Material World

This month I have mainly been testing glaze materials and porcelain clay bodies:

Every colourant, opacifier, some fritts, ashes, salts, ochers,engobes and some unknown glazes from my lab. This is what they look like before they've been fired

The shelf on the right contains materials collected from potters past, some I won't bother testing because there's not enough to be worthwhile, but I do like the eclectic collection of glass jars and materials.

Some of the materials from the shelf that I will be testing including a bag of 'Sunset Orange'. Looks like something from the 1950's.  The large bag of Smith & Smith glaze turned out to be a nice old school oatmeal glaze.

Side-handled teapot, teabowl and small breakfast bowl, along with some porcelain beakers from a recent firing. Glazed in celadon.

Cutting some shelves to size for the small kiln from old broken shelves discarded from an old pottery studio

All that remains of one of the bags of materials, I could just make out things like 'nepheline syenite' and 'kentucky ball clay'

Shrinkage and warpage bars after firing 1300 degrees C. The shrinkage bars were all under 15% shrinkage, made from throwing-consistency clay. The warpage bars all warped except the pipe clay. It was surprising that the commercial stoneware clays warped about the same amount as my own homemade stoneware and porcelain clay bodies.

These are some recently acquired materials that had been left in unlabelled bags. To get a better idea of what they all were, I had to fire a small amount of each one to see how each one reacted at 1300 degrees C. They all looked like white/grey powder but fell into four main categories: Nepheline Syenite, Silica, Kaolin and Ball Clay.

The main materials for clay bodies and glazes eg: silica, clays, fluxes, calcium carbonates, feldpars, fritts, etc.

These are the main colourants after firing to 1300 degrees. I added a strip of clear glaze down the left side of each example. Some of the colours burnt out at this high temperature but others didn't look too bad.

Unnamed materials test results. 1. is unknown pink stuff from Glenfalloch Pottery,  fired to an opaque white glaze, most closely resembling the Zircon Fritt, which you can see in the picture below. The Zircon Fritt was resourced from a retired potter. I haven't had any experience with these materials, as this is the first time they've been tested. 2 looks like Nepheline Syenite and 3 Silica.  7 is Kaolin.

Zircon Fritt is top left, pink stuff second from bottom right

Homemade porcelain clay. Throws well, vitrified with a good glaze fit. Glazed in my own clear glaze with no crazing. Not bad for a first attempt at porcelain, very simple, compared to the literature I've read that made it sound so complicated, ie, porcelain being not very plastic for throwing, prone to warping, 'glaze shivering' etc. etc. etc....

Kitten in a terracotta pot:  'My Cat Likes to Hide in Potses'

Clay Preparation Videos

I put up a video on YouTube here of the process I go through when making my own terracotta clay body, using nearby clay. (Apologies for the quality, the video was filmed on an old digital camera, but you get the idea!)

First, I dig up the clay, dry it, break it up with a sledgehammer and seive it through an old bed frame. Then I put it into buckets and saturate with water.

I 'blunge' it with an electric drill attachment and seive it again through a mouli seive. Then I pour off the excess water once the clay has settled. I put the clay on plaster bats to dry, then process it through a pug mill. (You could also wedge it by hand if you want). Finally, I store it in plastic bags.

Another YouTube video I like:  The Potters of San Marcos, showing a more traditional method of preparing clay from raw ingredients, by the potters of San Marcos Tlapatzola in Oaxaca, Mexico.

From the Ground Up

For the last few months, I've been working on finding and preparing my own clay bodies. Like most things, the price of clay has skyrocketed in the last few years, and knowing that I can throw as much clay as I want without worrying about the price has really freed up my creativity.

Dunedin with its ancient volcanic rock is a good source of plastic, terracotta clays - from weathered basalts up North East Valley to schists around Brighton. Each clay has its own personality, and using local clay gives a pot its own unique character, grounded in space and time. It can be refined, but is also really nice used raw with all its impurities.

Most how-to pottery books give the basics on finding clay. Road cuttings and creek beds are good places to look, sometimes local history and place names can hold clues as to where to start digging. Here I am prospecting in North East Valley, where I found a very nice terracotta:

And here I am digging behind my studio in Brighton, where I found another terracotta, also very good on its own.

Once dug, the clay needs to be tested for plasticity, shrinkage and firing range. A little minigama kiln is perfect for test-firing small amounts - and you can get some sauce dishes and beakers out of it, too. The local clay I've found so far seems to be good around Cone 02.  Here's some terracotta bowls I fired out of the first batch of NEV terracotta, with my minigama:

Minigama kiln and test pots

Processing the clay takes around two or three days. I break it up and dry it in the sun, crush it, sieve it, then put it in buckets covered with water and blunge it with an electric drill. I then pour it into a clay bath to settle, dry it on plaster bats, and  finally put it through a pug mill and into bags. (We're working on putting together a You Tube video which should show the whole thing). It's a messy business but not too hard once you get into a system. Not all that much different than recycling clay.

The next step for me will be to develop some Cone 02 glazes for my clay body, but in the meantime, the terracotta also looks gorgeous on its own.

It's perfect for flower pots and planters, which look great on a windowsill but will only get more beautiful the more they are left outside:

Rustic flowerpots

I also used it to make some amphorae and West African style pots, which I fired to Cone 02 (1120 Celsius) on Thursday:

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Opening the gas kiln, after work at NZ Post

Some African-style pots in front of the stack, sitting on the planters, amphorae in the background.

 North East Valley terracotta amphora

Close up showing the coarse natural texture of the clay