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Varnish Temperature

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39 minutes ago, scordatura said:

Thank you Davide. Hope you are doing well. I was planning of visiting you in Cremona in July. Not sure if my trip is happening. Probably not...

I'm fine, thanks.

I think it will be difficult for the situation to normalize for July, but who knows,  politicians are still navigating on sight here (not to use irreverent words) and making plans is unlikely.

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On 4/17/2020 at 6:14 PM, David Burgess said:

Does that create the possibility of ending up with a varnish which is excessively alkaline? Or does anything which isn't used up in the neutralization process fall out somehow?

In acquesous solution esterification (this includes cross-esterification) is base or acid catalyzed - I assume that is the same when you cook varnish in linseed oil. In my opinion what you do when you cook varnish is a cross-esterification. You remove the 3 fatty acids (linoleic acid, alpha-linolenic acid, and oleic acid.) from the glycerole, and you add some abietic acid, i.e. abietic acid replaces fatty acids on the glycerol, probably depending on the ratios. Oleic acid can not participate in the reaction and probably acts as a kind of softener. The base would work as a catalyst, speeding up this process. In some cases I belive I saw a bubbling effect when adding lime later, but this observation may be misleading, as I added it in water which may have caused all the bubbling.

When the varnish hardens it is mainly the alpha-linoleic acid, but to some degree also the linoleic acid that undergo reactiions via their conjugated double bonds (probably light induced Diels-Alder reactions). This is why linseed oil hardens even without rosin, i.e. without the abietic acid. But you get better cross-linking and a harder varnish with the abietic acid, which also has a conjugated double bond. There are a lot of older patents about this from the varnish industry as they use the same principle. For industrial varnishes the hardening is of course usually not light-induced but radical induced because it is faster. But the process is a different one.

If this is correct, and I admit it may not be, adding a tiny bit of KOH or carbonate should be even more effectve that adding lime to the cooking process. Lime is an odd choice as it leaves silicates and dirt that is difficult to be removed afterwards.

I would be interested to hear other opinions about this.

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On 4/16/2020 at 2:37 PM, David Burgess said:

I don't know, but my experience with rosin-based varnishes, without neutralization of the acidity, has been that they deteriorate rather rapidly.

I think I need to know more about this.  How much base (calcium hudroxide added?) do you add to the rosin?  The thing that confuses me is that rosin is 100% abietic acid; so it is 100% an organic acid.  The organic acid will react with a base such as calcium hydroxide or sodium hydroxide to make a soap., but I know you cannot react all the organic acid (rosin) and still make a varnish.  The varnish recipe books talk about a 5% addition of base to the rosin--is that what  you mean?.

I have been studying the literature to find out what is the chemical reaction between linseed oil and rosin when heated, but I cannot find the answer.  It may be the diels-alder reaction that allows them to chemically combine to make a varnish.  Drying of the varnish is by free radicals that crosslink the varnish using the unsaturation--they are naturally present in the cooked varnish and also form by oxygen that penetrates the film plus the action of UV light.  Metal oxides catalyze their formation such as are found in Japan Drier.

The Mariana manuscript 399 as mentioned by Mrs Merrifield mentions the addition of a small amount of alum to the cooked varnish, and this would supply a small amount of potassium and aluminum to react with the rosin.

The old varnish makers used a small addition of calcium hydroxide to make the varnish harder and more water resistant.

regards

Mike D

regards

Mike D

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13 hours ago, Mike_Danielson said:

The old varnish makers used a small addition of calcium hydroxide to make the varnish harder and more water resistant.

Harder, yes, but more resistant to water, I don't think. The reverse is more likely as calcium soaps or calcium rosinates are  water soluble. It is probably a question of quantity, perhaps in small percentages this effect is negligible, but I doubt it increases the resistance to humidity.

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19 hours ago, Mike_Danielson said:

I think I need to know more about this.  How much base (calcium hudroxide added?) do you add to the rosin? 

Apologies. I never went far enough into rosin varnishes to thoroughly sort this out, because early results were so unpromising, that I moved on to other things.

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8 hours ago, Davide Sora said:

Harder, yes, but more resistant to water, I don't think. The reverse is more likely as calcium soaps or calcium rosinates are  water soluble. It is probably a question of quantity, perhaps in small percentages this effect is negligible, but I doubt it increases the resistance to humidity.

Just  found out from Wikipedia that calcium soap is insoluble in water.   

Unlike K+ or Na+ soaps, Ca++ soap  is not soluble in water.  I guess it's because that calcium ion has two positive charges and bounds with two acids at a time. The two acids distribute evenly on both sides of calcium. And this makes the whole molecule non-polar therefore not soluble in water.

However, this leads to another question:  if we pour some potash solution ( as some recipes suggest ),  we get water soluble potassium soap which make our varnish humidity sensitive!? 

 

1634350480_2020-04-216_03_18.png.808b29b7371d3925717e605d0a9a9cda.png

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16 minutes ago, Kae said:

Just  found out from Wikipedia that calcium soap is insoluble in water.   

Unlike K+ or Na+ soaps, Ca++ soap  is not soluble in water.  I guess it's because that calcium ion has two positive charges and bounds with two acids at a time. The two acids distribute evenly on both sides of calcium. And this makes the whole molecule non-polar therefore not soluble in water.

However, this leads to another question:  if we pour some potash solution ( as some recipes suggest ),  we get water soluble potassium soap which make our varnish humidity sensitive!? 

 

1634350480_2020-04-216_03_18.png.808b29b7371d3925717e605d0a9a9cda.png

I don't know if calcium stearate is the same as calcium soap, but in any case you're right, I got confused with sodium soap, I'm sorry:unsure:

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2 hours ago, Davide Sora said:

I don't know if calcium stearate is the same as calcium soap, but in any case you're right, I got confused with sodium soap, I'm sorry:unsure:

My fault!  Calcium stearate is just one example of calcium soap. It's a calcium soap made from steric acid.   I'm not a chemist, but I would guess soap made from other fatty acid behave similarly. 

 

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On 4/20/2020 at 3:07 AM, Mike_Danielson said:

I think I need to know more about this.  How much base (calcium hudroxide added?) do you add to the rosin?  The thing that confuses me is that rosin is 100% abietic acid; so it is 100% an organic acid.  The organic acid will react with a base such as calcium hydroxide or sodium hydroxide to make a soap., but I know you cannot react all the organic acid (rosin) and still make a varnish.  The varnish recipe books talk about a 5% addition of base to the rosin--is that what  you mean?.

I have been studying the literature to find out what is the chemical reaction between linseed oil and rosin when heated, but I cannot find the answer.  It may be the diels-alder reaction that allows them to chemically combine to make a varnish.  Drying of the varnish is by free radicals that crosslink the varnish using the unsaturation--they are naturally present in the cooked varnish and also form by oxygen that penetrates the film plus the action of UV light.  Metal oxides catalyze their formation such as are found in Japan Drier.

The Mariana manuscript 399 as mentioned by Mrs Merrifield mentions the addition of a small amount of alum to the cooked varnish, and this would supply a small amount of potassium and aluminum to react with the rosin.

The old varnish makers used a small addition of calcium hydroxide to make the varnish harder and more water resistant.

regards

Mike D

regards

Mike D

I am not sure it makes sense to talk about a soap in this matrix as there is no water. Also abietic acid is not a lipid. But reaction with glycerol makes an ester gum which has drying properties. The glycerol will form mixed esters ( groups per glycerole) with the fatty acids of linseed oil and abietic acid. As abietic acis is present in excess it will push the equilibrium towards its ester product. But 2 of the 3 fatty acids from linseed oil can cross link with the fatty acids from linseed oil.

These reactions are well understood in acqueous solution, as used in Frye's method of making varnish. There you get a soap, and it feeld like soap before adding alumn. In the oily matrix alumn will mainly icorporate some metals. I have also added FeCl2 to get some red colour (you really want FeCl3 but it is oxidised in situ).

A base is probably simply a catalyst. It makes the process a but faster. Addng metab oxides probably also catalyses the 2nd reaction, the corss linking - the process is a completely different reaction, probably involving radicals.

 

 

Abietic acid is an organic acid

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This is way beyond what a violin maker wants to know, and I just read this varnish chemistry out of interest. From what I read I assume that adding a little phtalic anhydride would make varnish dry faster.

If someone wants my literature collection on varnishes let me know, I have a lot. There is an interesting master thesis on the internet entitled MODIFICATION OF TUNG OIL FOR BIO-BASED COATING by Narin Thanamongkollit presented to the University of Akron.

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On 4/17/2020 at 12:38 PM, Michael_Molnar said:

The excess lime precipitates and collects on cool surfaces. It’s a bit of a craps shoot. I have never seen anything suggesting too much lime. I imagine that some acidity is needed.

@joerobson is the expert here, not me.

 

Adding calcium hydroxide is a bit of a moving target.  Some rosins are more acidic than others.  Pre-cooking the rosin also changes the amount of lime that is necessary.  Excess lime can be seen in the fractured surface of the cooled resin.  If left in the resin it will combine with the linseed oil when you make varnish.  This will also alter the nature of the varnish.  In general, too much lime will give a chippy brittle varnish that is difficult to polish correctly.

on we go,

Joe

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The limed rosin IS slightly hygroscopic but almost not water soluble. It’s changes in real life are negligible. If you would lime all the acidity away, the melting point of the colophony might go into the high 280+ °C region. 
 

Potash colophony is extremely hygroscopic, therefore not usable for varnish without lake pigments or similar. On the other hand Potassium and Sodium hydroxide neutralized rosins can make a pretty transparent varnish with lakes, because they can react with lake pigments to colored rosinates. If you use enough lakes, they are not hygroscopic, too.


 

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It's worth noting that gum rosin, as sold by kremer or diamond g, is not 100% abietic acid. There is also pimaric acid and other stuff involved, but the abietic acid is the largest and most important component. 

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