Cleaning slip molds?

[DanG]

Hi, I was wondering if anyone has some advice on how to best clean slip residue from the delicate interior surfaces of a slip mold?

 

I have tried soaking them in water for 1/2 an hour and this seems to have some effect, but Im not sure if this damages the plaster.

 

Im a little nervous about directly wiping the inside of the mold as I dont want to damage them...

 

 

any suggestions?

 

with thanks,

 

Dan

[Lucille Oka]

 

Do not soak your molds!

 

 

It is best to use preventative maintenance and clean up your molds just after your casting sessions.

 

If your molds are made of Hydrocal White or a harder plaster they are very durable molds. You can use a misting of clean water and a clean ‘soft’ bristle toothbrush to clean textured surfaces and a soft sponge for smooth areas. Just mist the ‘clayed’ areas with clean water to soften the clay and wipe with a soft clean sponge. If the texture is very deep mist with water and use a clean soft fan brush to remove the wetted clay.

 

 

 

 

 

Dry out your molds completely. Do not use high heat!

 

 

 

 

 

Never, ever soak your molds.

 

 

 

 

 

[DanG]

Ok, thanks for the good advice.

 

What happens when you soak a mold? Does it reduce the life of the plaster or damage the interior surfaces?

 

I have heard of people 'priming' a bone dry mold for use by dipping it in water for a few seconds, is this necessary or is it fine to use a bone dry mold?

 

 

[Lucille Oka]

Soaking your mold besides being unnecessary can over time break down the integrity of the mold and make it soft, powdery, lose the definition and eventually make it prematurely useless.

 

Adding a light mist of water can help relieve the surface tension and will help the slip flow over the surface smoother. But, it increases the set up time and the damp surface can cause uneven drying and warping of the ‘vessel’.

 

If you are pressing moist clay in or on a mold it is not advisable to wet the surface, the clay may stick and take a longer time to set up and the uneven drying can also cause warping.

 

I am not sure why your clay is sticking to your mold unless you are ‘wetting’ it before you pour. Are you satisfied with the casts? Try not wetting the mold and see how the cast turns out. Depending upon where the clay is sticking, also check your mold for undercuts. You may have to ‘tweak’ the mold a little and make corrections.

 

I hope this helps you.

[Wind n Wing]

I did slip casting for over 20 years and found that a quick and easy way to clean clay residue off the inside of the mold was to use a piece of panty hose (the leg portion) dampen it slightly and LIGHTLY wipe. You can wrap the panty hose piece around a tool to get into tiny places. Again I repeat use a light touch. I also agree never soak a mold in water and yes it is much better to clean right after you remove your cast piece than to have to scrub to get the dry clay off. A molds life depends greatly on the care it is given. I had some molds I had used for 15 years.

[DanG]

Ok, thanks for the info.

 

Glad to hear that molds can last that long too.

 

I am still learning how best to make a mold that lasts... I noticed by accident that if you put a piece of bone dry plaster into the plaster of a newly cast mold (when its still liquid), it sucked the water out of the general mixture and made it rock hard.

 

I havent experimented with this in the slip casting phase, but im going to try making some molds within a precast 'sleeve' and see if the harder cast effects the way it interacts with the slip.

 

 

 

[Lucille Oka]

Why are you looking for a harder plaster? What kind of plaster are you using?

[AmeriSwede]

Ok, thanks for the info.

 

Glad to hear that molds can last that long too.

 

I am still learning how best to make a mold that lasts... I noticed by accident that if you put a piece of bone dry plaster into the plaster of a newly cast mold (when its still liquid), it sucked the water out of the general mixture and made it rock hard.

 

I havent experimented with this in the slip casting phase, but im going to try making some molds within a precast 'sleeve' and see if the harder cast effects the way it interacts with the slip.

 

 

 

 

 

A harder casting plaster (ie Hydrocal) is generally used to make the 'mother mold' from which the production molds (softer casting plaster) are made. To maintain the same size (allowing for shrinkage) of the finished pieces the mother mold is never used for production. As it is a harder plaster, if it is used for casting slip, because of its density, it will take much longer for the 'shell' of the slip to set up. Softer plasters (casting molds) have greater porosity and suck the moisture from the slip much faster allowing the shell of the casting to set up at a much quicker rate.

[Lucille Oka]

 

I hesitate to do this but- the term 'mother mold' applies more to the urethane/latex mold making processes where the flexible molds are 'cradled' in a harder outer shell for support.

The term I think you are referring to is 'case mold' from which plaster production molds are cast.

 

I recommend a very good book on mold making-

Mold Making for Ceramics by Donald E. Frith;

 

This book also contains the United States Gypsum Cement and Plaster Volume and Mix Guide, as well as The Frith Shrinkage Compensating Calculator enclosed in an envelope on the interior cover.

 

If you get the book used look for those two calculators in the book condition descriptions.

 

[Big Electric Cat]

Get The Essential guide to Mold Making and slip Casting by Andrew Martin. Andrew Martin is very good!

[AmeriSwede]

 

I hesitate to do this but- the term 'mother mold' applies more to the urethane/latex mold making processes where the flexible molds are 'cradled' in a harder outer shell for support.

The term I think you are referring to is 'case mold' from which plaster production molds are cast.

 

I recommend a very good book on mold making-

Mold Making for Ceramics by Donald E. Frith;

 

This book also contains the United States Gypsum Cement and Plaster Volume and Mix Guide, as well as The Frith Shrinkage Compensating Calculator enclosed in an envelope on the interior cover.

 

If you get the book used look for those two calculators in the book condition descriptions.

 

 

 

 

Yes... you are absolutely correct Lucille, possibly... if I'm wrong, I stand corrected. Been out of touch with the terminology for too many years. I also have Frith's 'Mold Making for Ceramics', and heartily recommend it for those desiring information on moldmaking. Additionally I would have to say that as much as I've used the included USG Cement and Plaster Volume/Mixing Calculator through the years ... that is probably worth the price of the book alone.

 

In Frith's book he defines a case mold as, '...Simply put, a case mold is a mold from which to make other molds.' which is as Lucille stated. However, in the book's index 'mother mold' is tabbed to an example (pg.4) and also referred to 'see case molds'. The example (a pictured entry) is of two identical fired clay molds (described), '...indicating that they were taken from one master mold'. In the text description's following page he goes on to state "...That the molds were evidently made from a 'mother mold' or 'case mold'....". (Also 1300 B.C. Palestine didn't offer too much urethane or latex, so that is possibly a more recent defining.) My thinking (am I wrong?) was that he was implying that a mother mold is also referred to as a case mold. Additionally I learned, during my formal education, the term, (if my recollection remains strong) mother mold as the mold from which the other duplicate molds are made. Kind of like a mother having children, that are replicas of herself.

 

 

Invariably, whatever one my choose or decide to call 'the main mold' to make additional molds from, a harder plaster is generally warranted as this will yield a more durable mold for reproducing many other molds.

 

 

[DanG]

Great, will look out for these books.

 

I have been thinking about the longevity of molds after I heard you can get about 50 casts from one before it is worn out.

But I guess they are meant to be soft otherwise they wont interact with tthe slip properly.

Ive just been using pottery plaster.

 

cheers.

 

 

[Lucille Oka]

AmeriSwede,

I mention the correct terminology only to encourage consistency, not to attack you. We answer a lot of questions from 'Newbies' I think it is less confusing if we have a standard of terms.

In ancient times, true, there weren't any urethane nor latex molds. They used animal bones to make gelatin for flexible molds. Though I don't know how far back this practice goes.

 

DanG,

United States Gypsum is a very helpful company with good customer service. They respond to email inquiries and help with questions of all types pertaining to plaster and their website is very helpful also.

I wish you the best in your work.