I started writing about leaching and glaze materials in my IG story about 2 years ago. You can find it in my IG highlight. What is leaching, you asked?
Leaching in glazes means that glaze is slightly-soluble into
food and liquids they come into contact with. There are a lot of things that we
don’t know about leaching in glazes. We think that glazes are glass that is timeless
and indestructible. Well… all glass leaches to some extent when it comes into
contact with even water. With acids, especially if the contact occurs over a
period of time; or the acid is hot, the effect is obviously greater.
Over time, glaze color can change! You can do a demonstration by trying this
combo: 33% CaO, 42% B2O3, 6% Al203 and 18%Si02 at 950C. It should fizz and
dissolve in vinegar within minutes even though it fires to a clear and
apparently hard surface!
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 or other minerals containing lithium, barium, lead, chrome,
etc. then safety and liability becomes a concern. I am very concerned about the
glazes that I am using in my tableware products. I make sure my glazes do not
contain known carcinogens or poisons. It is better to reduce the usage or not
to use at all materials that will harm user, such as Barium, Manganese, stains
with wide range of toxic metals (raw metal oxides stains), frits that contain
lead, Lithia, chrome.
I high fired my reduction/ oxidation fired pieces
(1250C-1280C), thus the glaze matured! If a glaze is not properly melt, such in
a case of low firing, you cannot expect the piece to be resistant to leaching. Feldspar
is leachable! I know adding a lot on your glaze will make a beautiful result,
but a glaze with high feldspar (+50%) is an example of allowing leaching. Same
case goes to “wanting the matte pcs instagramable for your food shot”; if an
inadequate glass (and alumina) plus intermediate oxides are done (oh yes, I
have a lot of cringe moments watching pieces on instagram post) the structure
of glass will not hold the oxides, thus it is leachable.
I would love to share more about underglaze, craze glaze and
how to test leaching.
Remember, when the same coffee tasted different when served
in a glass vs served in a glaze fired cup. You can start questioning the glaze
you use!
Also, be human and be kind, not for the sake of making money
or cut losses that you would sell your second pieces that have crawl glaze on the inside part where the food / liquid are in contact and
pinholes in the inside part where the food / liquid are in contact.
