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Mixing plaster of paris

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Mixing plaster of paris
Posted by Macman44 on Sunday, October 17, 2021 8:59 AM

I have just started using Woodland Scenics rock moulds, and am using plaster of paris.  I am unsure of the correct mixing ratio for the plaster.  The instructions on the box say to mix 1:1 with water, but when I do I end up with a white liquid about the consistency of milk.  Of course , it pours into the moulds easily, but when setting forms a pool of water on the surface which has to be soaked up.  On removing from the moulds, I have to be very careful not to break the casting, and thin casts just crumble in my fingers.  Even completed casts remain very soft.

So, is there a generally accepted wisdom about what ratio to mix the plaster?  What I am using seems very thin, espcially when compared to other scenery materials such as Sculptamold or WS Smooth-It.  Are there any other caveats about using this material that would be helpful to know?

Thanks,

Paul.

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Posted by richhotrain on Sunday, October 17, 2021 9:13 AM

Woodland Scenics recommends using its product, Light Hydrocal, in its rubber molds.  It is a lot more reliable than generic Plaster of Paris.

Rich

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Posted by woodone on Sunday, October 17, 2021 9:32 AM

I believe you will have better luck using 2 parts of POP (plaster of Paris) to 1 part water.

Also make sure your POP is fresh , if it feels grainy in you hand is has gotten moisture in it. NOT GOOD 

 

 

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Posted by Lastspikemike on Sunday, October 17, 2021 9:53 AM

The internet suggests 3:1 ratio is "ideal" but there isn't an ideal actually. It depends what you wish to use the mixture for. There is an ideal setting ratio chemically which is likely around three to one plaster to water. When mixing cement products generally you add water to the powder which makes it easier to reach the desired consistency. It's because you're pouring into a mould that WS is suggesting such thin plaster. It should still setup hard. Try a fresh container of powder, it doesn't last forever unless you can keep all the humidity out.

Alyth Yard

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Posted by Macman44 on Sunday, October 17, 2021 10:04 AM

Thanks, I'll try a thicker consistency next time.  My box of plaster still seems fresh enough, so no immediate concerns there.

I'll also give Lightweight Hydrocal a try when I next buy.

Paul.

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Posted by richhotrain on Sunday, October 17, 2021 10:31 AM

Good luck, Paul. Let us know how it all works out for you.

Rich

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Posted by MisterBeasley on Sunday, October 17, 2021 10:53 AM

I'm also a hydrocal fan for plaster casting.

If you find you like rock face and stone wall castings, you might also look at Bragdon Enterprises.  They use a two-part chemical resin that sets up very well but can actually be softened later with a hair dryer to allow the castings to be bent around curves later.  The castings are thin and light, and can even be cut with scissors.  The Bragdon rock molds are very finely detailed and the foam material brings out all the fine details well.

 

 

It takes an iron man to play with a toy iron horse. 

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Posted by CGW103 on Sunday, October 17, 2021 11:04 AM

I mix it about the consistancy of pancake batter, which works fine for me.

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Posted by BATMAN on Sunday, October 17, 2021 11:33 AM

If you have ever been to the casting room at the hospital as I have on many occasionsWhistling you will notice the Dr./Tech has a big apron and gloves to the elbow on and is often covered in plaster. Putting a cast on a broken leg or arm is a messy business and dipping the bandages in a vat of 1:1 mix ensures the material of the day is well impregnated with the PP. That being said I think the 1:1 mix suggestion is for dipping cloth in which I have done to make rolling hillsides. For use in moulds I think something a little thicker would be required and I am not sure how well it would hold together after coming out of the mould anyway. Hydrocal or some of the other suggestions would be a better choice I would think. Wait for Dr. Wayne to show up and see what he uses.

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1/videos 


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Posted by richhotrain on Sunday, October 17, 2021 11:35 AM

BATMAN

 Wait for Dr. Wayne to show up and see what he uses. 

Yes, that will be the defining moment. Stick out tongue

Rich

Alton Junction

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Posted by BATMAN on Sunday, October 17, 2021 11:39 AM

richhotrain

 

 
BATMAN

 Wait for Dr. Wayne to show up and see what he uses. 

 

 

Yes, that will be the defining moment. Stick out tongue

 

Rich

 

And you can buy a 50lb bag of the stuff for pennies compared to what WS will charge you.Laugh

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1/videos 


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Posted by richhotrain on Sunday, October 17, 2021 11:41 AM

BATMAN
 
richhotrain 
BATMAN

 Wait for Dr. Wayne to show up and see what he uses.  

Yes, that will be the defining moment. Stick out tongue 

Rich 

And you can buy a 50lb bag of the stuff for pennies compared to what WS will charge you.Laugh 

But, must you be a shareholder in GERN?

Rich

Alton Junction

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Posted by BATMAN on Sunday, October 17, 2021 11:51 AM

With all the reno's I have done over the years and not wanting anything to go to waste, I have put leftover tile thin-set and grout that was in the bucket on this portion of the layout. It is super hard and once painted and gets a little greenery sprinkled on it, I think it will look great.

The different colours are just because they are different products.

Whenever I was tiling or grouting I had the drop cloths down in the train room pryer to starting.Laugh

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1/videos 


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Posted by doctorwayne on Sunday, October 17, 2021 5:44 PM

I'm sorry, but I've spent over an hour trying to compose a reply, but the Forum has been freezing every few minutes, and eventually simply deleted what I had managed to add, including photos.

Wayne

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Posted by Macman44 on Sunday, October 17, 2021 5:54 PM

I tried a mix of pancake batter, and the results are much more successful.  It's not self-levelling, but it sets well and comes out of the mould very well.  I may experiment with maple syrup for my next effort.

Thanks, all.

Paul.

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Posted by SeeYou190 on Sunday, October 17, 2021 6:26 PM

Macman44
The instructions on the box say to mix 1:1 with water, but when I do I end up with a white liquid about the consistency of milk.

Most instructions for mixing plaster are by weight, not volume. This easily causes confusion. If you use an inexpensive digital kitchen scale you can pretty easily get the weight the same.

By volume, you will use far less water than plaster powder.

I have never used the Woodland Scenics plaster, so I cannot say for certain what their instructions specify. All the commercial hydrocal and ultracal I have bought specified the mixing ratios by weight.

EDIT--

Just found the instructions on the Woodland Scenics website. They are now stating to use a 2.5:1 ratio by volume.

I guess they updated their instructions.

-Kevin

Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by mobilman44 on Monday, October 18, 2021 5:57 AM

Hi, I'm a bit late to the party.........

I've used plaster of paris since 1956 when I had a basement Lionel layout.  I bought it for 5 cents a pound at the local hardware store.  Moving up the the 1990s, I picked up several of the rubber rock molds and made literally a couple of hundred castings for my HO layout at that time.  All I used was plaster of paris.  

The trick was to make a mix thick enough so as to not run, but to make sure it was thin enough to fill the cracks, and to make sure there were no lumps.  I never used a formula for the ratio, and the results were always good.  

Some comments......

  -  too thin a mix will lead to cracks when it eventually hardens

  -  you will find the casting warming up as it cures.  Be patient and pull after it starts to cool down.

  - make sure that the casting is well secured to a flat base.

  - any tinting or painting you do will end up darker than intended.

  - plaster of paris works well for the molds, but doesn't do well for roads and the like.  For that, I found Woodland Scenics "Smooth It" to work beautifully.

Hey, I'm not knocking other products, just saying plaster of paris is what I used for molds as it was what was available and I was experienced with.

ENJOY  !

 

Mobilman44

 

Living in southeast Texas, formerly modeling the "postwar" Santa Fe and Illinois Central 

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Posted by Lastspikemike on Monday, October 18, 2021 8:11 AM

The internet is all over the place on ratios.

The chemistry is such that the more water you add beyond 1.5:1 plaster to water by weight  the easier the plaster is to flow into the mould but  the faster it sets up and hardens. More water by ratio also leads to weaker plaster.

So, it appears you need to experiment. Start at about 1/3 water and 2/3 plaster. Then add water bit by bit to get the consistency you think is about right. Test the result by casting and adjust your ratio for next time. The actual chemical ratio is supposed to be 2/5 to 3/5 by weight but I doubt you need to be that accurate since you're going to adjust anyway. 

Adding water to plaster gives you more precise control over consistency without ending up with too big a batch. The bigger the volume the faster the set. The old surgical plaster bandage is deliberately light in plaster in part for this reason. Plaster can get quite hot as it cures.

Alyth Yard

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Posted by Water Level Route on Monday, October 18, 2021 8:37 AM

SeeYou190
I guess they updated their instructions.

Note the instructions say to add the plaster powder to premeasured water.  Numerous other manufacturers, suppliers, etc, advise the same.  For example, from DAP: Add DAP Plaster or Paris to clean water.

Mike

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Posted by richhotrain on Monday, October 18, 2021 8:43 AM

When the Hydrocal instructions say that working time is 5 minutes, they are not kidding. In my experience working with Hydrocal, you need to precisely follow the instructions and work fast.

Rich

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Posted by Lastspikemike on Monday, October 18, 2021 10:01 AM

Water Level Route

 

 
SeeYou190
I guess they updated their instructions.

 

Note the instructions say to add the plaster powder to premeasured water.  Numerous other manufacturers, suppliers, etc, advise the same.  For example, from DAP: Add DAP Plaster or Paris to clean water.

 

 

You can do it either way. The key is not to begin with too much water. The reaction creating the plaster results from adding the water. To get the product you want you need to be adding more water to the mix, not more plaster.

It may just be easier to mix the powder into the water, I've not experienced difficulties either way.

Alyth Yard

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Posted by rrebell on Monday, October 18, 2021 10:02 AM

You don't need to measure with plasters. it is more of an eye thing. Hydracale is best for rock molds and other castings and plaster for general scenery work. With plaster you start with powder and add water to consisancy you want, forget about Hydracal, been awhile.

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Posted by BATMAN on Monday, October 18, 2021 10:18 AM

Water Level Route
Note the instructions say to add the plaster powder to premeasured water.  Numerous other manufacturers, suppliers, etc, advise the same.  For example, from DAP: Add DAP Plaster or Paris to clean water.

Yes.Thumbs Up

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1/videos 


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Posted by Medina1128 on Monday, October 18, 2021 10:36 AM

I may have missed it in the earlier comments, but when using Woodland Scenics rock molds, I spray the mold with wet water before pouring the plaster in. This aids in removing the castings. I found another tip, especially when using large molds, is to let the plaster set up until it cracks when you flex the mold. Spray the area where you'll be adding the casting with wet water, then press the mold into place. Sometimes, I have to use push pins to hold the mold in place. Give the plaster a good hour or so to set up. Starting from the edges of the mold, gently peel the mold off. When you have all your castings in place, blend the castings together with more plaster.

 

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Posted by Macman44 on Monday, October 18, 2021 10:55 AM

Looks like that's where I initially went wrong.  The plaster of paris I am using isn't Woodland Scenics and it simply states to mix in a 1:1 ratio, without specifying by volume or by weight, especially since that is easier to do rather than weighing (who has access to small scales to weigh stuff, anyway?).  I would think that most people reading that would expect it to mean 1:1 by volume.  Shows how one needs to read and interpret instructions carefully, but I think manufacturers should be more explicit (as WS now seem to be).

At least I am now forewarned.

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Posted by SeeYou190 on Monday, October 18, 2021 11:23 AM

The Woodland Scenics product is simply indutrial Hydrocal manufactured by US Gypsum that they repackage into consumer-friendly sizes.

I am sure they just copied-and-pasted the instructions from the industrial product originally. In the industrial world weight is used because it is more accurate and easy to control. In the consumer world, it is confusing.

-Kevin

Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by Lastspikemike on Monday, October 18, 2021 2:13 PM

1:1 by volume will be about the same as 2:3 by weight. Plaster in powder form is lighter than water per unit volume by about that much.

Alyth Yard

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Posted by BigDaddy on Monday, October 18, 2021 4:05 PM

My experience is with hydrocal, but people use all sorts of plasters.  The pancake batter is what you want.  I add the water to the plaster and add less water than I think is neccesary and mix.  Then I add more water to get the right consistency.  I measure the plaster.  I don't think I ever measured the water.   I certainly never weighed anything. 

Henry

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Posted by kasskaboose on Monday, October 18, 2021 5:41 PM

This is a great combination.  I've used it also and try to bend the plastic mold a bit to help it not get stuck and unable to come out of the mold. 

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Posted by Lastspikemike on Tuesday, October 19, 2021 9:09 AM

Starting with water you need to measure the water or you may not get the right consistency and volume of plaster you need. If you know how much you will need fairly accurately sure it might be a bit easier to mix powder into the water (although I don't find that to be true for small quantities). 

If you start with about the right volume of powder and add water you really can't go wrong unless you add a bit too much water. If you start with a bit too much water you're stuck with making too much.

Chemically it really can't make a difference. Incidentally, the density of the powder varies by 5% to 10% which can be the difference between two batches made with the same volume of water. And nobody needs to weigh the water, just use a metric measuring cup. 

Alyth Yard

Canada

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