead they have a "glass transition temperature" (Tg) which is analogous.   Melt points usually refer to crystalline materials.  While melt points usually happen over a very short temperature range, the Tg happens over a wide temperature range.  A typical Tg for rosin ranges from 50 to 70 degC with the peak around 60 degC.  

 

Hardness is a vague term, but in this context it's often measured with some kind of needle probe such as a penetrometer or Shore hardness gauge.  At a temperature far enough below the Tg, the hardness isn't affected too much by the Tg, but as the temperature increases the material with the higher Tg will act harder longer.

 

Liming the rosin raises the Tg so it would appear harder at higher temperatures than unlimed rosin.

 

I'm curious, do people make varnish from rosin esters?  There are glycerol and penterythritol esters of rosin that raise the Tg and neutralize the rosin as much as liming does.  Esterification also stabilizes the rosin for improved oxidative resistance.

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Thanks for the info TQuinn. If you don't mind my asking, how do you know all this stuff? I take it you make varnishes yourself? I just know so little of who you are, you sound pretty intelligent!

A higher Tg sounds like what I want with a particular resin in mind, and hopefully the stuff will be brittle as normal stuff goes.

Tg sounds like a very useful term when it comes to talking about resins used for violin varnishes. So many descriptions of resins that I have read use the words like 'hard, soft' it doesn't really say a lot, and I see conflicting descriptions from some books or catalogues.

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Thanks for the info TQuinn. If you don't mind my asking, how do you know all this stuff? I take it you make varnishes yourself? I just know so little of who you are, you sound pretty intelligent!

A higher Tg sounds like what I want with a particular resin in mind, and hopefully the stuff will be brittle as normal stuff goes.

Tg sounds like a very useful term when it comes to talking about resins used for violin varnishes. So many descriptions of resins that I have read use the words like 'hard, soft' it doesn't really say a lot, and I see conflicting descriptions from some books or catalogues.

 

I've been an industrial chemist for 41 years and I work with resins like this all the time.  I've never made a varnish, but blending, mixing, reacting, and testing things like this are what I do.

 

On the side, I've been characterizing bow rosins and have just finished measuring the Tg's of 20 commercial ones.  I suspect, and some literature supports, that Tg is the single largest measurable characteristic that would correlate with "playability", whatever that is.

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Carlo, send chemmy an email and ask if him to join this thread. I enjoyed his side of the coin in previous discussions. He really seemed to know his stuff too and could perhaps shed some more light for or against this current topic. I'm very interested in more info even if it has been beat to death.

 

Thanks sailor...

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I have done a few tests today with the rosin I'm cooking...Today is day 22 in the crock pot. It does look like the red is developing more quickly. Perhaps it's at some kind of turning point stage. I took a chunk and thinned it down with some turp but there is basically little color. A light golden brown. So to use the thinned down resin only as a colorant is really not going anywhere. I would have to cook it for months and I think that is what Evan said to do many posts back. To bury it in the snow and forget it. I need varnish and oil now, this summer so...

 

Since it seems to be at some kind of turning stage I will cook it for 30 days max. I think the real color will develop upon cooking the oil into it. I'll try cooking it the same way as my last batch that is 500F or above. And a second batch at a lower temp just to blend and see how each other looks, dries and cures. It should be fun. It's been fun cooking this rosin down and noticing the changes from the prolonged cooking time. I think given enough time it would continue to darken.  I wish I had more time this summer...maybe someone else will do a longer cook and post their results.

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