Dan Saultman on sat 16 nov 02
I am getting a white residue (if that's the word) on my terracotta
pieces
that occurs and becomes evident only after a cone 6 bisque firing.
I have tried to scrub it off afterwards but it seems fired into the
body.
It is a drying issue I think because I use plastic to cover the edges
of my plaques and it doesn't happen there. It happens where the clay is
exposed longer to the air.
Even more surprising is that it is not just a surface issue since I have
sanded the greenware on some pieces
yet the residue pattern does not show any signs of change where the
sanding occurred.
Is there some kind of alkaline settling out happening? I recall someone
saying to try to spray
the greenware with a vinegar-water mixture but that sounds too easy.
(Ihaven't tried it) What causes this white chalky-like patina on
terracotta?
Dan Saultman
1st snow of the season in Detroit.
Pat Southwood on sun 17 nov 02
Dear Dan,
I was always told that scummy white stuff on Earthenware was due to salts
rising to the surface, which would perhaps explain why sanding it doesn't
help. They usually disapear under a glaze, even a transparent.
Pat.
pat@southwood4.fsnet.co.uk
Veena Raghavan on sun 17 nov 02
Hi Dan,
John Dellow posted some advice sometime ago. He suggested using Canola oi=
l
to remove the scumming from terracotta. I followed his advice and used it=
on low fire terracotta. It removed the scumming and it never came back on=
the pieces. =
Hope this helps.
Veena
Veena Raghavan
75124.2520@compuserve.com
Snail Scott on sun 17 nov 02
At 06:00 PM 11/16/02 -0500, you wrote:
>I am getting a white residue (if that's the word) on my terracotta
>pieces...
>Is there some kind of alkaline settling out happening?
Yep, that's pretty much it. It's called 'scumming'
and it's caused by the soluble salts in the clay
rising to the surface during drying. It's mainly
visible on earthenware. It's often said to not
occur on stoneware, but that isn't true. It's
merely that at sroneware temperatures, the salts
melt into the surface. They're not white anymore,
but can cause a visible alteration of the surface.
Slow drying tends to create more visible scumming,
since the salts have more time to migrate outward.
(It's a bit like the Egyptian Paste process, in a
way.)
Many clay manufacturers add tiny amounts of barium
to their recipes, to help suppress the effect.
(People who mix their own clay using water high in
soluble minerals also use this method.) This is
probably the most common means of combating
scummimg. It works pretty well (though I'm not
sure of the actual chemical process), but it's not
100% effective.
I've heard of the vinegar 'cure' you referred to,
but I've never used it myself. Anybody else?
-Snail
Steve Mills on sun 17 nov 02
This is not unknown over here with some of the Etruria Red clays; it
seems that handling the pots (not putting handles on them) draws salts
to the surface of the clay causing the white scumming. Once it is there
the only answer is to glaze the piece. Clay makers here in the UK cure
this occasional problem when it happens by adding a very very small
amount of Barium Carbonate to the clay in the mixing stage before it is
press-dried and pugged.
As a cure I'm not entirely happy with it, but it is effective!
Steve
Bath
UK
In message , Dan Saultman writes
>I am getting a white residue (if that's the word) on my terracotta
>pieces
>that occurs and becomes evident only after a cone 6 bisque firing.
>I have tried to scrub it off afterwards but it seems fired into the
>body.
>
>It is a drying issue I think because I use plastic to cover the edges
>of my plaques and it doesn't happen there. It happens where the clay is
>exposed longer to the air.
>Even more surprising is that it is not just a surface issue since I have
>sanded the greenware on some pieces
>yet the residue pattern does not show any signs of change where the
>sanding occurred.
>Is there some kind of alkaline settling out happening? I recall someone
>saying to try to spray
>the greenware with a vinegar-water mixture but that sounds too easy.
>(Ihaven't tried it) What causes this white chalky-like patina on
>terracotta?
>
>
>Dan Saultman
>1st snow of the season in Detroit.
--
Steve Mills
Bath
UK
Ron Roy on thu 21 nov 02
The answers to this are right on - scumming happens with white clays as
well but you don't notice it so it is not as big a problem. Many of the
clay s we use have scumming problems - some times it is there almost always
- some time now and them.
Yes - the soluble salts from clay and perhaps water - are carried to the
surface as water evaporates - and will fuse with the clay - sometimes
results in rims not taking the glaze during glazing properly - double
dipping rims is one way to deal with it.
Adding some Barium carb will tie up the salts in the body - make them
insoluble - 2lb in 1000lb dry body will usually do the trick - I don't
think there is anything else that will work to keep the salts in.
RR
>I am getting a white residue (if that's the word) on my terracotta
>pieces
>that occurs and becomes evident only after a cone 6 bisque firing.
>I have tried to scrub it off afterwards but it seems fired into the
>body.
>
>It is a drying issue I think because I use plastic to cover the edges
>of my plaques and it doesn't happen there. It happens where the clay is
>exposed longer to the air.
>Even more surprising is that it is not just a surface issue since I have
>sanded the greenware on some pieces
>yet the residue pattern does not show any signs of change where the
>sanding occurred.
>Is there some kind of alkaline settling out happening? I recall someone
>saying to try to spray
>the greenware with a vinegar-water mixture but that sounds too easy.
>(Ihaven't tried it) What causes this white chalky-like patina on
>terracotta?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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