wjskw@BELLSOUTH.NET on sat 22 jan 05
Ingeborg and Ronda and all:
As Kenneth pointed out, the problem is with the=20
mineral salts in the liquid in the plaster migrating to the surface
of the plaster once it begins to dry again.
This is a problem known in the floor tile industry as
"efflorescence". It may be caused by salts coming from your
clay body, or from the water used to mix the plaster.
It does not show up, sometimes for decades, then appears "as if by
magic" overnight. We also see the problem here in the Keys, where
early (sometimes unscrupulous) builders of concrete block homes
would use sea water to mix their cement, causing spalling to occur
long after they were paid and gone (the metal re-bar in the wall
rusts, and expands, popping out the cement. Not cheap to fix.)
In floor tiles, such as terracottas and Mexican saltillos
(you know, the unglazed brown tiles with the dog and chicken tracks
in them that are so popular in the southern US), the problem
normally occurs once someone, not knowing better, "cleans" the tile
by soaking it with water (or worse, acid-based compounds), lets it
"dry" (on the surface anyway) and then seals the surface with an
acrylic wax or floor finish. Even though the tile appears dry,
efflorescence once begun is a long term problem that continues to
re-occur. In our industry, floors that have begun to effloresce can
be repaired only with severely limited success. We usually
recommend that the tile be ripped out and replaced. That is the
only way to remove the problem completely. Of course, the owner
doesn't want to hear this, and tries all manner of "fixes",
including grinding the surface to remove the part of the tile with
the salts. Grinding in conjunction with...yeah...more water. In 30
years, we have NEVER found a satisfactory solution, other than
removal.
The solution?
Since it is a plaster table we speak of, not a $20,000 floor
installation, there is actually hope. Scrape, sand, grind (DRY!!)
the surface of the plaster. About an inch should do. Then repour
plaster on top. =20
Will the problem reoccur? Yes. The whole reason for the plaster is
to absorb water; in essence, you are creating the problem
intentionally by allowing the plaster to do what it does.
You can SLOW the problem of efflorescence by allowing the plaster to
dry completely (we're talking days or weeks here) between use, and
scraping the surface slightly before the next use, but you are not
going to eliminate it. For most potters, allowing that long a dry
time is not an option.
I'm working with a friend now to determine if one can pour a half
thickness of plaster (about 3/4 of an inch thick) for a base, on top
of that apply a newspaper or fabric layer which will allow moisture
to continue to wick through, on top of that will be poured a
"sacrificial layer" of plaster that can be easily removed (thanks to
the layer of paper or fabric and discarded when it begins to
effloresce. So far, that appears to work equally well with
newspaper, cotton, and 1/4 inch HardieBoard, though removal is
easiest with the HardieBoard, being a solid material. We are
letting it dry thoroughly between wettings, though.
Something you might want to consider for the future.
As for the "big stuff flaking off and dropping from the bottom
holes", are you sure you're allowing enough time for the plaster to
dry between uses? It sounds almost as if the plaster is beginning
to degrade from constant moisture exposure.
Hope that helps,
Wayne Seidl
Key West FL
PurpleLama@AOL.COM on sat 22 jan 05
It may be caused by salts coming from your
>clay body, or from the water used to mix the plaster.
>It does not show up, sometimes for decades, then appears "as if by
>magic" overnight.
Great information, Wayne, thanks. Do you think using distilled water when mixing the plaster would help? Of course, it won't help if the salts come from the clay.
Shula
Filthadelphia
AKA Philadelphia, PA USA
latest forecast is 10 to 16 inches of snow today and tomorrow. In this town, that's a great deal. And with the Eagles playing tomorrow . . .
One of my teachers(a nurse) told us on Friday that she will have a packed bag with her tonight when she goes to her job at a hospital.
Saw a nurse or two going in with small suitcases at my university hospital this morning.
Never thought about the need to camp out at the hospital if the weather is bad - can't abandon the patient.
Cindy in SD on sat 22 jan 05
Regarding the wedging table issue . . .
I wonder if a cement board, dur rock (sp?), etc. top would be more
tolerant of the salts? One would have to support it from beneath--maybe
slats made of green treated 2X4's on edge? Or maybe you wouldn't need to
have them treated. The various cement boards have fiberglass
reinforcement, so they might be able to take the salts better. Might be
worth a try, anyway. It's not that expensive. I haven't built my wedging
table yet, and I have a bit left over, so I think I'll give it a shot.
I just finished building a slab-roller/work table, and I did surface
that with dur rock. I wanted to pour a cement top for it, but figuring
out how to do that would have taken more time than I wanted to invest.
Will let you guys know how it looks after a year or so.
Best wishes,
Cindy in SD
John K Dellow on sun 23 jan 05
Here's a long shot, you could try wiping the plaster with canolia oil.
This works on
efflorescence on terracotta pots.
john
--
John Dellow "the flower pot man"
From the land down under
Home Page http://www.welcome.to/jkdellow
http://digitalfire.com/education/people/dellow/
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