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Mixing plaster of paris

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Posted by trainnut1250 on Wednesday, October 20, 2021 11:30 AM

Macman44

 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.

 

 

Paul,

(From someone who has mixed 100's of batches of plaster).

The symptoms you describe are from old plaster. It should "go off" (set up) with a definitive heat build up and should become pretty hard. Soft, crumbly castings generally mean the plaster is bad.  Plaster of Paris is softer than hydrocal but still sets pretty hard. If the box has been around for awhile it likely has absorbed water and gone bad.

Experiment with some measuring containers to get a good ratio of water to plaster and then use the same ratio everytime. It makes the process much easier. I mix my plaster to the "soft yogurt" thickness for rock castings.

I dont reccomend WS Light Hydrocal. It is very expensive and I dont like the finished product compared to "regular hydrocal" you buy at the plaster and lath supply store.

 

Have fun,

 

Guy

see stuff at: the Willoughby Line Site

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Posted by maxman on Tuesday, October 19, 2021 7:28 PM

selector
Also, consider light jiggling, even lifting and dropping short distances

I had one experience to do some plaster moulds when I belonged to a club.  I did the jiggling and dropping thing and can attest to the fact that the air came out.

Unfortunately that did nothing to help the plaster.

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Posted by BATMAN on Tuesday, October 19, 2021 3:49 PM

Brent

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

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


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Posted by Overmod on Tuesday, October 19, 2021 2:47 PM

Lastspikemike
Often it can help reduce cracking if you introduce fiber into the plaster.

This is good but I'd recommend using a skim coat in the mold first and then introducing the fiber-loaded material behind it.  This was SOP (in reverse) for high-quality wall plaster in the old horsehair days, and now for AVG-style shotcrete fabrication of inside walls without furring and rocking...

I confess to having made regular use of acrylic bonding agents in plaster casting for architectural purposes; it allows for thinner slip and stronger intricate details while also bonding nicely to the 'composite' material away from the face.

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Posted by Lastspikemike on Tuesday, October 19, 2021 2:32 PM

The problem experienced by the OP was not likely due to mixing method. The proportions were a bit on the wet side which does provide a likely cause for the difficulties experienced.

It literally does not matter which way round you mix the correct proportions for your intended application. The final consistency of the mix gives a good indication of how it will set. For moulding (casting really) you want a drier mix than you would use for say a broken arm splint.

Often it can help reduce cracking if you introduce fiber into the plaster. Short bits work better than long threads. This is commonly done in concrete work where steel mesh or rebar would be overkill. It is also used in some old fashioned residential plastering methods for the same reasons. 

Fiber reinforced plaster is an ancient form of what are now referred to as composite materials and the physics are the same.

Alyth Yard

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Posted by selector on Tuesday, October 19, 2021 12:48 PM

Only one quick contribution from me, based on experience:

Yes, use soapy water to coat the inside of the mould before you pour the mixture into it.

Also, consider light jiggling, even lifting and dropping short distances (3-4 mm, tapping) to get air to lift and move up to the surface once you have the pour in place.  This is not an easy thing to do, for obvious reasons, but it can greatly reduce the friable nature of the product when the material cures.

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Posted by richhotrain on Tuesday, October 19, 2021 11:14 AM

rrebell

Funny, many times the instructions are just wrong. 

Is that really the case?  I can speak for WS Light Hydrocal. The instructions for mixing are precise and accurate.

Rich

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Posted by rrebell on Tuesday, October 19, 2021 10:53 AM

Funny, many times the instructions are just wrong. With plasters the instructions they give are for (hopefully) never fail jobs of a larger nature, we can be much more percise to get what we need as it is ussually small batch. In other words a little more or a little less may be what works. This is because in small batches and the way we work, it may take a bit more time or less time to do a pour. This was learned by me when working in the 1-1 world.

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Posted by SeeYou190 on Tuesday, October 19, 2021 9:57 AM

Water Level Route
I will cease and desist in respect of that as well.  Thanks for pointing that out.

Our "smarter than everyone never wrong argues about everything" guy showed up in here, and the level of conversation took an immediate plunge.

Steve had to lock another thread yesterday after it was "Spiked" by this guy.

I am so tired of this merry-go-around in what should be the premier Model Railroading discussion forums on the internet.

-Kevin

Happily modeling in HO scale. 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 Water Level Route on Tuesday, October 19, 2021 9:47 AM

SeeYou190

The OP has his answer. 

Copy-and-paste answering is in full effect.

The merry-go-round is beginning.

-Kevin

 

Good point Kevin.  I will cease and desist in respect of that as well.  Thanks for pointing that out.

Mike

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Posted by Water Level Route on Tuesday, October 19, 2021 9:46 AM

I have always found it best when experiencing difficulties with something and not following manufacturer instructions, it's best to go back to square one and follow the directions.  Curiously, that sometimes solves the issue.

What gets added to what may not make a difference in many cases.  However, there are some where it makes a big difference, such as acid dilution.  Always add acid to water, not the other way around.  Logically shouldn't make a difference.  Reality shows otherwise.

Mike

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Posted by SeeYou190 on Tuesday, October 19, 2021 9:40 AM

The OP has his answer. 

Copy-and-paste answering is in full effect.

The merry-go-round is beginning.

-Kevin

Happily modeling in HO scale. 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 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

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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 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 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 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 in HO scale. 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 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 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 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

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


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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 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 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 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 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 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 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 in HO scale. 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 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 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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