Toxins

GLAZE OF THE WEEK 11

SCARY BARY AND CERAMIC TOXINS

• Which glaze components are toxic v What’s so bad about certain ones v What is good about some of them

• What is leaching

• Safety precautions

It is about time to talk about the toxic chemicals in glazes and how to handle them. Almost every glaze component can be harmful even if they are not toxic. The finely ground particles float lightly in the air like fiber glass where they can eventually enter the respiratory tract or mucous membranes and cause irritation. Some people are more easily challenged with this kind of irritation than others.

Both glaze makers and glaze users must exercize caution. The recommendation when making up glazes is to use a dust mask and get the chemicals wet as soon as possible by mixing them into water directly after weighing to minimize clouds of dust. When glazing pots, it is important to wipe up spills, more for safety than good housekeeping. After those spills dry up, get walked on, or leaned on, they become the same dust particles that can irritate you or someone else.

Barium Carbonate is a white chemical powder which is used to interact with other color forming compounds in glazes to create intense blues, greens, and purples. Barium glazes are so dramatic in color that it is hard not to use them. However, it is extremely toxic in very small quantities. Worse than lead. [OK, so Barium enemas are X-Ray radio opaque medical procedures, but a person is not likely to get very many of those in a month, and never in the respiratory tract. Also the Barium is mixed with thick stuff which is quickly eliminated.]

If Barium is absorbed into the system, it would only take a gram (approximately half a teaspoon) to be fatal, and no one is sure if it is additive from day to day. From here to fatality might not be too pleasant if Barium is absorbed in smaller amounts. It is said to be less fatal if inhaled, but it can cause chronic lung disease. Barium salts have been used as rat poison.

If Barium is used on pots that are used for food, leaching is inevitable because barium is soluble even after high firing. All foods have acids which can get through the microscopic pores of the glaze just like acids can eat through most kinds of glass eventually, and dissolve the Barium. In this way small amounts over time can be consumed and could cause problems even in high fired ware which has a hard surface.

Many potters use Barium in Chuns and other dazzling glazes. Toshiko Takaezu’s gorgeous big blue matte pieces have a Barium glaze on them. The key word here is “on” them, on the outside of the piece. No need to worry about ingesting any Barium if it is on a pot; we might worry about the potters though because of using a lot, but they should know how to handle it safely.

HPG does not use Barium in its glazes because of potential danger to the community when we sell our pots to unsuspecting buyers who assume our pots are safe to use for food. Our waste waters have been tested free of toxic materials therefore it can be said that HPG is not contaminting soils or waters. Barium has been removed from HPG’s inventory for eight years, and before that was used sparingly.

Strontium is a less toxic replacement for barium though the color development is not as good, so HPG has not considered the change worth it. No one will like it.

Other toxic metals need to be considered. All of them can be toxic, but not fatal, therefore it is essential to use gloves and mask when weighing and mixing these compounds. The frequent use of these compounds in a water suspension such as in slips and stains

1

brushed on for decoration is not dangerous because of their containment in water. Care must be taken when they are in powder form. Top of the list of the ones we use is Copper, either the carbonate or oxide, for blues, reds and greens. The raku glazes are very copper rich. A plant potted directly in a copper rich raku pot will die. Cobalt is high on the list as well to get the blues we all like. Don’t let it get absorbed through the skin.

Nickel (browns, blacks), Vanadium (orange and browns) and Cadmium (yellows and oranges) are called “heavy metals” which means they are not dissolved and excreted by the body; they just stick in certain organs where they can do harm like cause cancer in some people. Lead is used for color and a flux in low fired glazes, but is quite toxic. HPG glazes do not have any of these compounds in use, and they are removed from the inventory.

Lithium (in Petalite and other stuff), Zinc, Tin, Manganese, Magnesium, Titanium (in Rutile), Zirconium (in Zircopax) and Chromium are commonly used in glazes and stains for color of other effects, and must be considered potentially harmful. Aluminum is in all glazes for its as a hardening agent as well as a flux. Silica is glass, something not good to inhale. Iron is especially common in HPG glazes and does no particular harm. Almost all of them are trace metals essential for the body, but the body doesn’t know what to do with large amounts.

About HPG Frits: These are scintered glasses containing fluxing chemicals like borax. None of the HPG frits contain Barium nor Lead. Some contain Lithium. They enhance melting and colors in the glaze.

Don’t be scared, just be careful.
Pat Harwood 7/17/2005 ed January 2009 ed April 23, 2009 Nov 2010

2