As my technical project, which informs my Subjects, I tried to devise my own in-glaze (Maiolica or Faience), surface decoration process.
I wanted to emphasise the rich texture of my hand-build terracotta sculptures, crisscrossed with veins-like imprints of my palm. The Tin glaze allowing Iron rich terracotta undulations to show through the opaqueness, but at the same time providing a light background for bright colours to flourish on the newly formed skin of my growths.
looking for the perfect opaque
I tried 10 different transparent or white glazes from various sources, firing each at 1060C and 1160C, as well as their response to opacifiers (either Tin Oxide, Zirconium Silicate, or a combination of them).
All of the glazes were based on High Alkali Frit (or as a replacement to Ferro Frits), Lead Bisilicate, Standard Borax Frit, Calcium Borate Frit, or a combination of them.
They all performed just fine, when the firing was actually good, with only some showing common problems: forming bubbles/foaming, mainly at thicker places, or having uneven, ugly milkiness, or present cracks.
Some had interesting idiosyncrasies, such as the Morgan Hall’s Maiolica glaze where the the small addition of Lithium Carbonate brought forward bright iron oranges, through the white glaze. The very simple clear terracotta glaze combining Calcium Borate Frit (65) and Red Clay (35) had a very good fit on terracotta between the temperatures of 1100-1150 C. It produced high gloss bright terracotta, almost honey like, but with greenish tint. However, with the (slightly higher) addition of opacifiers (Tin Oxide 8 and Zirconium Silicate 6), it could only produce murky browns.
Testing first 7 glazes in 2 temperatures, as clear and as opaque.
From the tests I could combine the best performing transparent base recipe (a combination of mainly Standard Borax Frit and small addition of Calcium Borate Frit), and the best performing opacifier (an equal addition of Tin Oxide and Zirconium Silicate).
My preference was a non-Lead glaze, that can be used on functional ware and can withstand the highest range of terracotta (1150-1170). This allows the clay to reach its fullest maturity – fullest strength and rich, toasty colour.
I’ve put the fired samples under stress test too, soaking them in water for prolonged time and then drying them, either naturally or by the means of a heat gun. I also heated them high with the heat gun and then submerging in cold water, as well as scratched the glazed surface with sharp metal objects.
I wanted to test the functionality and strength of the glaze, if it would chip too easily or develop cracks. The samples withstood the stress tests marvellously, possibly because of the addition of Zirconium Silicate as opacifier, which can strengthen a glaze. However, it’s important to note that the size of the samples were quite small, so tests on larger objects with larger glazed surface area might behave slightly differently, amplifying all forces.
colour tests and tests and tests
First colour tests. Applying oxides or stains mixed either with just water, water and frit, or water and transparent base glaze. There was not much difference, so I went with the most recommended version of water and transparent base glaze, adjusting the ratios as I went along in the research project. Starting with ratios of 1:1:1 – colouring powders:transparent glaze:water to 1:3:2 and later even more water or glaze as I was adding Rutile by 1/8’s.
For faint colour effects, it was easy to thin the colour mixes with a brush and water during decorating, very much like water colours.
When starting to test in-glaze colour additions, the chosen base glaze proved not stiff enough, making brush strokes bleed and finer detail disappear in the highest range of temperatures (1150-70).
I tried to adjust the recipe and the mixture of colouring glaze, but at the end I settled with the commercial transparent glaze available in the glaze workshop as the best base, with the addition of the opacifier mix.
The commercial clear glaze provides just the perfect surface and firing range to use flexibly (from 1000-1160C but survives slight over-firing too).
Having the base glaze and colour mixture finalised, I could start looking for materials and mixtures creating a range of colours.
I started with the traditional oxides used to produce a number of colours throughout the history of in-glaze.
Cobalt Oxide (also Carbonate) for blues, Chrome Oxide for greens, Copper Oxide for blue-greens, Manganese Oxide for browns to purples, Iron Oxide for red-browns, Rutile for rusty orange, and the combination of Lead and Antimony for Naples yellow.
yellows, marigold to oranges
The search for the traditional Naples Yellow has been, unfortunately unsuccessful. I tried various rations of Lead Sesquisilicate or Lead Bisilicate with Antimony but only achieved faint yellows on a bubbly surface. This could be due to the lack of Lead in the opaque background glaze with which Antimony normally reacts with to create the colour, or just by me consistently missing the ratios.
For the oranges I tried mixtures of Vanadium (O4), Rutile (O7), with tiny additions of Cobalt and Chrome oxides, or mixtures of Synthetic Iron Oxide, Titanium Dioxide and Manganese (O9, O9b).
Non of them were really satisfactory, not even pure Orange stain for its un-toned bright colour (O6). However, using small amounts of Rutile as addition to the stain was the perfect way to control the colour, creating a gradient to more gentle and warmer tones.
Rutile proved useful as addition to yellow stains to find the interesting colour ranges between yellow and orange.
Furthermore, diluting the stains and Rutile in the opaque glaze produced beautiful pastel versions of any colour.
lavender to purples
Manganese Oxide is traditionally used to produce plum purples to browns, however I haven’t achieved any good colour here either, perhaps the lack of Lead has a role here too.
The P10 and P10b was the closest I could come to plum purple made up of Manganese Carbonate and small additions of Cobalt Carbonate.
With 1/8 additions of Rutile the Manganese/Cobalt purple as well as purple and lavender stains turned beautiful blue grey, but any larger additions of Rutile turned the samples towards browns.
For the first time I also tried to add 1/8 of Red Iron Oxide, instead of Rutile. As Rutile is impure version of Iron Oxide, the effect was just the same but amplified, making the samples turn to orange browns with smaller additions than Rutile. Continue reading Technical write up on in-glaze