mold degradation, powdery plaster

[Shane]

Hi All,  I'm wondering if anyone can help me solve a bit of a ceramic mystery,

I'm a slip caster from Australia. I've poured thousands of molds over the last thirty or so years and this issue has only occured on two of them (so far).

Essentially small parts of the mold (that appear identical to the rest) can be rubbed away with light finger pressure. It doesn't rub away in chunks but seems to have reverted back to it's original powder form somehow. These molds are different brands (scioto and albertas) and are otherwise in good condition. It doesn't appear to be water damage as I've seen what that looks like. They are also stored in the same area with thousands of other molds that do not exhibit this issue. I know the scioto mold had the problem from day 1 but I'm pretty sure the albertas mold has only developed it recently.

My only thought is that both companies may have simultaneously used a dodgy batch of plaster, seems a bit of a stretch though.

Anyone else got any ideas?

 

Cheers,

Shane

[Jeff Longtin]

Hello Shane, welcome to the Forum.

My guess is that a liquid was spilled onto the molds. Perhaps coffee or an acidic cola?

In the 70's my late father worked for a large advertising agency that was in the process of opening offices around the world. (BBDO) One of their more successful locations was in Sydney. He visited several times. He always came back with cans of Fosters beer. The opera house had just been built so he also came back with postcards of it. He enjoyed his visits to Australia.

[Shane]

Thanks Jeff,

You may be right about the acidic liquid, who knows what the molds went through before they found their way to me.

Out of curiosity I think I'll find a mold that's ready for the bin and give it some coffee and cola and try to replicate the issue.

Maybe we'll start a whole new thing, people will need to have an acid etched plaster cast piece to complete their collections : )

I'll let you know how the experiment goes.

[Shane]

For those who'd like to know I tried lemonade which is apparently more acidic than coke and coffee, no effect.

I think I've got a likely explanation. The mold maker probably just threw some badly quick mixed plaster in at the end of the pour when they came up a little short.

They pour many at once I imagine so surely there would be occasional miscalculations and the care factor for that last mold on a Friday afternoon may be approaching zero.

Anyhow two molds out of thousands is pretty good odds I suppose, I just sealed the (outside only) affected sections of the mold with a glue and they seem to hold up okay.

 

If in years to come I see acid etched plaster cast pieces floating around out there I'll know where they got the idea : )

[Callie Beller Diesel]

Is there a possibility that it could have been exposed to excessive heat that would cause the plaster to re-calcine?

[Jeff Longtin]

A few years ago I had just completed a Ceramical press mold when I brought it out and saw several previously made molds with a light brown staining. As I looked at them in surprise the young gal pressing, at the time, apologized and said she spilled coffee on a few molds. At first it seemed it was just a staining of coffee and nothing more.

It was only a few days later, when another person tried to use a stained mold, that the deterioration became evident. I had to remake the mold as a result.

Later that week I poured some black tea onto a pottery plaster mold, in my studio. At first no damage,  but after a few more days, the pottery plaster was soft.

Not really sure why it happened but I do try to keep beverages away from molds as a result.

[Shane]

Hi Callie, I'm not 100% sure about the excessive heat exposure but there's reasons I doubt that's the problem. The issue only occurs on one half of a two piece mold on a few places on that same half. Seems likely that the whole mold (or whole half) would be damaged if it was exposed to the temps required to re-calcine plaster. Bare in mind also that the worst effected mold appeared almost new and had obviously only been poured a few times and had no visible marks or stains that differentiated the effected sections from those unaffected.

 

Howdy Jeff,  Yes I did wonder if time may be a factor so I'll keep an eye on the lemonaded mold for a while and see what happens. It will have to be quite a transformation however to replicate the effect I was looking for. I'll let you know if the miraculous occurs.

[PeterH]

14 hours ago, Jeff Longtin said:

Not really sure why it happened but I do try to keep beverages away from molds as a result.

Could the fact that "dried" coffee is deliquescent have anything to to with it? [Tending to absorb moisture from the air and dissolve in it.]

Do molds stored where they are permanently damp have similar problems? Obviously multiple wet/dry cycles are not a problem.