Karl P. Platt on sun 7 jul 96
I first of all want to point out that the tile works near here typically dry and
fire their 40cm x 40 cm tile in about 80 mins. Granted these are dry pressed
with a teeny bit of water, but there are a couple lessons in what happens there.
Likewise, the olarias here dry tijolos (hollow extruded structural clay tile)
and telhas ( extruded and pressed ceramic roofing tile) without warpage in about
2 days depending on heat,wind and humidity.
What's happening in these cases that isn't happening where the tiles are warped
likely has a lot less to do with the clay body, which is a real factor, than
with how the tile are dried. Uniformity is the key, and placing the tiles to dry
on any sort of flat *solid* surface is asking for problems as drying must
necessarily proceed from only one side of the tile: the top. The stresses thus
instilled during drying are only accentuated in firing.
Solution:
Place the tile on a stainless steel screen or wood slats, like they do at the
olarias, so they can dry from both sides -- uniformly.
Firing in a tile setter as opposed to firing the tile on a flat shelf likewise
provides uniform conditions that will also minimize the liklihood of warpage.
Cesco in Crooksville, Ohio furnishes tile setters.
And on to BaO, again.....
Bao Is a highly useful consituent in ceramic glazes for a lot of reasons. The
lone fact that it's there says absolutely nothing to whether the glaze has good
chemical durability. Likewise with lead. For the 50th time here I'll note that
the US tableware industry for years used PbO bearing frits in their glazes and
I'll defy anyone to show me a case of toxification subsequent to the use of this
ware. It simply doesn't exist. Why? Well, a lot of research was done to
determine the mechanism of leaching and to determine the est methods of avoiding
it. Why? Dead customers don't buy much and lawyers are more expensive than
research.
So, that said, let's set some ground rules. If we're going to discuss this let's
agree that simply saying this or that glaze contains BaO actually says nothing.
This is like saying water contains hydrogen and thus it is prone to burn. There
are a lot of other factors at work and if the discussion is to be useful the
entire composition of the glaze should be noted (preferably in mol%). As well,
it would be good to know how hard the glaze was fired and onto what.
We need to also understand that what, if any, hazards are due to Ba ingestion
are very poorly understood. Which is not to say they might not exist. In point
of fact, it remains that outside of entities with a financial interest in
discovering this or that "hazard", almost nothing exists in the literature
relative to poisoning by Ba ingestion -- outside of the extremely rare and very
extreme cases of exposure. The scale of discrepancies on discovers are indicated
by the fact that the USEPA TCLP/RCRA (leaching) standard for Ba is 20 times
greater than that for Pb, Cr or As and 100 times that for Cd or Se. Likewise, Ba
finds a lot of use in medicine, for example, as an x-ray absorber -- and apart
from using the terminally ill and old as guinea pigs for unproven medicines, the
medical profession isn't known for killing people.
Now let's consider leaching generally. There is no absolute measure of chemical
durability and glazes are judged by exposing them to some standard conditions
and comparing the results. The nature of the conditions has a huge role to play
in the results obtained. Factors such as temperature, pH, the ratio of the glaze
surface to solution volume, vaporation of the corroding solution, time, etc.
>>.... just put vinegar in a bowl glazed with a recipe containing the barium
frit and let sit for 12+ hours. If the glaze surface takes on a slight milky
hue, then the acid in the vinegar is reacting with the alkaline earth ions
(barium ions mainly).<<
This is hooey. It suggests a lot of things that either aren't substantiated
under any circumstances or at the least are open to question. All that is
indicated is that the glaze if very poorly formulated. It is also possible that
a glaze could release substantial quantities of Ba and never show a film of
corrosion.
If one has any sort of glaze that'd deteriorate to this (or any) extent during
12 hours of contact with vinegar -- *really weak* acid-- any number of things
could be producing the said milkiness other than Ba. This is not to say that Ba
might not be released into the film.
The film on the glaze is more likely silica hydrate and it probably does contain
Ba along with some of everyone else in the glaze. Yet the main reason not to use
a glaze this crummy om anything utilitarian is simply that it is useless for
anything other than looking at. I wouldn't use such a glaze on a brick. As well,
if it corrodes so drastically in vinegar, the glaze was also likely highly
crazed, which would condemn it from food use before any other consideration.
Consider that if anything is going to be extracted from a glaze, it's going to
be the most weakly held substance. In any glaze/glass, the alkalies are most
weakly held. Nothing is extracted in preference to the alkalies.
In extremely general terms, corrosion of glass surfaces happens in stages and
layers. Alkalies are first removed, weakening the network, and providing the
opportunity for other species in the glaze/glass to be set free. Once the alkali
leaves, a Si rich film is established in its place. In order for anything to
make it into solution, it must pass through this film. How fast it comes through
depends on its size, who else is in the leaching solution, and the affinity of
the solution to have the given ion within it. Ba, being a rather bulky ion, is
much less favored for extraction than any of the other alkaline earths -- at
room temperature. As well, from a thermodynamic point of view, all of the other
alkaline earths are favored for exchange than Ba - see any table relating to the
energy required to set free alkaline earth ions from silicate glass.
I want also to mention the time factor. In a reasonably formulated glaze, the
amount of anything that could be leached in the amount of time a salad, soup or
steak sits on it is *extremely* small. Extremely small.
This probably all gets back to considering what it is that makes a durable
glaze, and maybe this should be another thread -- before I get chewed for making
an extremely long post.
Regards from 27 degrees south
KPP -- eating a bad pizza
Eric Lindgren on mon 8 jul 96
>What's happening in these cases that isn't happening where the tiles are warped
>likely has a lot less to do with the clay body, which is a real factor, than
>with how the tile are dried. Uniformity is the key, and placing the tiles
>to dry
>on any sort of flat *solid* surface is asking for problems as drying must
>necessarily proceed from only one side of the tile: the top. The stresses thus
>instilled during drying are only accentuated in firing.
>
>Solution:
>
>Place the tile on a stainless steel screen or wood slats, like they do at the
>olarias, so they can dry from both sides -- uniformly.
>
>Firing in a tile setter as opposed to firing the tile on a flat shelf likewise
>provides uniform conditions that will also minimize the liklihood of warpage.
>Cesco in Crooksville, Ohio furnishes tile setters.
>
-----------------------------
In my experience, it does not matter how uniformly the tile is dried, if
the body is not well formulated for the forming method used. I make tile
from plastic clay: stresses are produced in forming that will manifest
themselves as distortion if the body were not formulated to minimize that
likelihood, i.e. an open structure with as little longitudinally aligned
particle orientation as possible (for instance using a primary kaolin -
with rounder particles).
It would be best to have even drying conditions, and hot humid conditions
would go a long way towards that, but most of us are working in less than
ideal circumstances. I dry tile on paper on untreated plywood boards. Those
absorbent materials help to even the drying rate, back to front. Also I do
not slow the drying, just flip once the day after making. No matter how
even the conditions, the corners dry out faster.
There are two basic types of tile setters: span and leaf (or supporting)
setters. Karl are you referring to span setters? For those of us firing
stoneware, or any glazed (preceded by a lower temperature bisque firing),
or once-fired glazed, this wouldn't be advisable as slumping and/or sifting
from the shrinking tiles would be likely.
For those of you interested, there is an association of tile-makers being
formed in Canada (where it is rarely hot and humid and we work mostly
indoors). Contact me please, if you'd like to be kept informed of its
progress.
Eric Lindgren
lindgren@muskoka.com
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