Natural glazes

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I’ve been invited to sell my work in my local hometown and want the glazes to reflect the colours of the rural landscape of North Wales. These thrown vessels have been painted with the glazes I made at the start of the year as part of the local clay project. The bowl has been pained with a glaze made from a 2:3 ratio of Potash feldspar to my clay while the rounded pot has a 2:1 mixture of Whiting and my clay. It flows very much like an ash glaze but luckily wasn’t too runny that it stuck to the kiln shelf. I’m going to add more whiting and feldspar to these glazes because the colours are much darker than I expected.

Testing my clay

Firing tests
The test tiles I made from clay sourced from my local area in North Wales were fired at different temperatures between 1000C and 1280C. It’s possible to see just from the images below that this clay is low-firing since it even begins to warp and turns a dark  red/purple at 1100C. Any higher than this and it begins to melt to fill the tray, bubbling and becoming metallic. Visible on the tile fired to 1000C are white deposits on the clay which are probably sodium and potassium salts. These act as fluxes and indicate the clay may start to melt as it gets to higher temperatures, which is proven on the higher fired tiles. Clays high in silica tend to puddle at higher temperatures. My raw clay is a dark green-grey colour which suggests the presence of carbon.

Raw clay
1280C Oxidation
1280C Reduction

To test for porosity I boiled the test tile fired to 1000C for 1 hour. I weighed it before and after boiling. The tile turned out to weigh exactly the same before and after (80.2g), showing when fired to this temperature it has a porosity of 0. The clay has reached it’s maturing temperature at just 1000C so it’s a very low firing clay.

After air drying the 10cm line on the tiles shrunk to 9.4cm showing a shrinkage of 6%. However, after firing to 1000C it shrunk further to 8.5cm which is a shrinkage of 15%.

I also weighed out a 100g ball of the clay and fired it to 1000C. The fired ball weighed 76.3g – a 23.7% decrease in weight.

Acid test
To test for the presence of limestone (calcium carbonate)  in my clay, I placed a small lump in dilute hydrochloric acid. No fizzing (carbon dioxide released) took place which proves there was no limestone in the clay. This is supported by the fact geological survey maps show no evidence of carbonate materials in the area I sourced it.

Glaze tests

The three tiles above show slip made from my clay brushed on the left hand side. Different amounts of glaze materials have been mixed with the clay along the tiles. The first tile shows my clay as it mixes with wood ash. A ratio of one part clay to two parts wood ash produces a warm khaki colour. Equal measure of both creates a dark brown while a ratio of two parts clay to one part wood ash makes a dark purple/brown. The wood ash on its own is on the right hand side of the tile.

Below this is the clay mixed with potash feldspar. A small amount of this feldspar mixed with my clay produces very reflective brown glazes, however a very small amount of clay produces a subtle and attractive pale blue-green like a blackbird’s egg. Potash feldspar (also known as Orthoclase) is the commonest of the 12 types of feldspar. It’s an important glaze material and is used as a flux in bodies.

Below this is my clay mixed with whiting creating different shades of green. My application of the glazes is a bit splotchy, next time I know to paint on more than one layer and aim for a more even coating. I like the effects of mixing two parts potash or whiting to one part clay and I plan to make up a batch of these greens to use on vessels.


Clay in the Dolgellau area

North West Wales is not known for it’s clay. The area is dominated by slate and igneous rock and as a result Dolgellau is a town (a very grey town) built almost exclusively of granite walls and slate roofs. I could find no evidence of potteries nearby although it has a rich history of mining and other industries. Granite from quarries on the Lleyn peninsula supplied stone to pave the streets in English cities to the east such as Manchester and Liverpool.


Dolgellau nestles inside the  Snowdonia National Park so it’s no surprise that tourism is the major industry in the area. Much of the surroundings is rural countryside and sheep farming is another important industry. Interestingly, gold was discovered here in the 1850s and as a result the area was hit by a mini gold rush. Two of the most famous gold mines are Gwynfynydd near Ganllwyd and the Clogau St David’s mine in Bontddu. Clogau was originally opened as a copper and lead mine but gold veins were soon discovered and at one point the mine employed over 500 workers and the total output from the mine so far has been recorded at 4 tonnes. All rings for the royal weddings since the Queen Mother’s marriage to King George VI have been made from Clogau gold.

Earlier, during the 18th century the area had a thriving wool trade and coarse wool from sheep in the surrounding hills was exported to New Zealand via the Mawddach Estuary. Leather tanning was another big industry in the town and the large tannery business closed as late as the 1980s.

Superficial deposits in Dolgellau

The map above was taken from the Geology of Britain Viewer on the British Geological Survey website. The faded yellow on either side of the town, along the river Wnion indicates alluvium deposits of clay, silt and sand left by flowing floodwater in a river valley (producing fertile soil). In the bottom left peat has formed which suggests the environment nearby below the slopes of Cader Idris was dominated by swamps and bogs. The town itself sits on an alluvial fan deposit of sand and gravel (in orange) which formed up to 3 million years ago.

My first effort to find clay took me to the banks of the Mawddach Estuary, down one of the tributaries. In the riverbank here I found a small amount of greyish clay but mixed in with it was lots of sand, brown soil and vegetation. The texture was crumbly and it didn’t have much plasticity, so I decided to look elsewhere.

Mawddach River from top of Moel Faner hill fort
River feeding in to the Mawddach at Arthog where I found clay
Clay with lots of vegetation

After consulting Natural Resources Wales I decided to look a little further afield. I finally came across what I’d been looking for. Just over 10 miles away from Dolgellau in the direction of Trawsfynydd just off the straight line of the Roman road, lies the village of Bronaber with it’s collection of holiday chalets. Turning right here we followed a road into the hills and drove parallel to the Afon Gain until coming to a landslide at a turn in the river. It was clear from a distance that this bank was oozing with blue-grey clay. This clay turned out to be much more plastic than what I found previously. Although it had no vegetation in it, it turned out to be full of small pebbles which I sieved out later while processing it.

Sun setting on the Afon Gain

Clay is formed when igneous rocks (such as granite and basalt) break down over millions of years. It’s made up of three main ingredients: alumina, silica and water. 

Clay in the bank of the Afon Gain

Around a mile away from this site at Pen y Stryd there are the remains of two Roman tile kilns which would have been in use between the 1st and 4th centuries AD. Nearby is a waste heap into which the broken tiles and bricks went. It’s believed these kilns once supplied tiles to the Roman fort of Tomen y Mur above Trawsfynydd.

The cross on the map below indicates where the clay by the Afon Gain was found (grid reference: SH743321). The yellow colour running along the river indicates a superficial deposit of alluvium. Alluvium is composed of silty clay which can also contain sand, peat and gravel. Further downstream the circle indicates the site where Roman tile kilns were found. These also sit upon a deposit of clay silt, sand and gravel which is indicated by the purple colour.

Superficial geology only

The map below shows the same area from above but with only the bedrock visible. The grey green seam that the clay site sits on is Maentwrog mudstone: sedimentary rock of sandtone and siltstone. The nearby purple seams represent Igneous rock formed in silica rich magma and the blue lines are part of the Clogau formation of mudstone and sedimentary bedrock formed on the ocean floor 502-508 million years go.

Bedrock only
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Dot on the end of the arrow shows where I found clay on the banks of the Afon Gain
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Clay straight from the bag and ready to be processed.