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vinegar test leaching

updated sat 5 dec 09


Bill Merrill on fri 4 dec 09

Here is a short informative article on vinegar test leaching....





Many glaze recipes employ hazardous materials, especially heavy metals.
Additionally, unsafe firing practices can lead to immature glazed
surfaces, which jeopardize the relative safety of a ceramic piece for
use in food service. The following policy describes several unsafe
materials commonly found in decorative, non-functional ceramic pieces,
and addresses firing techniques to ensure that glazed ceramic ware is
safe for the U.S. market.

The US FDA (Food and Drug Administration) tests ceramic ware extensively
for leaching metals. While the US FDA is ready to take action against
offenders, it is much more anxious to see initiatives by companies and
potters to learn how to recognize leach-likely processes, materials, and
formulations and learn how to fix the problems.=3D20


Although glazes are glass, they are not indestructible. All glass
leaches to some extent, even in contact with water. Acids, like vinegar
and citrus, magnify glaze leaching problems. If a glaze is made from
harmless materials like silica, dolomite, kaolin, feldspar, whiting,
ball clay, etc., leaching is only a functional and aesthetic issue. But
if the glaze employs metallic colorants (other than iron) or other
minerals containing lithium, barium, lead, chrome, or other heavy
metals, then safety and legal liability become concerns.=3D20

Leaching issues are not limited to the relative danger of glaze
ingredients. The issue is complex and involves materials preparation and
firing processes, as well as glaze formulations. It is possible to use
toxic materials safely and it is possible to compromise an otherwise
safe glaze by creating unbalanced mixtures.=3D20

Glaze Maturity

Testing glaze maturity is essential in the prevention of leaching. If a
glaze is not properly melted then it will leach. It is recommended to
fire all glazes to one and two pyrometric cones higher and lower than
the anticipated temperature for production. Line visual inspection of
the results will typically reveal any problems. This test is especially
important with the use of matte glazes, which do not reveal their
maturity as readily as glossy glass glazes.


Glaze Recipe Balance

Glazes are also leachable if they contain inadequate glass former and
alumina. Flux saturated glazes are very common because they develop
surfaces usually associated with non-homogeneous melts of more and less
fluid components. If a glaze formula calls for a high proportion of
flux, it is likely that silica and alumina are lacking. A glaze with
high feldspar (50%+) is a classic example (feldspar by itself is
leachable). If inadequate glass and intermediate oxides are available,
coloring oxides that might otherwise be securely held in the glass
structure may also leach. Producers should always calculate the formulas
of their glazes and compare them with limit charts to ensure enough
glass former and alumina are present. Sometimes it is necessary to
compromise visual character to produce a product more resistant to acid

Another simple thing to check for is material amounts that do not seem
normal. It is common, for example, to see 5% talc in a glaze, but 30% is
definitely not normal. Likewise more than 5% lithium carbonate or zinc
is strange and needs explaining. Watch also for high amounts of Gerstley
borate or boron frit (more than 10%) in high fire glazes, this is not
normal. In addition, every glaze should have as much silica and kaolin
as it can tolerate. High and middle temperature glazes with little or no
silica or kaolin need an explanation. Low fire glazes must have lots of
boron sourcing material like frit or they simply will not melt. Many
frits are quite balanced as a low fire glaze (e.g. Ferro Frit 3195),
others are not (e.g. Frit 3134) and need more added silica and kaolin.

Firing Temperature

Firing a glaze to a high temperature does not ensure its stability.
Although most high-fire glazes contain less flux and more silica and
alumina, many popular high-fire recipes still contain a high percentage
of de-stabilizing ingredients, such as metal oxides, barium, and


Carcinogens and Poisons

Many glaze recipes contain toxic ingredients that are difficult to
stabilize, and are extremely dangerous if allowed to leach. Barium,
manganese, copper (it increases glaze solubility), unknown frits (may
contain lead), chrome and lithia are all relatively common, and all
poisonous. Glazes containing these ingredients should be avoided. Stains
can contain a wide range of toxic metals, but they are optimized
chemically to produce the intended color and therefore tend to contain
less toxic metals than would be needed if you tried to produce the same
color using raw metal oxides. In addition stains are smelted materials;
they are mixtures of metals and stabilizers and are therefore inherently
less soluble.


A liner glaze

One great way to ensure glaze safety without compromising aesthetic
character is to use a liner glaze on all surfaces of pieces that come
into contact with food or liquid. Liner glazes made from materials like
kaolin, ball clay, bentonite, silica, feldspar, whiting, wollastonite,
dolomite, nepheline syenite, Gerstley borate or boron frits (having only
B2O3, Na2O, K2O, CaO, MgO, Al2O3, SiO2), talc, iron oxide, rutile, and
zircon, titanium and tin opacifiers are safe. Frits are also safe as
long as they contain oxides common to the above materials (no lead nor
barium). Be sure that colored under slips or glazes are not diffusing
metals up into the liner glaze, especially where the liner might be


Glaze Crazing

Crazed glazes create a bacterial 'breeding ground', as they are porous.
If the craze lines are of adequate width, bacteria will thrive if the
clay body supplies absorbed moisture from below. To test for water
absorption, soak the piece in question for a few days, then put the
empty vessel in the microwave. If it gets hot (and it is not of a high
iron clay or glaze) then it has absorbed water. Weighing a new piece
before and after soaking will verify this.=3D20

Another factor to consider: Glazes craze because they have a high
thermal expansion. Often high expansions are associated with low silica
and alumina contents. Stability in glazes is often linked to the amount
of silica and alumina-- the more the better. Thus, crazing can indicate
a potential leaching problem.


Simple Vinegar Leaching Test

Fill a glazed container half full of vinegar and leave it for several
days. Dry the item and compare the color and surface character (e.g.
lustre, gloss, texture) above and below the liquid line. Any difference
indicates that the glaze is subject to leaching. There is a way that you
can amplify the result: If your glaze is white or transparent add 5%
cobalt and do a vinegar leaching test on it (if you are using a colored
glaze, add enough of the colorant to get a bright color before doing the
test). Excessively unstable glazes can easily turn white with an
overnight vinegar leach.



* Make sure glazes melt well enough to form stable glass.=3D20
* Use as much alumina and silica as possible, while ensuring a
thorough melt.=3D20
* Glazes containing an oxide that has a reputation for
destabilizing behavior (like copper), have it professionally tested.
* If any metallic oxide colorants are used, be wary of large
* Be wary of all glazes with heavy metal oxides (barium, lithium,
manganese, and lead).=3D20
* Use liner glazes.

Further reading

* Keeping Clay Work Safe and Legal by Monona Rossol
Industrial Hygienist, Arts, Crafts and Theater Safety, 181
Thompson Street, #23, New York, NY 10012-2596=3D20
* At the 2001 NCECA conference in Charlotte, NC, John Hesselberth
gave a presentation on "Stable Glazes for Functional Pottery". Over 500
people heard this presentation and showed a great deal of interest in
the subject. John is making available a fully-scripted teacher's set of
35mm slides of this talk in the hopes that ceramic educators will begin
to teach this important subject to their students. For more information
visit John's web site at:
* Glaze Stability to Acidic Foods and Basic Cleansing Agents: A
Review of the Literature
A study in progress by John Hesselberth at
* California State Proposition 65 and State Assembly Bill 3659
lead/cadmium guidelines 1991 Glazes are glass and tend
to be thought of as timeless, indestructible. However all glass leaches
to some extent when it comes into contact with water or acids,
especially if the contact occurs over a period of time or the acid is