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  #1  
Old 05-30-2007, 10:28 AM
ronfroim ronfroim is offline
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additives to make plaster stornger

hi, i'm looking for some kind of material that i can add to the mix of water and plaster to get a stronger material.
thanks
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  #2  
Old 05-30-2007, 11:08 AM
loupgarou loupgarou is offline
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Lightbulb Re: additives to make plaster stornger

Check out Sculpture House in New York and Skillman NJ. They sell a bone meal product--liquid--which gives added strength to plaster. But, make it easy on yourself and use Hydrocal or Hydrostone--both plaster based--that are miles ahead of plaster of paris in hardness.

Loys
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  #3  
Old 05-30-2007, 12:40 PM
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Landseer Landseer is offline
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Re: additives to make plaster stornger

"stronger" does not necessarily mean HARDER, and harder does not necessarily mean it won't chip or break.

Plaster of Paris is very soft, hydocal is much harder and you could lay a slab of it on a flat driveway and drive a car over it, but stand a heavy sculpture up on a hard surface on a corner or just bump it against something hard like another cast and it chips easy.
The USG specs say something like hydrocal will hold 5,000 pounds per sq inch compressive strength, sounds impressive doesn't it? but one little '"tap" on a hard surface and corners, sharp edges or fine details break off pretty easy.
Hydrocal's specs are stronger than concrete's specs, but both will chip.

I just discarded a toilet yesterday, I believe these are or were made of porcellain or at least high fire slip, well all it takes is a slight "tap" with a small hammer on the side to break the whole side, yet it will support a 300# fatty no problem, and withstand clorox, acids etc.

There's such a thing as TOO hard, and too hard can be brittle- thick in terms of cast iron and bolts/screws that are really hard and you torque them a little too tight and they just SNAP like cast iron instead of bend or "give"
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  #4  
Old 05-31-2007, 06:55 AM
ronfroim ronfroim is offline
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Re: additives to make plaster stornger

thanks for the important notes about the differernces between stronger and harder. what i need is stronger plaster,so a little tap of hard material on it wont break it. i'm making a circle mold made out of slices , and i cant take the risk that it will break. by the way,i'm israely,and i buy a plaster made in israel. from what you described,i seems like it is closer to the paris one.
buy i'm not sure about this.if you have any suggestion,i'll be glad
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  #5  
Old 05-31-2007, 10:06 AM
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Merlion Merlion is offline
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Re: additives to make plaster stornger

Plaster is strong in compression. But it is prone to crack and break when bent or knocked sideways. To avoid this, I tried mixing in short chopped glass fibers. It works well.

This is the picture of my bending test on a plaster plate. On the left you can see some chopped glass fibers.
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  #6  
Old 05-31-2007, 01:25 PM
Rojellio Rojellio is offline
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Re: additives to make plaster stornger

You might try "stealth" microfiber Microfiber for concrete Also some ready mix plants will sell it over the counter, often they have fibermesh 150. 1 pound is enough for 1 yard of concrete.

I use it in all of my concrete. The fiber does not show on the molded surfaces. I often expose the aggregate, which will also expose fiber "hairs".. a quick torching takes care of that.

For glass fibers, fiberglass supply places are the place to go. There are several different grades.

Either way it doesn't really strengthen anything. but it does help control cracks. If something does crack, or break.. it will not break away clean, the fibers will hold it together. Fiber improves the matrix, with millions of tiny fibers to bond to.
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  #7  
Old 05-31-2007, 11:25 PM
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Re: additives to make plaster stornger

hi all i use a material that is called M1 from rohm and haas compsites that a local suplaer inports it is a mineral crystals within an acrylic envelope i donot have there webstie as jet but will go look for it and then post it later. very strong
anton
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  #8  
Old 08-16-2007, 11:05 AM
MedFX MedFX is offline
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Re: additives to make plaster stornger

Acrylic additives like Stone Mason's Concrete Fortifier or Acryl 60 (sold at home centers like Lowes and Home Depot) can be added and will make the finished product both harder and more resistant to chipping and cracking. The downside is that the additives also make the plaster more prone to trapping air bubbles.

When you mix your plaster, keeping the water to a minimum will also improve the strength of the end product. You can usually get by with less than the amount recommended in the manufacturer's guidelines. Again the downside is that the product will be less flowable and will trap air bubbles more readily. Chopped sisal, hemp, or fiberglass will help prevent hairline cracks from becoming all out fractures and may add some structural strength when prying details out of the mold, etc. but they are not the answer to prevent surface damage.

Some plasters and cements can be hardened by baking them out at a relatively low temperature but the best hardener of gypsum is time. Keep your pieces in a warm dry environment for several days and the strength will greatly increase over the green state it was in when it came out of the mold. Along those same lines, don't get overly anxious to pull the sculpt out of the mold. Unless you are fighting production deadlines, you are better of ignoring your casting for a while before pulling it. Remember that the manufacturer's demold estimates are the minimum time, not necessarily the optimum time.
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  #9  
Old 08-16-2007, 12:28 PM
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Landseer Landseer is offline
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Re: additives to make plaster stornger

Quote:
Originally Posted by MedFX
Acrylic additives like Stone Mason's Concrete Fortifier or Acryl 60 (sold at home centers like Lowes and Home Depot) can be added
Ordinary white glue probably works just as well and I have seen it suggested for this purpose.

Quote:
When you mix your plaster, keeping the water to a minimum will also improve the strength of the end product.
If you go out of the recommended USG specs for specific water to plaster ratios this will affect both hardness AND set time, as will using warm water for set time.

Quote:
You can usually get by with less than the amount recommended in the manufacturer's guidelines.
The water ratio I have changed considerably when I wanted the stuff to set/harden FAST and be trowelable immediately, if I wanted these properties for something I use hot water, very little of it and mix, and the stuff while mixing is like dense whipped cream- ready to apply to a vertical surface and starts setting in a minute or two, and sets rock hard.

Quote:
Chopped sisal, hemp, or fiberglass will help prevent hairline cracks from becoming all out fractures and may add some structural strength when prying details out of the mold, etc. but they are not the answer to prevent surface damage.
I'm not clear on what is bing produced here that is THAT fragile, I never use sisal, reinforcements or fiberglass in any of my casts and as soon as I see steam rising from the back of the cast I pull it out of the mold, I don't have breakage.
Pulling it quicker is better for the mold not to be exposed to the continued heat, and the cast isnt going to be any harder while moist than it is when the steam is coming off and it's hot, so waiting till it cools is not going to be any stronger.

With the USG hydrocal, hydrostone etc you MUST mix with a mechanical mixer, and you must mix it for the time it says if you want the full properties. The MIXING is what creates a lot of the hardness according to USG.

Hydrostone and hydrocal B-11 I found I had to mix for several minutes, especially the B-11 which I had to mix with a drill mixer for 6 minutes per batch. If I mixed less than that time the casts would start showing water rising on the back in small columns like oil wells spurting up- that's showing its going to be a weaker cast and more porus, it would also take loger to set. The literature shows a graph for this stuff and maximum strength with about 12 minutes of mixing, I don't have patience to stand there that long mixing plaster.

With the hydrocal white it never does this, I mix for 1:45 and that's plenty, it starts setting in a minute or two.


Quote:
Some plasters and cements can be hardened by baking them out at a relatively low temperature but the best hardener of gypsum is time. Keep your pieces in a warm dry environment for several days and the strength will greatly increase over the green state it was in when it came out of the
You don't want to BAKE plasters, if the water evaporates in any portion before the rest and it gets above about 115 or so degrees it will start calcining and breaking down.
I once put a cast on top of the steam radiator to dry and broke in half when I lifted it up, it was little more than powder.
All you really need is a warm room and a fan on the cast turned on high the fan will dry it fast.
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  #10  
Old 08-17-2007, 12:37 AM
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Re: additives to make plaster stornger

hi all hope your all well . have you tryed forton mg casting sisteam goto www.lifecasting .org you mite have to sine up as member then goto the libbery site and look for forton mg give you the formula's ect
hope it helps
anton
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  #11  
Old 08-17-2007, 04:35 AM
furby furby is offline
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Re: additives to make plaster stornger

I was taught as an art student that egg whites make the plaster stronger.
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  #12  
Old 08-17-2007, 11:54 AM
MedFX MedFX is offline
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Re: additives to make plaster stornger

Quote: Ordinary white glue probably works just as well and I have seen it suggested for this purpose.

White glue is a vinyl product. The vinyl additives do reduce breakage, especially to impact but do not impart the surface hardness nor increase compression strength to the degree that acrylic does. Both products have benefits, but they are not the same. Be aware that vinyl can affect adhesion of some finishes. Cost per volume, vinyl to acrylic, is about the same when you take into account the ratios of water to additive.

Quote: I'm not clear on what is being produced here that is THAT fragile, I never use sisal, reinforcements or fiberglass in any of my casts and as soon as I see steam rising from the back of the cast I pull it out of the mold, I don't have breakage.

The original post did not specify what was being produced, castings, mother molds, etc. Fiber additives are useful for a number of purposes including mother molds and backfilling large objects. They are obviously not the choice for facing finished castings, or for making small to medium size castings. I looked back at how I posted that and it was poorly worded as it is not so much "details" as "protrusions" that was intended.

Quote: Pulling it quicker is better for the mold not to be exposed to the continued heat, and the cast isn't going to be any harder while moist than it is when the steam is coming off and it's hot, so waiting till it cools is not going to be any stronger.

All of my molds are silicone, most are addition cure silicone, with either ultracal or fiberglass mothers. True, even silicone is affected by heat but I have never had a problem giving my pieces a reasonable amount of time in the mold. I also cast urethane and epoxy which can get very hot. By "overly anxious" I mean to pull the cast before a reasonable amount of strength is reach and yes, even an extra 20-30 minutes can make a difference.

Quote: You don't want to BAKE plasters, if the water evaporates in any portion before the rest and it gets above about 115 or so degrees it will start calcining and breaking down.

Actually the optimum drying temperature for Hydrocal and UC30 is 110-120 F (direct from USG) but that doesn't mean to slap it on top of a radiator. I have built an insulated box with a heating element and circulating fan with a variable thermostat from 95 - 150 F. You can avoid calcination by watching your humidity and slowly lowering the temperature as the piece dries. USG has an entire tech sheet on drying including temperature/humidity curves. My cabinet cost less than $100 and I use it for everything from softening clay and wax, to "baking" silicone and gypsum. Perhaps I should have been more specific about "relatively low" temperatures, but the curve varies so much from product to product and project to project, that I hate to mix specifics with general information. (Please note that by "baking" I did not mean to toss it in the oven at 350 F until golden brown )

Quote: With the USG hydrocal, hydrostone etc you MUST mix with a mechanical mixer, and you must mix it for the time it says if you want the full properties. The MIXING is what creates a lot of the hardness according to USG.

Absolutely correct and I thank you for adding that.

There are a lot of different gypsum products and applications, and for each application there may be multiple acceptable techniques. The original post was quite general in asking about increased "strength" so in good faith I posted what readily came to mind as possibilities depending on the application.
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  #13  
Old 08-17-2007, 12:37 PM
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Landseer Landseer is offline
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Re: additives to make plaster stornger

Quote:
Originally Posted by MedFX
Be aware that vinyl can affect adhesion of some finishes.
Correct, that is why I don't use it, because one of my finishes is a STAIN and the end results would be affected by the additives.

Quote:
All of my molds are silicone, most are addition cure silicone, with either ultracal or fiberglass mothers.
Mine are all silicone as well, but we have to remember that heat, sunlight and chemicals do shorten the rubber's life. Some thick casts can get so hot you can't handle them, I never measured the temperature but I would say hydrocal etc probably approach 140 degrees at the upper end of the scale when casting large very thick walled pieces- the steam can be seen just pouring out and the room quickly gets humid from it after a couple of casts.
That heat if nothing else certainly doesn't help the rubber any.


Quote:
Originally Posted by landseer
You don't want to BAKE plasters, if the water evaporates in any portion before the rest and it gets above about 115 or so degrees it will start calcining and breaking down.
Quote:
Actually the optimum drying temperature for Hydrocal and UC30 is 110-120 F (direct from USG) but that doesn't mean to slap it on top of a radiator. I have built an insulated box with a heating element and circulating fan with a variable thermostat from 95 - 150 F.
correct, I should have looked at my USG papers right here in the drawer but I was lazy to look up the exact temperature, it also depends on how much moisture is in the cast as it dries and as you say you can start higher and reduce as it dries, but most people don't want to have to mess with keeping time/track of temperature/moisture/time on casts.
Commercial plaster studio I worked at 25 years ago used a closet with wire racks in it and a gas heater, dehumidifier and fan, it would dry all the casts overnight.
I don't think the dehumidifier helped because above a certain temperature as in that closed up closet it just can't cool the coils enough.


Quote:
My cabinet cost less than $100 and I use it for everything from softening clay and wax, to "baking" silicone and gypsum.
I nifty cheap method might be a box with several 100 watt light bulbs in it each connected so they can be controlled individually, bulbs put out more heat than light.
I don't really care for the term "baking" which implies cooking at high temperature like baking a cake at 350 degrees, conversely many women seem to use the term for firing clay in a kiln, I'm not sure you can really call 2000 degrees "baking" it's closer to burning.
I would say you are warming the casts or better yet; force drying them with heated air is less confusing to those not in the know.

Quote:
(Please note that by "baking" I did not mean to toss it in the oven at 350 F until golden brown )
You know, years ago when I worked in a ceramics firm and fired the kilns, I did an experiment with some hydrocal, I put a small dry cast in the kiln to see what would happen to it. I think we fired to cone 05 if I remember right, well when the cast came out of the kiln it had shrunk considerably and I think it had a bunch of hairline cracks and distorsion. I don't remember it being soft and powdery it still had strength, so I'm guessing the original rock after being processed has to be ground up- it doesn't just fall apart into powder- I had thought the stuff would just crumble into a heap like sugar.
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  #14  
Old 08-17-2007, 07:26 PM
MedFX MedFX is offline
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Re: additives to make plaster stornger

I thought about the incandescent bulbs for a heat source, and I think they would work well, but I stumbled across another source at a garage sale. An American Harvest Food Dehydrator. I built the cabinet out of 3/4 MDF, lined with 1/2 inch styrofoam insulation. A couple of adjustable shelves made from the plastic grid, used to diffuse fluorescent lighting in suspended ceilings, on MDF frames completed the cabinet. I added a 160 F thermal breaker for safety and mounted the dehydrator through a hole in the bottom of the cabinet so that the heating unit and controls are on the outside. Temperature is monitored with a cooking/meat thermometer with the probe inserted through a hole in the side of the cabinet about half way up.

The food dehydrator is nice because it is a fan and thermostatically controlled heater all in one. The top setting is 145 F but if I leave it on full blast the three plus cubic foot cabinet stays at a toasty 150F. The dehydrator stays comfortable to the touch and show no signs of overheating such as darkened or warped plastic.
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Old 08-17-2007, 07:57 PM
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ahirschman ahirschman is offline
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Re: additives to make plaster stornger

Since we are talking about heat sources, I sometimes use two bulbs (high wattage) in series. This cuts the light output by a great amount, and produces 1/2 of the indicated wattage. The bulbs should last forever used in this manner.
(Series connection, for those not electrically inclined - is connecting all the wires in a straight line ----O---- -----O----- so that the light bulbs shine a lot dimmer (Twice the resistance)
Ari.
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