Dry glaze application

[luis goldfarb]

Hello! I read a post about dry glaze application. I cannot find it now, but wanted to ask how do you apply these dry glazes, for example to a glazed plate. Do yo apply the dry powder with a sponge onto the newly glazed bisque ware? Can it be applied on both sides, up and under it?

I am trying to investigate with glaze superpositions to achieve variegating effects. Is there an alternative way to obtain this? (I am attaching a picture of what I would like to get)

Tahnk you very much. Regards from Argentina!!

[Norm Stuart]

I've always applied glaze wet.

Porcelain factories often apply finely powdered dry frit glaze to plates and bowls which they have charged electrolytically so the glaze sticks thinly and uniformly due to the static electrical force just before they enter the kiln. The ware is typically also hot from the prior firing so the frit melts partially as it touches the bowl or mug.

Tile factories in Spain apply dry frit glaze by gravity through sifters to large floor tiles just prior to firing. Obviously this sort of application can only be used on a a relatively flat surface and only on one side.

Tile makers are major users of specialized oxide granules sold by firms like Rabco Specs, which are also known to potters as "glaze crystals" to create the speckled look of granite or similar effects. Each color is pre-milled into a variety of sizes.

http://www.rabcospecks.com/

Your photo might also be made with one wet engobe layered over another then physically distressed, or it could be a two-layer oil spot like glaze.

My personal guess is this is a photo of a sintered ceramic product without any glaze applied - similar to the sifter used to apply dry glaze to tile, or shoveled into the molds, but this would be ceramic body material of two different colors rather than a glaze.  The dry body material would also very fairly granular rather than fine. Where did you find the photo? 

[luis goldfarb]

Thank you Norm. It's very interesting what you tell about industrial dry glaze application. I am trying different glazing techniques for a number of plates and bowls. A friend of mine who is a chef at a restaurant would like to have some dinnerware with the appearance of that in the photo he sent me. I did obtain some variegating glazes by glaze superposition.

For the first layer of glaze, I am using the following formula:

17.9 Whiting

8.0 Zinc oxide

49.2 Potash Feldspar

19.9 Kaolin

5.0 Quartz

Add:

6.7 Iron Oxide

1.3 Cobalt Oxide

And for the second layer:

50 Soda Feldspar

30 Dolomite

5 Whiting

20 Kaolin

5 Talc

Add:

6% Tin Oxide

Here I show some tests I made.

These test were fired to cone 6 and with a slow cooling so that crystals develope.

The problem is that it seems that due to the amount of iron in the first glaze, all colours result brownish. And I need to obtain Gray tones.

I don't know if what makes the underlying glaze to be reactive and break through the second white layer is the amount of iron and the crystal developement.

I tried different glaze superpositions but the only variegated results were achieved with the high iron glaze.

[Norm Stuart]

Making plates for a chef, your primary concern should be food safety - so eliminate the 8% Zinc Oxide, the 1.3% Cobalt Oxide and the 6% Tin Oxide.

What you need is a "Mason Stain" for the Black and Mason Stain or Ziconium Silicate for the white. Neither will leach into food. 

Mason Stain - Best Black 6600

Mason Stain - White 6700    or   Zirconiumm Silicate - brand name Zircopax Plus

The Mason Stain manufacturer

Although the black Mason Stain or other colors may contain many of the same ingredients, Mason Stain colors are encapsulated in zirconium glass, or processed in a different manner, none of which melt below Cone 10 - so they are safe.

Mason Stains are primarily used to color masonary or cement, so you may be able to find these at cement makers.  But not all colorants for cement are food safe.  The "Mason Stain" brand is also made to be food safe for ceramics.

It is possible to use toxic ingredients in a glaze if the result of the firing traps them inside as stable glass like the zirconium does.  But this requires testing with acids and bases, and wear testing.

Food Safety - Digitalfire

Not all glass is stable, Using copper as a colorant, will make glass particularly vulnerable to erosion over time.

How glass is attacked by food

"Limit Formulas" to make safe glass

When using metal oxides as a colorant, even iron oxide can be unsafe if it is not pure.  Many iron oxides also include toxic elements like Manganese and Barium.