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Plaster of Paris ground


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On 5/19/2021 at 11:07 AM, ctanzio said:

Plaster of Paris is Calcium Sulfate: a highly basic (corrosive) solid that undergoes an strong release of heat when water is added.

Calcium Sulfate combines with water to form Calcium Sulfate diHydrate: a pH neutral, low solubility transparent crystal with an index of refraction compatible with most common violin varnishes. It occurs naturally in nature under the name "gypsum".

Because the diHydrate as two water molecules attached to it, I seem to recall it becomes slightly "polar", meaning it can attract nearby water molecules to form a suspension with the consistency of mud. But once all the excess water evaporates, it will harden into a firm, solid plaster.

The "clear" water  from step 1 of your preparation will be a saturated solution of the dihydrate. You can dab this onto the wood surface and after the water evaporates, the dihydrate will crystalize into a fine, white haze that can fill even the smallest voids in the wood surface.

One can repeat this procedure to completely seal the surface to varnishes so none of the varnish will seep into the wood. Once varnish is applied, the white haze completely vanishes and the transparency of the finish is a function only of the transparency of the varnish itself. The downside is that this method of application tends to raise the grain. So you might want to prepare the surface before application to minimize this.

You can also use the "mud" to apply a much thicker coat in one go, and rub it down to thin the coat and get the desired sealing effect. Same risk of grain raise. The main reason for multiple washings of the mud is to make sure all of the PoP is fully hydrate because the unhydrated PoP can be corrosive. I am not sure that much washing is really needed. But since my experience as an analytical chemist ended many years ago, and water is relatively cheap and easy to get, then wash away!

I prefer using the saturated solution. It is easier to control the degree of application, and it creates less of a mess on the wood. My personal feeling is one does not need a thick layer from using the mud to get a good seal on the wood, and it also avoids the application of impurities in the PoP to the surface.

Of course, impurities in the mud is not an issue if one starts with high purity PoP, and one can hardly be faulted for following the method of a master, like Roger Hargrave.

 

 

 

 

 

 

Well said.  Thanks.

POP, gypsum,  pumice, or others are all workable as grain fillers mixed with water, linseed oil, or varnish as you prefer.

The trade offs in texture and illumination must be considered.

When I was using and experimenting with this process my preference was Kaolin.  It absorbs oil or varnish very well.  It's particle size is small. The IR is 1.57.

on we go,

Joe

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On 5/19/2021 at 4:52 PM, JacksonMaberry said:

I have used the plaster of Paris ground extensively, and I find that it is in general an inferior option. It does tend to overdo the sealing of the wood surface. Given the recent research from Bridget Brandmair and Jean Philippe Echardard, I would suggest that an oil/resin ground is closer to what the ancients would have used and in my experience produces a superior result.

I would try that but concerned about end grain. Do you not fill endgrain with anything?
So the oil/resin ground is basically a varnish but with a higher ratio of oil? 

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On 5/19/2021 at 4:52 PM, JacksonMaberry said:

I have used the plaster of Paris ground extensively, and I find that it is in general an inferior option. It does tend to overdo the sealing of the wood surface. Given the recent research from Bridget Brandmair and Jean Philippe Echardard, I would suggest that an oil/resin ground is closer to what the ancients would have used and in my experience produces a superior result.

I would try that but concerned about end grain. Do you not fill endgrain with anything?
So the oil/resin ground is basically a varnish but with a higher ratio of oil? 

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1 hour ago, Sapiens said:

I would try that but concerned about end grain. Do you not fill endgrain with anything?
So the oil/resin ground is basically a varnish but with a higher ratio of oil? 

test everything on scrap , several times, prior to using it on work

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5 hours ago, Michael_Molnar said:

Can someone save me some time and list the research references that found PoP?

Are you asking for references that found PoP in the finishes of old violins? I do not think you will find any, as PoP is processed gypsum. By the time the PoP finds its way onto a wood surface, it has been hydrated back to gypsum.

Various chemical analyses and spectral scans identify concentrations of calcium and sulfur in some Strads, Amatis and Gaurneris that exceed what would occur naturally in spruce and maple. So the inference is that some chemical compound commonly available at the time was used hat contained these elements. That could be calcium sulfate which occurs naturally as the mineral "gypsum" and was known in the 17th and 18th centuries.

There are also higher than expected amounts of Aluminum and Silicon in some old violins, which would suggest pumice sourced from volcanic areas, or the natural clay commonly called kaolin. But kaolin tends to be opaque, while pumice can be translucent.

Mercury is another element detected in unusual amounts for natural wood, that and the presence of sulfur would suggest a red dye, like cinnabar. This is a highly poisonous coloring and is no longer used.

Lead is also detected, which was a common dryer added to oil varnishes.

How the calcium sulfate or pumice, if used, was actually applied is anyone's guess. Although I believe I read an article that found writings in ancient times of making a paste of drying oils and crushed minerals to be used as a wood sealer prior to varnishing.

It might make an interesting research project to gather the many research references in one place, but modern varnishing materials are so superior to what was used back then, why bother? The real secret to varnish is mostly in the application, IMNSHO.

 

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7 hours ago, Michael_Molnar said:

Can someone save me some time and list the research references that found PoP?

Thanks

See p.8 etc., in this paper: https://www.sciencedirect.com/science/article/abs/pii/S0026265X19321204?via%3Dihub

Also see: https://www.spectroscopyeurope.com/article/surfing-through-coating-system-historic-bowed-instruments-spectroscopic-perspective

If(???) some form of gypsum is indeed involved, it could well be in a form other than POP.

Not a violin study but may still be of interest: https://link.springer.com/article/10.1007/s00339-008-4449-7

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5 hours ago, John Harte said:

See p.8 etc., in this paper: https://www.sciencedirect.com/science/article/abs/pii/S0026265X19321204?via%3Dihub

Also see: https://www.spectroscopyeurope.com/article/surfing-through-coating-system-historic-bowed-instruments-spectroscopic-perspective

If(???) some form of gypsum is indeed involved, it could well be in a form other than POP.

Not a violin study but may still be of interest: https://link.springer.com/article/10.1007/s00339-008-4449-7

Thanks John.

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7 hours ago, John Harte said:

See p.8 etc., in this paper: https://www.sciencedirect.com/science/article/abs/pii/S0026265X19321204?via%3Dihub

Also see: https://www.spectroscopyeurope.com/article/surfing-through-coating-system-historic-bowed-instruments-spectroscopic-perspective

If(???) some form of gypsum is indeed involved, it could well be in a form other than POP.

Not a violin study but may still be of interest: https://link.springer.com/article/10.1007/s00339-008-4449-7

Knew you would come through with the facts. Thanks a million.

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9 hours ago, ctanzio said:

Are you asking for references that found PoP in the finishes of old violins? I do not think you will find any, as PoP is processed gypsum. By the time the PoP finds its way onto a wood surface, it has been hydrated back to gypsum.

Various chemical analyses and spectral scans identify concentrations of calcium and sulfur in some Strads, Amatis and Gaurneris that exceed what would occur naturally in spruce and maple. So the inference is that some chemical compound commonly available at the time was used hat contained these elements. That could be calcium sulfate which occurs naturally as the mineral "gypsum" and was known in the 17th and 18th centuries.

There are also higher than expected amounts of Aluminum and Silicon in some old violins, which would suggest pumice sourced from volcanic areas, or the natural clay commonly called kaolin. But kaolin tends to be opaque, while pumice can be translucent.

Mercury is another element detected in unusual amounts for natural wood, that and the presence of sulfur would suggest a red dye, like cinnabar. This is a highly poisonous coloring and is no longer used.

Lead is also detected, which was a common dryer added to oil varnishes.

How the calcium sulfate or pumice, if used, was actually applied is anyone's guess. Although I believe I read an article that found writings in ancient times of making a paste of drying oils and crushed minerals to be used as a wood sealer prior to varnishing.

It might make an interesting research project to gather the many research references in one place, but modern varnishing materials are so superior to what was used back then, why bother? The real secret to varnish is mostly in the application, IMNSHO.

 

I was looking for references, but thanks anyhow.
Listing detections gives the erroneous implication that ALL or MANY instruments had these. That is not true. The maker and date must be attributed as well. Don’t forget that fiddling is human nature (pun). 

Your last sentence is my mantra. 


 

 

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5 hours ago, Michael_Molnar said:

I was looking for references, but thanks anyhow.
Listing detections gives the erroneous implication that ALL or MANY instruments had these. That is not true. The maker and date must be attributed as well. Don’t forget that fiddling is human nature (pun). 

Your last sentence is my mantra. 


 

 

True Michael.  There is also the implications that these substances were intentionally applied as powders mixed with something to create a sealing boundary between the wood and the varnish.

on we go,

Joe

 

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Yeah, refractive index has become trendy now.

I think; This violin is grounded with ruby dust, sells better than that the ground has a refractive index if X

But seriously, transparency is much more important than refractive index.

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On 5/21/2021 at 9:01 AM, Michael_Molnar said:

I was looking for references, but thanks anyhow.
Listing detections gives the erroneous implication that ALL or MANY instruments had these. That is not true. The maker and date must be attributed as well. Don’t forget that fiddling is human nature (pun). 

Your last sentence is my mantra. 


 

 

 

On 5/21/2021 at 2:50 PM, joerobson said:

True Michael.  There is also the implications that these substances were intentionally applied as powders mixed with something to create a sealing boundary between the wood and the varnish.

on we go,

Joe

 

Joe,

I was replying and referring to ctanzio’s list that included mercury, lead, etc. My point is that one detection does not mean it was used extensively.

You brought up a interesting point that sounds like you support a model with gypsum. 

 

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50 minutes ago, Michael_Molnar said:

 

Joe,

I was replying and referring to ctanzio’s list that included mercury, lead, etc. My point is that one detection does not mean it was used extensively.

You brought up a interesting point that sounds like you support a model with gypsum. 

 

I'll be interested to hear Joe's reply. Judging by his product line, I'd guess he's not a mineral guy, but I could be (and often am) wrong.

For me, it's oil and resin all the way. Rocks are for geologists.

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6 hours ago, joerobson said:

True.

Joe,

Did you know, that thanks to you, I'm a varnish maker nowadays!

Not to complain, you have posted all your trade secrets over the years :)

Nevertheless, I learn more constantly. This summer I'm going to cook large amount of lighter varnish for various things, non violin related stuff.

Varnish making is a lost forgotten "not so secret"

THANK YOU!

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Joe is extremely generous with his hard won experience. We are all in debt to him and his products remain the favorites of some of the world's greatest makers. I'd be willing to bet he has some methods and insight that he keeps close to the vest, and I believe that's as it should be. 

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