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tango

After plaster of Paris

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Hi

 

This question is about some doubts I have about adderence.
I saw some scratches in my violins. Of course I can´t assure that is due to weak adderence or the owner careless.
 
So:
Is necessary to sand some after the "plaster of Paris" before the first coat of oil varnish coat?
 
I am afraid that a little of remanider sanding dust over a little rough surface would be worse.
 
 

post-38774-0-42729600-1476705591_thumb.jpg

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I didn't do any sanding, just tapping with my finger. If the plaster of paris is extrafine you won't have any problem. I used some yellow chalk (from a crayola sidewalk chalk :ph34r: ), but even if you use white chalk, it will turn yellow after some time. Mixed with linseed oil, it just became like any other pigment, ketchup consistency, easy to spread.

post-29508-0-80791600-1476707678_thumb.jpg

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Hi Pold

 

Yes, this "plaster of Paris" made with colophonia oil varnish and plaster  has a yellow hue.

The maple wood is some golden. I expect it will yield a nice brown when I will add coats.

Thanks for reply

Tango

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Hi Pold

 

Yes, this "plaster of Paris" made with colophonia oil varnish and plaster  has a yellow hue.

The maple wood is some golden. I expect it will yield a nice brown when I will add coats.

Thanks for reply

Tango

You are welcome, I didn't use any rosin (like Rembrandt...), only linseed oil.

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Tango, It looks to me like you have some harlequin effect going on, and some thing in the lower bass corner ? what number is this violin for you? overall it looks OK to my poor eyes, I'd be inclined to move on , except maybe a real light scrubbing with very fine abrasive , pumice on cloth,nothing to cut below the surface or you chance disturbing the uniformity of the surface. as far as adherence goes if you are referring to the light on one side dark on the other , that's a property of the wood and it's cut, nothing you do will change that.    

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Hi James

 

That is only dirt. Is a little of fresh plaster with varnish I took off when saw the photo in the pc.

The dark zone of the middle bout is very dense wood, very different from the clear wood.

Thanks for Reply

Tango

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A question for all the folks using plaster of paris: Are you using it as-is, or washing the lime out as Roger described in his bass building thread?

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Hi

Bill.
I made three times exactly as Roger explained. One with water and two with oil varnish.
 
Thanks Not telling for compliments.
 
Regards
Tango

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A question for all the folks using plaster of paris: Are you using it as-is, or washing the lime out as Roger described in his bass building thread?

I don't believe any lime gets "washed out" the plaster is being slaked , while being stirred this produces small grains of cured Plaster that do not stick together when dried. I've used it both dried and mixed w/ varnish and suspended in water, both methods seem to have advantage and disadvantage. 

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I don't believe any lime gets "washed out" the plaster is being slaked , while being stirred this produces small grains of cured Plaster that do not stick together when dried. I've used it both dried and mixed w/ varnish and suspended in water, both methods seem to have advantage and disadvantage. 

Isn't Plaster of Paris Calcium Sulfate,  anhydrous?    Lime and slaked lime do not have the sulphur.  

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Would there be any disadvantage to just purchasing hydrated plaster powder in the first place, rather than making it yourself?

 

Here's one source which might have something to say about the various forms of hydrated plaster, including acoustical effects.

http://pubs.acs.org/doi/pdfplus/10.1021/jp210601a

 

I haven't paid to read anything beyond the "abstract", but maybe someone else here has free access to the full paper, and could post a summary, including an opinion on whether this could be in any way useful to us.

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why not to buy calcium carbonate on ebay?

Arguably, because calcium carbonate isn't the same thing as calcium sulfate? Not that I know whether or not it matters, for what we're doing.

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Arguably, because calcium carbonate isn't the same thing as calcium sulfate? Not that I know whether or not it matters, for what we're doing.

I think it doesn't matter whether is sulfate or carbonate, I would buy both, the price is the same, maybe the calcium carbonate is easier to find, coarse or extra fine. I don't know for food, but in painting, lime and gypsum can be interchanged for each other.

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Isn't Plaster of Paris Calcium Sulfate,  anhydrous?    Lime and slaked lime do not have the sulphur.  

correct,according to wiki ...always something to learn.... .. I guess I wasn't trying to be to specific as much as to point out that nothing was leaving the solution , the process is designed to cure the plaster into fine partials fit to fill the pores . 

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Would there be any disadvantage to just purchasing hydrated plaster powder in the first place, rather than making it yourself?

 

Here's one source which might have something to say about the various forms of hydrated plaster, including acoustical effects.

http://pubs.acs.org/doi/pdfplus/10.1021/jp210601a

 

I haven't paid to read anything beyond the "abstract", but maybe someone else here has free access to the full paper, and could post a summary, including an opinion on whether this could be in any way useful to us.

Hydrated plaster is hard, dried plaster.   The added water is incorporated as "waters of crystalization"  and it is why plaster "sets up."   I find that plaster of paris plus a little ordinary (both anhydrous) seem to fill well.   After the coat is rubbed to the surface,  one can see the wood and also places where the material is in the end-grain.  

 

I read the link top the experiment.  I wonder if the idea is to do this in a LOT of water.

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  I find that plaster of paris plus a little ordinary (both anhydrous) seem to fill well. 

What do you mean by ordinary, gypsum? You were mixing the two components together?

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Tango,

 

Have you tried mixing the plaster with thin hide glue, gelatin etc. instead of water? Roger's plaster is the noble material which made up the ground on so many of the great paintings from the Renaissance onward, and they invariably mixed it with glue of some sort (the wooden panels were also previously glue sized as well).

 

Additionally, they most often would apply another very thin coat of glue on top of this to entirely exclude oil staining the ground, a practice which some later authorities thought unnecessary. But this practice originated presumably from Cennini's warning that "oil corrupts gesso."  However, I did try brushing a very thin layer of hide glue over my usual ground (with particulate but not plaster) on one half of a piece of spruce, and I was quite surprised by the result: the depth of the figure seemed to disappear when varnish was applied to the glue side, but not the side without glue. I very much preferred the look of the side without the glue because of the visually more interesting appearance (I've got a picture of this test if you're interested in seeing it).

 

It is also possible to color this top glue layer by adding saffron to water, soaking it until the desired intensity of color is achieved, and then removing it, adding the hide glue to swell as usual. The tint is most beautiful and the lower R.I. of the glue produces a luminous glow when varnish (with a higher R.I.) is applied, which I think is superior to that from tinting varnish. I think some here have also tried this.

 

I don't like the idea of mixing plaster with varnish: I think it's simply not that effective at absorbing the varnish mixed with it, let alone the varnish which is applied on top. 

 

As for this idea of adding glue, yes, applying it thinly and evenly will take some practice and experimentation, such as whether to apply it by brush or rubbing it in. Likewise, the precise plaster to glue ratio would be a matter of personal preference.

 

Regarding your question about sanding, I would think the wrong grit would leave scratches, so "water planing" would be an option if using glue. You make a pad (as in French polishing), wet it with water and rub the surface to remove surplus plaster. You have to make more than one pad as the plaster will eventually wear through after rubbing. I do understand your reluctance to use water near the glue joint, though. Also, I produced more than one failed ground where the particulate remained defiantly noticeable and did not disappear after varnish was applied, so for this reason I abandoned the idea of using anything approaching plaster in consistency. However, some of my grounds were nearly cement, and these became water resistant when dry, and therefore couldn't be easily reworked even when very thin.  

 

Also, regarding the making of the slaked plaster, Lady Christiana Herringham has in her 1899 translation of Cennini some very interesting notes and commentary. She has this to say about slaking plaster:

 

 

"The Italian name for burnt or baked gypsum is gesso grosso; when it is slaked it is gesso sottile. Slaking is wetting the gesso grosso with so much water that it is unable to set. It recrystallises into needle-shaped filaments, which in a way felting together, help to give it the pliancy and toughness which it possesses in a marked degree, and which gesso grosso and whitening are quite deficient in, they being composed of amorphous particles like sand, in no way cohesive. It seems to me a mistake to grind gesso sottile, as nothing should be done to disturb its crystalline conformation." (Emphasis mine, p. 233 "Grounds and Size" chapter)

 

 

Sorry if I've run on, but you may find this interesting.

 

 

 

Dave 

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 I very much preferred the look of the side without the glue because of the visually more interesting appearance (I've got a picture of this test if you're interested in seeing it).

 

Hi Dave

 

I will translate slowly to understand well what you explain. And yes, please upload the photo to see resulta.

Thanks for your very interesting aproach.

 

Regads

Tango

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What do you mean by ordinary, gypsum? You were mixing the two components together?

Whatever is used on walls.  Plaster of Paris was too  fragile.  By the way,  I did not use 100% water.   I mixed either or both with 50-50 water/alcohol to insure that the set-up did not make a completely finished crystaline substance.  (I guessed that forming waters of crystalization would be impeded with the 50% alcohol,  and that turned out to be true....... (lucky me.)    I think that this approach bypasses the tedious business of long-term slaking of lime or other attempts to form micro-crystals before application.   

 

The "ordinary" plaster makes the applied mix with Plaster of Paris  more durable.  I want it to be firm but not crumbly.   I rub it down to flush with the wood after it dries.  (With dry paper towels)

 

Not many people reply to my posts.  But BELIEVE ME,   interrupting the formation of full-crystaline gessos with alcohol is a very helpful thing.  I like chemistry,  and knowing what happens with materials can give great hints for experimentation.

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In reading about plaster on Wikipedia, it is mentioned that lime plaster was commonly used for plaster and lath walls in buildings, often mixed with Pozzalan to speed up the curing time. My only concern is they indicate a ph level of about 12 in the liquid state. Long term this may be some concern over wood deterioration.

 

The gypsum plaster sounds more benign in comparison.

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In reading about plaster on Wikipedia, it is mentioned that lime plaster was commonly used for plaster and lath walls in buildings, often mixed with Pozzalan to speed up the curing time. My only concern is they indicate a ph level of about 12 in the liquid state. Long term this may be some concern over wood deterioration.

 

The gypsum plaster sounds more benign in comparison.

 

I use what is called "plaster" in the modern sense.  (Finishing plaster)   I think it is likely of the benign sourt.  The amount used is only about one fifth of the total solids.  In any case,  It is not in acqueas state.  Any sealer ought to isolate it from wood or varnish.  I have had no problems of interaction with varnish.

 

It is also extremely low penetration,  which of course is the idea in first place.  For lath walls and directly over the external wall tile (with lath-like ridges) like I have here in my old house, there is a coarse base-coat.  I don't think that it is straight plaster as in finishing plaster.  It looks a little more like concrete/mortar.

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