Plaster slabs for reclaiming clay

[Jennifer F]

I have poured two 3 inch thick slabs about 18 x 24 inches of pottery plaster for reclaiming clay. They have cured about 3 months. When I run my finger across the surface a white residue still comes off. They are hard and smooth. I can scratch the surface a little with my finger nail. Are they okay to use or did I mess up mixing the plaster. I hate to fire a load and loose everything. 

[Jeff Longtin]

Good Morning Jennifer,

Welcome to the Forum.

Is this your first time mixing plaster? That's a lot of plaster to mix up on your first outing.

"Cured 3 months" - That surprises me a little. Are you looking for "completely dry" for your purposes? A small fan can really help plaster dry when its wet. (Either from weather conditions or from wedging lots of clay.)

"White residue" - That's a tough one. Have you wiped the plaster surface with a damp sponge? Fresh plaster might have a SLIGHT white residue but nothing that a sponge won't remove. If its more than just a LIGHT residue I wonder about your mixing process.

When I mix plaster, for casting molds, I mix just long enough to create a durable mold but not so long that air bubbles become an issue. When I mix plaster for a wedging surface I mix longer as I know I don't have any "detail" to worry about. (A longer mix encourages a stronger plaster surface.)

Hope this helps? 

 

[Pyewackette]

What KIND of plaster was it?  Was it Plaster of Paris or was it #1 Potters Plaster, Hydrostone, or Hydrocal (or something else)?

Plaster of paris is not appropriate for use in the studio.  My favorite for a drying table is Hydrostone.  I'm hoping to pick some of that up soon during a plant/clay buying trip - tricky since the pottery supply place closes at noon and my appointment at the Native Plant sale is at 11.  And its a 5 hour drive from where I live, sooo... I may have to leave at 4 AM if I want to pick up clay stuff AND plants.  *sigh*

You can also use Hardiebacker but if you do lay the clay out in strips to dry.  I have found that if I lay out a large square of wet clay it won't dry as fast in the middle.  Probably because the "half inch" hardiebacker is .42" thick, so not as much mass as a 3" thick slab of (the proper) plaster.  I don't know if using more than one layer of the Hardiebacker to dry clay would help, I only had the one piece at the time.  

[Callie Beller Diesel]

Also, what method did you use for mixing your plaster? Did you weight it, or do the “sift to peaks” method? 

[Jennifer F]

I used #1 pottery plaster, and yes about 3 months. I already have lots of clay to work with so I just kept putting it off since I really didn’t have anyone to ask. I did weigh the plaster and sifted it in thru a course screen. I used my drill with a paint mixer. I poured the slabs at two different times. 

[Rae Reich]

Might be time to do a test. Wedge enough clay on your new plaster (wipe plaster down first as suggested with a damp cloth) to make one of each item that you usually make. Carve TEST on the pieces and fire with your regular ware.

[Jeff Longtin]

Your response suggests you did everything the correct and you're probably fine to use the slabs. The only puzzling aspect of your posting is the phrase, "residue still comes off."

The one aspect that remains unknown is the quality of your water. Municipal water, throughout the country, differs. (If there is a brownish ring in your toilet tank you have iron in your water, if the ring is green in color, you have chrome in your water.) While the influence of these minerals is minimal, they can have an affect upon the pottery process. It's possible the residue is the result of such minerals?

[Pyewackette]

What about hard water?  Lots of places around the country are experiencing harder water as a result of drought and strains on auquifers.

[Rae Reich]

Hard water leaves a whitish mineral deposit when it evaporates. Vinegar will dissolve it. 
Though I haven’t worked with plaster for a while, I do seem to remember some advice about washing a plaster slab with a vinegar/water solution. That may have been advice for our region; we have hard enough water to necessitate regular purchase of vinegar. 

[Pyewackette]

OP reports a whitish residue that is still rubbing off the top - could that be a mineral deposit from hard water, or might the plaster have not set properly due to hard water?

[Piedmont Pottery]

You can also cover the plaster with canvas, which helps to keep from abrading the plaster and potentially getting bits into your clay when reclaiming.

[Callie Beller Diesel]

50 minutes ago, Piedmont Pottery said:

can also cover the plaster with canvas,

In a lot of the communal situations I’ve been in, you needed to cover the reclaim bats with a thrift store sheet to prevent cross contamination anyways. For the longest time I thought some form of cloth was necessary.

[oldlady]

please not canvas.  find a thin cotton fabric that can be cleaned in a washing machine if it needs it.   canvas is too thick and those dry powdery bits that show up in your lungs come from those tiny spaces between the big, thick threads in the weave.  old cotton sheets and pillowcases do a good job of separating clay from plaster and can be washed easily.

if you want to see the bad stuff, shake any fabric you have been using.   do it in the dark with a flashlight turned on so you can see the flying debris.  your lungs get the invisible tiny, tiny bits.

[Jennifer F]

We do have hard water in our area. If I had thought about it I would have used distilled water. I will definitely wipe off the slab with a water vinegar mixture and cover with an old cotton sheet. I have a large gas kiln I bought from the local community college so it will take me a while to fill it up I haven’t even fired anything yet. Obviously I’m not a professional, so all your advice is very much appreciated.