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Plaster of Paris ground


violins88
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I read Roger Hargraves’ method, then did my own experiment. Three tablespoons of POP in one Liter of water in a glass jar. I shook the mixture intermittently for one hour. Then let it settle for 10 minutes. There is a settling line visible. Pour the top portion into a second container. Now, into the first container, put clean water and fill to the top. Shake, and allow settling for 10 minutes. Pour off the top portion into a third container. Discard the contents of the third container, but NOT into a sink or drain.

Refill, shake, let settle. Pour off the upper portion as before. Repeat several more times.

Finally, pour into a coffee filter. 
 

Now it gets interesting. The white portion seems to hold the water, instead of draining, like sand would do. 
 

For our maestronet chemists, what is happening, please?

 

Thanks.

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A similar thing happened when I made madder lake. I think the particles actually plug up the filter and prevent it from draining.

Perhaps let it dry in the filter and crush it with a mortar and pestle after. If you washed out enough of the lime, it shouldn't really harden into a solid mass.

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I think there were some extensive discussions about that and how to make that more effectively.

The whole process is carried not too allow creation or large crystals of the plaster. You are constantly breaking it and only small crystals are created.

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3 hours ago, HoGo said:

I think there were some extensive discussions about that and how to make that more effectively.

The whole process is carried not too allow creation or large crystals of the plaster. You are constantly breaking it and only small crystals are created.

Yes, you have created hydrated plaster "mud".

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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.

 

 

 

 

 

 

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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.

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1 hour ago, Shunyata said:

Does anyone else find that plaster ground kills the chayotancy (shimmering flames) of highly figured woods?  I have quit using plaster ground because of this.

I had the same impression, compared with pumice the latter seemed decidedly superior from this point of view (not to kill the chatoyancy).

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I haven't found any chatoyance issues. However, I apply the gypsum mixed with my varnish in a paste like consistency (no water). Or maybe my varnish (colophony/linseed oil) is a better refractive index match to the gypsum?

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37 minutes ago, Jim Bress said:

I haven't found any chatoyance issues. However, I apply the gypsum mixed with my varnish in a paste like consistency (no water). Or maybe my varnish (colophony/linseed oil) is a better refractive index match to the gypsum?

Perhaps, but in any case I didn't mean that gypsum kills refraction, just that pumice does less, at least mixed in my mastic and rosin-based ground with just turpentine without oil

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52 minutes ago, JacksonMaberry said:

Your gypsum thing (you showed me at my place in Indiana) seems to work much better than the PoP mud for whatever reason

I'm lazy, or at least have very little free time so I purchased my gypsum from Kremer. I did side by side test samples on spruce and maple using the Kremer gypsum mixed with varnish, applied with water first, and POP (wet) taken from Joe Thrifts bucket. End results were the same. I just like working without water better. The difference between the home made POP and what I purchased is that the Kremer stuff has finer particle size, so maybe it has better (more) pore penetration. As I apply it, 1 mm thick test samples were sealed with no penetration to the other side. I don't know if this is the best approach, but I've become familiar with the application method and it works well for me.

https://shop.kremerpigments.com/us/shop/pigments/11810-selenite-marienglas-fine.html

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6 hours ago, Torbjörn Zethelius said:

You're supposed to leave it in water for a month. It will break down (rot) by itself into a fine powder.

this, >or like she said, "you're doing it all wrong" :lol:....understand the "basic science" and then everything else is easy....you have stuff that is meant to be for lack of better terms "chalky concrete" and you can "decompose" or make it so its just chalky and not concrete by simply mixing it up , leaving it in water submerged, then pouring off the top water after a bit {a week}then topping it off with fresh water a couple times, stir it a few times, let it resettle, and then after about a month or so the stuff that would have made it bind together and dry like a rock has been washed away and left ineffective for it's initial purpose. But now it makes a very nice "transparent" grain fill that will also introduce "minerals" into the varnish base coat , there are other ways to get basically the same thing but it is hard to beat the "workability" of POP when doing grain fill on the wood surface.

related to varnish layer systems this is NOT quite the same "effect" that one would get if they did a shellac and pumice grain fill, as that puts the mineral layer above the first initial finish layer and is not the same type of mineral and does not have the same "wet'ed" refractive index as the POP which will yield a better looking system imo

 

 

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3 minutes ago, jezzupe said:

this, >or like she said, "you're doing it all wrong" :lol:....understand the "basic science" and then everything else is easy....you have stuff that is meant to be for lack of better terms "chalky concrete" and you can "decompose" or make it so its just chalky and not concrete by simply mixing it up , leaving it in water submerged, then pouring off the top water after a bit {a week}then topping it off with fresh water a couple times, stir it a few times, let it resettle, and then after about a month or so the stuff that would have made it bind together and dry like a rock has been washed away and left ineffective for it's initial purpose. But now it makes a very nice "transparent" grain fill that will also introduce "minerals" into the varnish base coat , there are other ways to get basically the same thing but it is hard to beat the "workability" of POP when doing grain fill on the wood surface.

related to varnish layer systems this is NOT quite the same "effect" that one would get if they did a shellac and pumice grain fill, as that puts the mineral layer above the first initial finish layer and is not the same type of mineral and does not have the same "wet'ed" refractive index as the POP which will yield a better looking system imo

 

 

edit, I Just read Davide response, and I can't say one way or another which would yield a "clearer" layered system as there may be quite a wide variety in specific products that we all may be using. AND fwiw I do primarily do pumice as I find it less "intense" to deal with AND so much of this gets down to the individual grain in the material you are using, AND that bigger instruments tend to have wider grain section that dramatically benefit from some type of grain fill, whereas some violin material is so fine that it almost repels grain filling 

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5 hours ago, 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.

Too controversial Jackson. Best keep it a secret. 

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I've tried applying the POP wet (mixed with water like what Roger showed in the bass book) and mixed with varnish also discussed in the book but not illustrated -  found the latter easier for me

Wear a mask - the POP stuff is really fine and probably not great to breath in!

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Thanks to all. Especially ctanzio, because now I feel like I should experiment with the saturated solution. Sounds neat. But I don’t want too much water on the violin. I presume a sealer must be applied before the saturated solution is applied.

New idea, since I have been reading on the subject. What about using plain chalk? Calcite.

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What is the RI of chalk?

I think if the crystals are stable and have RI close to varnish than they would work very similarly.

Perhaps the exact shape of the crystals matters, but I haven't seen any discussion about that.

I just found that transparent calcite crystals show optical property called birefringence... That could be interesting new point for marketing the violins made with that in ground :-)   (after all the talk about dichromatism and dichroism).

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Now I will partly reply to my own post....

The POP crystals formed by Hargrave method are clear and with RI= 1.52

varnish is somewhere  between 1.52 and 1.57 so the crystals become invisible once the varnish surrounds them.

The ordinary chalk is not formed by clear crystals and would muddy the varnish.

Clear crystals of calcite have RI somewhere in 1.5-1.6 range so could be of use as well. You would have to either grind them finely or create them in a proces similar to hargrave method...

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Modern chalk is actually gypsum. Although traditionally, it was made with calcium carbonate. So you need to  be careful to get actual calcium carbonate and not decide solely on the labeling as "chalk".

Calcium carbonate has a variety of crystal structures and not all of them are transparent. Although I know gypsum will precipitate as a clear crystal, I am not certain that will happen with solutions of calcium carbonate prepared at room temperature. Unless it precipitates as a clear crystal, it will not give a transparent finish to the wood when varnish is applied to it.

The refractive index of calcium carbonate is slightly higher than that of gypsum, but it should be "close enough" to typical varnishes to work if the crystals are transparent.

 

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4 hours ago, HoGo said:

I just found that transparent calcite crystals show optical property called birefringence... That could be interesting new point for marketing the violins made with that in ground :-)   (after all the talk about dichromatism and dichroism).

I would consider bi-refringence to be undesirable, unless there is a way of lining up all the crystals on the same axis. Otherwise, multiple changes of the refractive index from different orientations of the crystals would tend toward making the the varnish film more occluded, muddy, or opaque.

However, there are multiple forms of calcium carbonate, with some having extreme bi-refringent properties, and others much closer to none.  (I have one large crystal which when held over a newspaper or magazine, will turn the print upside down). Some combination of rendering an object right-side-up, and rendering it upside down, will probably not result in optimal clarity.

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