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Lake of cochineal is violet; I've been lied to


Deo Lawson
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21 hours ago, Michael_Molnar said:

I’ll emphasize what others have said. Cochineal’s color is dependent on the metal used to make the lake, the pH of the mix used to precipitate the lake, and the pH of the medium in which it is mixed. The strong low pH (acid) of oil varnishes pulls the color red-side. However, the acidity can make the lake fugitive. This is a difficult balance to achieve. I love the colors of cochineal but avoid it because it is so temperamental. 

Everything I've been able to find to read online has supported this. If I decide to go any further beyond this current project, I'm sure I'll get Kirby et al's book. I'm going to follow Bursch's recipe for carmine lake, who says "The  majority  of  the  recipes  for  carmine,...  differ  but  little  from  the  above [Cenette's method] ; acid  potassium  tartrate  is  used  instead  of  the  oxalate [I'll be using cream of tartar],  but  the latter  is  to  be  preferred  because  of  the  slight  solubility  of  the tartrate.  It  is  important  not  to  use  too  strong  decoctions  of cochineal,  and  to  add  only  small  quantities  of  alum" (I will likely err on the shy side with the tartar). He does not specify a quantity of alkali solution to use, and I'm thinking that's because you only should add a drop or 2 at a time, and let the dye bath foam up and recede, and then another few drops. 

My plan is to make 2l of dye bath from 40g ground cochineal, in two batches, one with 1/2 the tartar. I'll filter and divide those 1l batches in half and add a little less and a little more alum to each pair of containers. (All the recipes say to use shallow pans for precipitating the lake; Mason jars is what I have, fingers crossed that it will work with the relatively small amount of dye bath I'm making.) The jars are a quart capacity; It seems to me like that should work, knock on wood. Bursch says (if I'm reading it right) to add the alum to the simmering dye bath and simmer for another several minutes; but every other source I found seems to recommend dissolving the alum in boiling water and then adding it to the dye bath after filtering. I'm leaning toward adding the alum after filtering. 

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On 2/6/2022 at 5:23 PM, Don Noon said:

In a few days I'll be making some Michelman resin with cochineal.  Stay tuned, as Mike says.

Simplified, a base (KOH) was added to the cochineal to dissolve it, resulting in purple (as expected).  Acid (HCl) was added to the precipitation phase to neutralize the base, giving a nice scarlet.  However, in washing and filtering the resin, most of the red component washed out, giving a very purple resin.  Useless, until purple violins become a huge fad.

I'll stick with alizarin.  It's cheaper, too.

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6 hours ago, Don Noon said:

Simplified, a base (KOH) was added to the cochineal to dissolve it, resulting in purple (as expected).  Acid (HCl) was added to the precipitation phase to neutralize the base, giving a nice scarlet.  However, in washing and filtering the resin, most of the red component washed out, giving a very purple resin.  Useless, until purple violins become a huge fad.

I'll stick with alizarin.  It's cheaper, too.

Alizarin is (arguably) the most stable of natural organic red pigments. (PR 83)

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