Technical write up on in-glaze

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.

Identifying few base glazes and tips from Matt on Maiolica

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.

Further base glaze tests as well as first colour response, mixing Cobalt Carbonate in mixture of the transparent base glaze and water.

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.

Testing on larger scale, with first instability problem arising.

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).

Commercial transparent glaze with opacifiers and test colour strip.

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.


One of many firings to test colour mixtures. Looking for Turquoises to blue, Yellows, Greens, Oranges.
Additions of Rutile by 1/8 increments to various stains.


Toning Yellow, Orange, and Marigold Yellow stains with Rutile. Also, using the opaque glaze to dilute the stains and Rutile creating pastel tones. Also showing some first material mixtures to find oranges.

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.

Looking for purples to gentle lavender.

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


Colour in Glazes Technically

Returning to Subject Module, I started to explore colour in glazes on my own, to help me develop specific colour pallet for my Subject work.

Orange being the most prevalent colour in Penguin Books designs, it provides a great depth of symbolism and context associated with the cultural impact Penguin Books had in UK and other English speaking countries. It brought inexpensive fiction and non-fiction to the mass market, educating, and having impact on public debate in Britain, through its books on British culture, politics, the arts, and science.
Therefore, I’m using orange as colour of possibility for social and personal growth.
I found number of good colour pallets around orange, but one with split complementary colours: blue and purple stood out visually, and belonging in the Penguin colour scheme with dark blue representing Biography and Purple representing Essays.

I started with a search for multiple recipes of the chosen colours in EW and SW, which would give me glaze bases and colours which I could then combine and refine.

I tested 4 SW recipes with one very successful Barium based glaze producing interesting dark blues with orangey halo on White Saint Tomas, and another glossy Soto Amber with interesting double colouration, but in green and browns.

Adapting the 2 base glazes, I stained them with commercial stains, which however burned out or in case of Lilac and Rosso Red Stains just reduced in vibrancy. 20170518_111126-COLLAGE

Therefore, I started experimenting with combinations of oxides to mixed into my leftover stained batches, referencing oxide combination sheet in our glaze room.

In the Barium based glaze I used:
Vanadium + Cobalt Carbonate + Titanium Dioxide produced matter, more even, lighter blue.
Manganese Carbonate produced very dark uneven purple.
Vanadium + Rutile didn’t show up and the glaze remained white.

20170518_110437-COLLAGEIn Soto Amber I used:
YIO + Rutile + Vanadium producing just light brown with streaks of blue.
Cobalt Carbonate + Manganese Carbonate + Rutile producing dark background with interesting blue streaks.
And RIO, browning the Rosso Red stain.








I had more luck in the EW glazes due to the simplicity of using commercial stains.


Finding another 4 different glazes, mainly Lead based, but one with Wollastonite andIMG_7803-COLLAGE Strontium carbonate as fluxes which proved as most stable on different clays, pleasant surface and gloss, not settling down and good colour response.

I was able to use colorant combinations from the other glazes to adapt the non-toxic base glaze, and experiment with other combinations, to produce a range of oranges, yellows and dark blue purples.

If I had more time I would really go for a bit more scientific/controlled way of testing the additions to perfect the colours, rather than my very quick, mostly intuitive decision making.

The fact that I opted for spraying my final work, made the glaze much brighter, but at least I’m not having brush streaks or loosing any details in texture.


The latest test piece glazed with my Penguin Purple, Penguin Yellow, Penguin White and Penguin Orange applied by spraying were acceptable. The one directional application even suggested sun exposure, as if sun rays sprayed the piece, giving direction to the growth.






NICHOLAS JOICEY; A Paperback Guide to Progress: Penguin Books 1935–c.1951. 20 Century Br Hist 1993; 4 (1): 25-56. doi: 10.1093/tcbh/4.1.25 []

Colour in Slips technically

Starting with Slip technical, exploring colours in slips with Morgan alongside Field module.


I started with a search across library books and the internet for base, white slip recipes of which we mixed 7.

We applied them on terracotta and White Saint Thomas tiles in 3 various layers fired to EW and SW temperatures, half glazed. Also fired on its own.
The blobs of slip were created by piping about 20ml of the individual slips onto plaster bat and gently smearing them. However, they proved extremely fragile after drying, some of them quickly cracking in the process of drying. Anyway, the colour was the same as 3 coats on the tiles; on the other hand, we wanted the blobs of slips for more interesting and easier presentation of our outcomes, giving us possibility to use them to create an installation piece.


The experiment allowed us to see that the simple recipe used in our glaze room: 50/50 of Ball and China clay has the best result in producing white slip in EW temperature, meanwhile using Porcelain powder slip is whitest for SW temperature.

IMG_7383Morgan’s temperature test showed us the colour changes in number of oxides and staines in EW and SW temperature.
Using the chosen 50/50 slip he stained it with Red, Yellow, Purple and Synthetic Iron Oxides, as well as Coral stain and Chrome Oxide.
Half glazed, they reveal a dramatic change in colour.
This test made us stick with EW temperatures, with added bonus of reducing cost and environmental impacts, but being aware of increased fragility of EW products.

To further see the colour response of each slip recipe, we added 10% of Yellow stain to each recipe, bisque fired and half glazed at EW temperature, on terracotta tiles.
Most of them look the same, except 1 weird recipe that melts on SW.

These experiments gave us really just the starting point, establishing processes and testing techniques, as well as backing up choice for base slip and temperature.
Rigorous testing of combinations of stains and oxides.

We want to be able to produce a specific colour pallet in specific shades, similar to Jin Eui Kim’s carefully mixed tonal range of his engobes, to create an illusion of curves and voids. ltvs-jineuikim-5.jpg

Technical Exploration

Technical exploration within my Subject project and Technical Research project;
exploring slips, layering, and simple EW melt test and glazes.


Looking at my object from the Ken Stradling Collection, I wanted to try and recreate the 20161024_100056plywood that created the curves and form of the Penguin Donkey in clay.
Using slips seemed like the easiest option to start with, applying it onto a plaster bat with a brush, layer upon layer of blue and white as well as black and white, drying them slightly with a heat gun to prevent the colours mixing.
The problem arise when taking the ply-clay slabs from the bat. I tried to use thin metal kidney, but the slip continued to tear and distort even when drier.

Screen Shot 2016-12-06 at 00.35.59.pngI assume the difficulty of removing slabs from plaster are due to the thinnes of the pieces. Anyway, I wanted the overall thickness, as well as the thickness of individual layers to be much thicker, closer to the thickness of real plywood.

I mixed a bigger quantities of slip to experiment with, with different colours to try various colour combinations.
I chose a specific colour palette based on the most widespread Penguin Books colour – Orange, and its split complementary colours to work nicely together, layered upon each other.
In Penguin Books’ categorising, Orange is representing fiction, whereas Blue is for bibliographies and Purple for essays.
Further colours close to the 3 split complementary that I will consider to use is Yellow (miscellaneous), Cerise (travel and adventure) and Pink (drama).

I made multiple thin guides in the wood workshop to guide me at making the individual layers uniformed. IMG_1510
The scraping problem didn’t reoccur again, and the thickness and uniformity was ideal.
However, I did use quite a lot of slip; but more importantly, I wasn’t able to bend the slabs easily at all.
I was only able to create a small curved pieces, with quite a lot of cracks.

I think for next trial I’ll have to investigate adding a paper pulp to the slip to make it more 20161027_161418flexible when forming, as well as create a plaster moulds to bend the slabs over into desirable form.


20161122_153049 I also tried to paint the slip over a pieces of paper to help remove the slip slabs from plaster and make the forming easier. I attempted to recreate and enlarge some of previous tests, but it didn’t work that well, having difficulty to join the sides and hold the shape.











A series of 10 melt test of simple combination at EW temperature (1050-1070C).
I started with China Clay and Flint with additions of Borax, High Alkali or Calcium Frit, or Potash Feldspar, Wollastonite or Strontium Carbonate, with additions of orange stain or Rutile for colour.

I played with their values on to predict how they could behave.

Few of them didn’t fuse, but most of them turned into a glaze with different matt, porridge like or shiny transparent effects.

Processing my dug clay

Initial processing of my dug clay from Fforest Fawr near Tongwynlais, and shaping it for experiments and analysis.

After drying up my clay at home and braking it into smaller pieces, I brought the dried clay to the studio where I covered it with water to soak and brake the dried pieces up again.

Leaving it for a few days to sit and braking it up a bit more with hands I noticed how my substance is more like a sandy mud, than a clay.
I decided to screen it first through 30 mesh sieve to remove any larger sand, rocks and organic debris more easily, and then again through finer 40 mesh sieve.

My very short clay, most likely very high in organic compounds.

The process was very strenuous and incredibly smelly, the clay substance releasing powerful sewage odours, giving me more proof that what I have is a highly organic and sulphury substance with little clay in it.

When I removed all larger sand and organic debris with the sieves, I poured the wet mud onto a plaster bat to soak up excess water, and tried to wedge it into a one ball.
The substance felt rather sandy and as sand is a non-plastic part of a clay, I knew that my clay would be very short and lack plasticity.
This would make it difficult to shape my clay into tiles for further testing, therefore I decided to integrate some clay available in university to make it more plastic and malleable and see how the two clays would react.

There could be another option on how to separate and retreat the clay from my dug substance, using levigation. However, that was the more time consuming process and I already felt slightly behind with the task.

I decided to introduce the same amount of porcelain as my dug clay; choosing porcelain for it’s plasticity and whiteness; so that any colouring from my raw clay would be more noticeable.
Wedging the two clays together, the mixture was still too short for my likings, so I added a bit more porcelain.

Totally my clay mixture consists of 1296 grams of my raw dug clay and the same amount 1296 grams of porcelain with extra 300 grams, as the mixture was still too short. That is 44.81% of clay sourced in Fforest Fawr and 55.19% university’s porcelain.

Raw clay test tiles
The test tiles and 100 grams balls.


I ended up with a more plastic and malleable clay, but also more porcelain than my dug clay.

I managed to shape the original raw clay at least into a ball of 100 grams, to test the water content in the dug substance.
The clay mixture was also shaped into a 100grams ball to see the weight difference compared to the pure raw clay, bone dry and then bisque fired; suggesting the amount of water as well as chemically bonded water in the clays.

The test tiles from my porcelain and raw clay mix will be fired to different temperatures raging from just bone dry, biscuit fired to 1280°C reduction.
This will demonstrate changes across the different temperatures as well as the shrinkage rate, when measuring the 10cm long line marked on the tiles.

After drying for few weeks and then weighting the 100 grams balls again, the difference in water content was 33.3 grams for raw clay and 26.1 grams for clay mixture with porcelain.




Source of my Dug Clay

8GB-Old-Map-British-IslesAs a summer project before starting the degree we were asked to dig up some local clay for further testing, analysing and experimenting with.
Also undertaking a short research into how the area was used before for production; socio-historic timeframe, history and geology.

I dug up my clay in one of my favourite explorational places around Cardiff, where I tend to go on a short bicycle trip to relax and explore.
Fforest Fawr with the Castle Coch nesting nearby, above the village of Tongwynlais.

Landscape – Site of Specific Scientific Interest – the woods surrounding the Castle Coch know as the Taff Gorge complex, are amongst the most westerly natural beech woodlands in the British Isles. … The area has unusual rock outcrops, which show the point where Devonian Old Red Sandstone and Carboniferous Limestone beds meet.

Screenshot of my location from Geology of Britain viewer on British Geological Survey website.

In the area of Tongwynlais, coal, limestone and iron ore deposits could be found in close proximity to each other, allowing creation of early industrial landscape for iron production.
The mine entrances known as “The Three Arches” (or The Three Bears Cave) descending of up to 20 metres deep into the thick bedded limestone, is still visible, but fenced. The walls of the cave reveal how the mines were carved, with cylindrical features; former drill holes for dynamite to blow open the seam.
The iron works ceased in 1879 due to competition of surrounding towns using cheaper methods of extraction.

Looking at British Geological Survey’ viewer, I could pinpoint the location of where I dug the clay substance and look at the geological composition of the area.
Unfortunately, there’s no specific information on surface composition of exactly where I dug the clay; but “Alluvium – clay and slit,…” deposits very close-by in the village.
The bedrock of the forest area is sedimentary Dolomitic Limestone, formed about 326-359 million years ago.

Me trying to dig some clay.

Equipped with only a small frog trowel, big buckets and a dear friend driving me to the chosen location, I enthusiastically set out for the investigational task.
With the advice from the letter stating the summer project, we’ve found a small stream in the forest. Trying to dig approximately 40cm deep before excavating the clay proved to be challenging without a proper spade. So after about 20cm of excavation I couldn’t go any deeper so collected any, at least a bit plastic seeming substances.

Fforest Fawr-2
Carved wooden sculpture next to the mine entrance, also known as the Three Bears Cave.
Fforest Fawr-1
Castle Coch next to the Fforest Fawr, with my brother in the foreground.

Back home I spread the mud substance, removed any larger rocks, twigs and leaves and let it dry. In the process a strong smell of mainly sulphur creeped across my house.


Clay Suppliers

In our weekly ‘Material Alchemy’ lecture we were encourage to look into clay suppliers and the description for their clays.

One of the most widely used clays in the CSAD are from POTCLAYS. Established in 1932 as a clay mining company in Brownhills, South Staffordshire; it now creates premium-quality clay bodies with a worldwide reputation.

The supplier has very good website with large selection of products for potters, however the only technical information for their clays is the recommended firing temperature; which for the clays used in CSAD is 1150°C – 1290°C for Buff Stoneware and 1160°C – 1300°C for White St Thomas.

Valentine Clays is another British family manufacturer of clays, from which the CSAD gets its Red Terracotta clays.
The website also states the basic properties such as texture, appropriate use, colour and firing range:1080°C – 1180°C

Screen Shot 2015-10-13 at 21.15.44

I haven’t manage to find any more first hand manufacturers of clay with good website and online shop.

However COMMERCIAL CLAY LTD is another manufacturer from Stoke-on-Trend established in 1982. Its old-fashioned website shows all of their clays with some Screen Shot 2015-10-13 at 21.30.43data sheets information.

I searched for suppliers in Slovakia too, finding only one KERAMIKA BIELA HORA s.r.o. manufacturing a multi-purpose clay, however the website doesn’t even inform if it’s stoneware or earthenware.
Most of the suppliers buy the clays produced in Germany or other European countries.
Local clay depositories and previous mining activity such as in Pozdisovce has apparently been closed, mostly due to low competitiveness and demand, as well as bad business practices from Communist era.