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Plaster Moulds Transfering Layers Of Plaster Onto Clay

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Hi there, I thought someone here may have the answer to this frustrating issue.

 

I have been slip casting and all of a sudden some of these molds started transfering layers of plaster to the porcelain slip after using them for some time. Some of these moulds were a couple of months old some were a couple of years but they all started doing this at the same time. 

 

I made new moulds and they started doing the same thing after about 15 casts.They all staretd ddoingit at the smae time again. Loosing fine layers of the plaster onto the porcelain slip cast clay. 

 

I have attached some images below both of the moulds and the porcelain with the plaster attached.

 

Also some extra into

 

Also there is no other obvious deterioration of the mould on the outside. Only the small fine layer that has peeled off inside.

I am casting a piece a day.

It is winter here, but I have a heated studio.

I'm using imperial porcelain slip.

These moulds I made myself out of potters plaster and are part moulds.

I cast very fine works 2-3mm thick

They are from different batches of plaster the first and second time. I alowed the moulds to completely dry before using them (no dampness transfered onto newsprint overnight)

The moulds that I use plastic clay in are fine!

 

Any ideas? I have asked around a bit in my circle but so far everyone seems perplexed. One thought is that I'm not letting the moulds dry out enough between times. That maybe casting once a day is to much for a plaster mold to take? I' needing to make a lot of work using these designs so I am casting daily and letting the moulds sit over night empty, but it is cold. I'm desperate to keep using them.

post-86537-0-95734300-1498911670_thumb.jpgpost-86537-0-09763900-1498911714_thumb.jpg

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did you mix the plaster yourself? could you have added too much water in the mix? I think the plaster is breaking down. Is it possible there is sonething in the slip you're using? If the plastic clay doesn't cause a problem , it is likely the slip you are using. 

Marcia

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Marcia:

Something troubles me about this particular circumstance. All the molds started doing it at the same time; some new, some several years old.

1. Have you changed slip supplier or recipe?

2. Did you recently store household chemicals nearby? ( looking for acids evaporating)

3. Did they freeze and thaw several times?

 

Nerd

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Agree with Nerd - logically, it isn't a problem with a change in the plaster mix as some older molds started exhibiting the behavior at the same time. The only thing that is new is the slip. As Nerd suggests - a change in the slip supplier if commercial slip is being used, or change in the recipe if mixing one's own slip. If it is a pre-mixed commercial slip, there could be a manufacturing change that is not published. For self-mixed recipes, there is a possibility of a change in the water. If on a well system, is there a water softening system that the filter was recently refreshed? If on public water, did the utility start using a different conditioner.

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Marcia:

Something troubles me about this particular circumstance. All the molds started doing it at the same time; some new, some several years old.

1. Have you changed slip supplier or recipe?

2. Did you recently store household chemicals nearby? ( looking for acids evaporating)

3. Did they freeze and thaw several times?

 

Nerd

 

I'm using a comercial slip. This is a new slip for me and I'm just about to finish the bucket 10 L... A friend is also using this slip probably not the same batch and she is not having an issue. It's a popular slip here in Australia.

No, no freezing of the moulds, they are kept in a studio indoors. They are all kept on a bench so nowhere near any chemicals. The is also no deteriation on the outside only on the internal surface which is breaking down :/

 

The ony other difference is that I have never worked my mould so hard before with daily use and neither has my friend that also uses the same slip. She will go hard for a couple of days and then nothing for a week for so where I'm pouring into them daily (once only) they take awhile to dry,  but they dont seem to be getting that wet at all. This last batch of moulds I have  been using since the 15th of June so for a few weeks! The last pour yesterday the were all fine and out of the 5 moulds yesterday that I removed the form from 3 had left some plaster behind. I do aways pour these 5 togther though so i must be a usage thing rather than a plaster ratio which I'm sure is fine. The moulds don have signs of being week.

 

The other thing with that plastic clay is that it doesn'tt have anywhere near the water content so if its a matter of being over used then this wouldn't cause the same issue. I don't use these moulds with plastic clay only slip.

 

:/

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Plaster molds do age and have a limited life if used a lot. I was in the slip cast business for 12-14 years once and we wore out molds from heavy use. .and had to continuly make new ones to replace worn out ones. 

The other issue is the plaster you made them from and how you mixed it-was it fresh and did you get the water to plaster ratio exactly right?

The are issues that only you can answer really.

When plaster breaks down it it wears or far;ls apart or both.

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Mark, some of the molds were new. She just made them. I think it may be the new slip. Possibly a bad batch with something off.

Something that sticks to the plaster ever so slightly. What do you think would cause that?

Marcia

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Dick and nerd,

I agree since old and new molds were reacting this way. The only variable if the new slip. Plastic clay did not peel off the plaster. Only the new slip did this.

But what in it would cause the slip to peel the plaster?

Marcia

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Marcia:

My initial instinct is a hot ( sodium) batch of slip. Etch stated this is a new slip, and also a different batch than their friend. I think hydrolysis is occurring; which is softening areas of immediate contact with the slip. I would suggest an immediate rinse with straight vinegar to reduce the high PH levels of caustic sodium levels. Followed by a rinse of baking soda/ water to bring the PH to neutral. Afterwards, find a new slip.

 

etch: put 1/4 cup of this new slip on a plate, and slowly pour some vinegar on it. Look closely to see if scumming or a if a film builds as the vinegar hits it. Then let it dry completely: baking it till dry is even better. Grind it back to powder and look for white crystalline residue. Think you have a caustic level of sodium in this batch.

 

Nerd

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Since the slip is not made by her its hard to say what is going on-I would make my own slip then you can have control of the slip.

Thats all I can suggest-unless she made an error making those molds as I noted above.

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Are you sure it's plaster and not just a thin layer of clay that was stuck to the mold from previous use?

Yes I initially thought/hoped  that originally but no plaster missing from mould and plaster is plaster... :(

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I spoke to the slip supplier and he had never heard of this happening except once in the 90's with bought moulds and earthernware slip, he said they never found the cause but he suspected it was faulty moulds. He aslo feels that as I did my plaster mixing by eye (waiting for it to form a mountin etc ) this is the cause. But the thing is this made a strong plaster mix, there was no water left to settle o top when I poured the moulds so wouldnt this be less week?

 

Anyway he said that as there have not been any oher complaints that it can't be the slip as the produce a huge amount per batch?

 

I now have to make new moulds, I have a new batch of the slip and 1ltr left of the old. I'm going to use one of the new moulds and just use the old slip on it and the rest will have the new slip. If it was the slip the set aside mould will fail while he others will be fine. I will also mesure out the water and weigh the plaster this time.

 

I still don't see how old original mould that was fine until I used this slip started to laminate if it was a plaster issue :?

 

Thankyou all for your advice :)

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Mixing plaster by eye is the worst way to do it. Plaster and water need a specific ratio for the final product to be as strong as it should be. Adding plaster to the water until forms a mountain is completely inaccurate. The shape of the container and the speed at which you add the plaster will both add to the inaccuracy of the mix. For pottery plaster No.1, you need 10 parts plaster to 7 parts water. for other types of plaster the ratio is different.

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