search  current discussion  categories  safety - toxicity 

hardness and leaching

updated sat 27 feb 99


John Hesselberth on fri 26 feb 99

Peter Atwood wrote:

>My question
>is, would a harder glaze be more durable and would a more durable glaze
>tend to leach less metals?
There is not a one to one correlation. Hardness will relate to scratch
resistance in use. A high boron-containing glaze I have tested, which is
slightly softer than others I have tested is one of the most resistant to
leaching. Your 5 hardness glaze will probably show visible signs of wear
sooner than the others.

With respect to your comments on copper, you may well be right. Of
course, people were saying the same thing about lead not too many years
ago. With more sophisticated analytical tools available who knows what
will be discovered in the future. For me, it is a matter of professional
craftsmanship to make my glazes beautiful AND stable to the use for which
they are intended. To me that means minimizing leaching of metals in
addition to criteria like an easily cleanable surface, stability to
dishwasher detergents, no metal marking on plates, etc.

By the way I hope you are not remembering your chemist uncle correctly.
ppm and mg/l are numerically the same. 20 of one equals 20 of the other.

John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: web site:

"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer