Searching for the right fake celadon glaze: part 3
I thought I had found the perfect transparent high gloss glaze that could double as a fake celadon by just adding Copper Oxide. However, when passing on the recipe to one of my fellow students, she pointed out to me that it contained Barium Carbonate, a substance which may not be food safe. So, whereas it may be perfectly safe to use on my sculptural pieces it may not be appropriate for my little pinch pots. After all Barium Carbonate is used in rat poison, doh!
So, I trawled the internet to see what pottery sources had to say about Barium Carbonate's safety and found opinions not 100% consistent but the advice generally is to err on the side of caution and avoid it for pots that may get into contact with foodstuffs.
Thus I'm trying out other glazes, which hopefully contain no toxic substances that could leach out and contaminate food therein. My aim is to reduce the number of glazes I'm having to relay upon across my sculptural pieces and my more utilitarian pinch pots, i.e. to have one nice clear transparent shiny glaze that can also be adapted as a fake celadon.
Here are the results for my first four glazes all tested on Valentine Clays PF700G (white stoneware porcelain grog) to 1260°c in oxidation.
On the left: colourless base glaze over blue, black and red underglazes to gauge its transparency.
Middle and right: same glaze with just 1% or 2% Copper Oxide added over white design liner.
The only glaze that works for me is Glaze I, which is transparent, very glossy and produces the right colour with 1% Copper Oxide added. The others are either too opaque, not glossy enough or produce the wrong sort of celadon look. Next I will try to match a bigger batch and glaze some pots and compare the outcome with my Barium Carbonate glaze.
The original Glaze I recipe was an American one containing Ferro Frit 3124 and Feldspar (Soda F4) which I substituted with Borax Frit and Soda Feldspar. I could alway tinker around with the quantities of the ingredients to get a better result should it not be quite right. For instance, the firing temperature was given as Cone 6 (=ca 1230°c) but I fired my test tile to a higher 1260c. On a vertical surface this glaze may run off. Let's see.
An alternative approach would be to see whether I can find a way of adapting my original glaze recipe (Glaze C1) by substituting the Barium Carbonate with a safer ingredient. Again, the internet offers helpful suggestions. For example, according to the online Digitalfire Reference Library (https://digitalfire.com/4sight/material/barium_carbonate_86.html):
"BaO can often be sourced from a barium frit (like Ferro CC-257) instead of raw barium carbonate, provided that the percentage is not too high. Do not assume that BaO is necessary in every glaze in which it appears. In some recipes BaO can be substituted for SrO or CaO and even MgO (using glaze chemistry software of course) without losing the color or surface (bright glossy blues, for example)."
However, go ahead and do such substitutions I need to find out more about chemistry and as one cannot simply substitute the ingredients by weight but needs to base the calculations on a molar (molecular) basis to ensure that the correct ratios of total fluxes to alumina and silica are maintained - thus the need for glaze chemistry software. The Ceramics Arts Network website has the interesting article Leaving Bariumville: Replacing Barium Carbonate in Cone 10 Glazes (17 Nov 2009) by Jennifer Harnetty in which she summarises her findings when she substituted Barium Carbonate with Whiting and/or Strontium Carbonate. She does so fairly successfully with her Celadon glaze which is fired to cone 10, i.e. higher than my firing temperatures. I assume that the tests were fired in oxidation, even though it is not stated anywhere, as they don't contain the classic Red Iron Oxide but a combination of Yellow Iron and Chrome.
Anyway, this may be a future avenue to investigate depending on the outcome of my own tests.