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violins88

DAP plaster of paris

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I used DAP plaster of Paris. 

Mix 4 pounds with 4 gallons water.

Stir for one hour, then stir each half hou for 6 hours.

Next day, stir and let settle for 30 minutes.

Pour top two gallons into another 5 gallon container. This will be smaller particles.

Add 2 gallons water to this smaller particle mix.  Stir. Let settle 30 minutes. 

Pour top two gallons into an empty 5 gal bucket.

Let this this settle for a few hours.

Pour off the clear liquid.

Strain the remainder through a cloth.

At this point, for me, it got interesting. This substance has the consistency of yoghurt. When I took some of this and applied it to wood, it had much too much water. So I removed some water by patting the mass with paper towels, thus removing water. Eventually I achieved a useful consistency.. 

 any thoughts from chemists on this?

 

thanks

 

 

 

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No. Just use it wet. Well, barely wet. It seems easier that way. I will store it in a plastic bag and see if it molds, mildews or decays.

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If you are trying to use Roger Hargraves formula I think you need pure gypsum. Dap POP has other things in it. I have stirred it for a week, on a drill press with a paint stirrer changing the water twice a day and never achieved anything that would not still set up like concrete,

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PoP is only one type of filler. There are many you can choose -- I've used cheap Behlen pumice on the last two and it worked out well. I mixed it into the varnish and rubbed it in (see Gegg Alf article in the Strad)

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I believe Dap is "pure" plaster of paris. According to it's msds. It has no vinyl or other filler/plasticizers  

https://www.dap.com/media/33826/00071008001english.pdf

I would say that the "key" to pop is to make sure it has "decomposed" , meaning freshly made pop has a cement like quality to it where it will act like , well, plaster, and harden, the key with degraded pop as a pre grain fill/ mineral ground is that you want to "kill" any of the cement like quality of the plaster, basically by mixing it up, thickish, letting it almost dry out all the way, kinda kneading it a bit during the first drying stages , then re soaking the dried out pop, then after drying it out the second time around, it will no longer harden like plaster. this is somewhat important for application so as to not have any build up and to be able to be wiped off down to just leaving it in the grain. If it's done wrong it can dry on the instrument with a whitish uneven haze that will require resurfacing to remove it

so the way to make it work is to mix it pretty dry, to knead it once mixed during the drying process, to let it dry out almost way, that is super important, you need to let the chemical process start, and just before it turns into a brick, and is all dry and crumbly like dried out playdo, then it can go back onto the water to soak...take a small amount out, and let it dry, shape it into a pinched hershey kiss, once completely dry, you should be able to smash the "kiss" with one finger, and or it should be more like chalk that got wet, holds a little, but easily re-powdered, if it dries hard like plaster, you need redo the process, making sure it's quite dry before re-wetting 

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I think the whole point of washing, washing, and washing some more is to eliminate the lime from the gypsum. It's the lime that makes it solidify.

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3 hours ago, Bill Yacey said:

I think the whole point of washing, washing, and washing some more is to eliminate the lime from the gypsum. It's the lime that makes it solidify.

That is important. I need to check the pH.

Thanks

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10 minutes ago, not telling said:

You might be able to buy dehydrated lime at the hardware/garden store, which might be essentially the same, minus the work. Worth a try?

POP is based on calcium sulfate while lime is forms calcium carbonate.

POP often contains some percentage of lime powder and quartz as fillers.

3 hours ago, violins88 said:

That is important. I need to check the pH.

Thanks

Isn't lime added to linseed oil or rosin to reduce acidity? Perhaps the small amount of calcium carbonate won't do a harm...

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Interesting idea to reduce the size of the particles. 

What I know about micro particles is basically because the overall surface is bigger as compared to the same mass formed in one particle, it behaves differently. I would think the yoghurt like consistency comes from there because more water molecules can adhere to the surface of the small particles.

I would try to wipe tests thinly on a glass plate to see if it gets transparent with drying out.

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There was a thread about this few years ago. I remember that the crystals are transparent but the light scattering on the surfaces of the powder makes it look white. Once it is fully embedded in varnish or similar medium it becomes transparent (or almost, depending on RI match)

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14 minutes ago, JacksonMaberry said:

I like the kremer super fine marienglas. Nothing but gypsum, and a jar will last you ages. No stirring required! 

Mixed with varnish to a tooth paste consistency, I think it took less than a teaspoon of marienglas to ground/seal a viola.  One jar will definitely last a while.

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Gypsum, which is what is formed when plaster of paris reacts with water, has a strange but possibly useful property: The solubility decreases with increasing temperature. So if you let a cold solution stand until it is saturated, it will become milky with superfine crystals when heated even slightly.

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If you are using the paste, you are probably putting it on way too thick and run the risk of including opaque contaminants on the wood.

Gypsum has decent solubility in water. So just add a spoonful of PoP or gypsum at a time and shake vigorously until no more will dissolve and paste starts to rapidly precipitate out. Let it stand, then filter the "clear" liquid

This clear liquid is a saturated solution of gypsum. Pad it on the wood and let it dry. As it dries, the gypsum will precipitate out as fine crystals and looks like a white, crusty haze on the wood. You can pad on additional layers until you get a consistent covering, especially on end grain.

The refractive index (RI) of the crystal is around 1.53, about the same as most spirit and oil varnishes. Since a crystal is also transparent, once you cover the haze with a varnish with a similar RI, the full transparency of the crystals is restored.

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