Preparing rocks for glazes

Unlike when using commercial materials, and you can just open a bag of powdered ingredients, there is a long process involved in preparing rocks for use in glazes. This marks the second stage in my making cycle. The first being preparing and pugging the clay bodies ready for making.

First of all I sort out samples of rocks that I am getting low on in powder form - generally around 2 - 5kg, though less if it is a particularly rare rock, and up to 10kg if it is one that I use regularly. If the rocks are in large pieces I will break them up into more manageable pieces with a steel mallet.

The next stage depends on what type of rock I have to prepare. Some, like the ones shown here, benefit from calcining. This involves heating them to above red heat and allowing them to cool down. I generally put them on the floor of the kiln in a normal bisque firing. This is particularly useful for rocks with big crystals like the granite shown here. The pale white/clear crystals are quartz and these expand and contract at a different rate to the others, on heating. As a result, when you take the granite out of the kiln, it can often be crumbled by hand into its individual crystals.

Other rocks that benefit from calcining are granodiorite (left) and diorite (right). These rocks, like gabbro have a coarse crystal structure that breaks up on heating.

Other rocks, have to crushed to a powder without calcining, as it has no effect, such as with the quartz to the left. the volcanic tuff (right) has such a fine crystal structure that heating makes no difference in its hardness, while clay (far right) will just get harder.

I will go into more detail about this in my forthcoming book, Rock Glazes Unearthed.

Roman kilns next to the A14

Last year I was fortunate to be able to go out a few times with the archaeology teams that were running a series of digs as part of the A14 redevelopment. As land was cleared and topsoil removed for construction works the history of this small area of East Anglia, just north of Cambridge, was revealed.

Settlements from the Bronze Age and Iron Age were discovered, right through to Roman, and beyond. A medieval village was uncovered that had been abandoned during the time of the Black Death, which is apparently not uncommon. The Roman remains were the most interesting for me with over 30 kilns discovered showing that the area had been an important site of pottery manufacture. The central and right hand image show two of the kilns, one with a single tunnel connecting the fire pit with the chamber, where the pots were packed, and the other with two. The kilns were of a simple ‘figure of eight’ or ‘dumb bell’ construction, with a small circular clay-lined firing chamber or oven and an associated stokehole. Generally, the kilns were aligned east to west or south-east to north-west with the firing chamber to the west and flue leading to the stokehole to the east. Remains of pottery found within the kilns, which include wasters, suggest the kilns were primarily used to produce a limited range of utilitarian lid-seated jar forms, with associated lids.

The Roman kilns were a vast technological improvement on the Bronze Age and Iron Age kilns, which were little more than enclosed bonfires in a shallow pit. These new kilns separated the pots from the firewood by using a firebox. This not only protected the pots from sudden temperature rises as the wood caught fire, but enabled the Roman potters to continually add more wood and control the air flow through the kiln. The result was that higher temperatures could be achieved, making stronger ware, and a higher success rate. The pots were packed on a suspended perforated floor inside the chamber of the kiln, the supports of which can be seen in the right hand image.

The question is, what clays did these potters use? The bedrock here changes from Oxford clay to Kimmeridge clay but there are layers of alluvial and boulder clays on top.

  Piles of pots were found along with over 3 tons of shards, that all have to be cleaned and studied. The image on the right shows one of the alluvial clays close to some of the kilns. Of all the pottery shards collected by the Archaeologists the most common clay body found is a sandy grey or oxidised ware with varying amounts of additional sand and fine flint fragments added as additional mixing agents or temper. In addition the shards contain the remains of the fired clay kiln superstructure and its kiln furniture. This material was found to have been made using three different clay bodies. I have been testing the various clays that I collected from the sites and this mug is covered with a glaze developed from the Oxford clay.

How to make test tiles for glaze tests.

Biaxial glaze tests.
I'm using the enforced lonely time in the workshop to do various things, including expanding the testing of clays and rocks that I have recently collected. I have a slab of plaster that I have scored with a grid of 25 squares. 

The size of the squares is up to you but they need to be large enough to see the glaze quality but not so large that they take a lot of glaze to fill Mine are 4 x 3cm for my rock glazes and 4 x 4cm for commercial materials.

The lines are drawn on in pencil and then.scored in using a lino cutting tool and a steel ruler.

Either roll out a slab slightly thicker than you want the tile to end up, with an area around that of the mould -

or, if you are planning on making multiple tiles, prepare a block of clay and cut slices from it (as shown the left).

Use a rolling pin to press the slab very firmly over the plaster mould in all 4 directions. You might need a piece of material on top of the clay to stop it sticking to the rolling pin.

This image shows the impression of the grid on the tile, which needs to be deep enough to contain the individual glazes. (Don't bend your own tile back like this as clay has a memory and will warp on drying.

Carefully peel off the clay tile and place it on an absorbent surface like a wooden board. Cut away the excess clay but make sure you leave enough to mark the tile with the glaze test details.

I use the same mould to make lineblend tests, just pressing a smaller strip of clay along a 5 square section of the plaster mould.

Happy testing!

The old must give way to the new

I built my little bourry-box as a small test kiln so that I could fire samples of new rock glazes and also clays that I collected in different firing schedules than I use in my main wood kiln. Some of these firing produced good results, though all were interesting and allowed me to experiment far more than I would have been able without wasting a lot of materials and time. Particularly successful were the firings that I did to cone 6 with a reduction cooling. The high iron clays I used in these firings produced some amazing purple and red colours and surface crystallisation.While useful, the kiln had a few problems. The firebox design proved unsuitable. A Bourry box is stoked with wood up on hobs in the firebox, meaning that it must be cut to precise lengths. You can see the partly dismantled firebox in the image on the left. Because the wood I collect is scrap or unwanted, it is not possible to get enough that can be cut in this way



The image on the right shows the arch being taken down.


I also found the kiln too inefficient. It used around 1/2 as much wood to in a shorter firing than my main kiln, for around 1/8 the packing space. The chamber is simply too small for the mass of bricks used to build the kiln.


So..... I've been talking about rebuilding it for a couple of years and now, with all shows and exhibitions cancelled, I have the time to actually get on and do it. There are already a lot of second hand bricks in this kiln, which makes taking it apart much harder because of all the mortar used to make the bricks sit flush together. The dense bricks in the base are only 1 inch thick. I got them almost 20 years ago from a potter in Suffolk. The next kiln will be the third that they have been used in.




So what's the next kiln going to be? It's still on the drawing board but I'm planning a raised catenary arch chamber with a large open firebox area attached, where the wood will sit on hobs above a grate system. As always, I am aiming to maximise the effects of wood in the chamber, lots of ash and direct flame contact, in relatively short (and hopefully easy) firings. 

Winter comes early in Cornwall

Hoping for a glorious out-of-school-holiday summer break in Cornwall we spent the first two weeks of September exploring our most south-westerly county, as the storms and squalls relentlessly rolled in from the Atlantic. Luckily the bursts of rain were interspersed with enough dryish blustery weather to get us out of the car and out into the countryside. It really is a beautiful place, lined with jaw dropping rugged coastline.

First was The Lizard, with its rare geology, partly created from a massive chunk of exposed mantle rock. It’s pretty dark down there: igneous metamorphosed. Mantle and gabbro (which is slow cooled basalt.


Like the rest of England, Cornwall is littered with quarries, though far more densely. Only a few of the rock quarries are still operational, mostly for road base or other aggregate (an inglorious end for some exceptional rocks!)

The rest of Cornwall is made up of bands of sedimentary rocks: Devonian age mudstones and sandstones mostly, with intrusions and dykes of igneous rocks such as granite. Along with these intrusions is heavy mineralisation and the associated tin and copper mining – the remains of which can be seen almost everywhere along the north coast.

Yet another stunning coastal view…..and I finally found copper, though sadly none that I could tea home with me.

I have, however, come home with many bags of interesting new rocks. Let the testing begin!

Drips and blushes

After the unpack, grinding, polishing and cleaning from the last firing, and the pinging noises of crackling glazes has subsided, I start to photograph the pots. My nervous adrenaline has dissipated and I am able to really spend some time with each surviving successful pot, out in the daylight, and start to get to know it. To my huge relief it was a fantastic firing and I have many beautiful pots with runs, drips, heavy or scattered crystallisation and flame blushes.

This pot was glazed with rocks that I collected during my February visit to Cumbria. It was fired on its side at the front of the firebox stack and was buried in embers for much of the last 24 hours.


This next pot was from the second chamber stack. It’s made from a body based on kaolin that is derived from Dartmoor granite, with added milled quartz (from the granite) and the parent rock itself. The pale glaze is based on an intrusion within the same granite. It was fired on its side on shells from Devon. I’m loving the drips on this one, including the one just hanging from the rim.



As well as my place specific rock glazed pots I also fired a lot of functional pots made from a couple of bodies that I have been testing: a ‘dirty’ porcelain that flashes very well, and a dark body that turns black with ash deposits. The black body pots that survived are rather lovely but many dunted or gave a very rough, almost flaky surface with the ash. The porcelain, however, came out beautifully. These faceted cups will be going with me to Earth and Fire in June.


All that glitters not gold. It's probably mica!
I was being optimistic thinking that the rock I collected a couple of weeks ago from the same parent body but 40 miles away from the place I visited before would be the same. This rock is a beautiful varied mass of rose pink and white crystals. It is very high in silica and surprisingly free from impurities.

The colours it gives in the wood kiln can be seen best in this image of one of my disc forms. The pure, high silica, low iron, high potassium feldspar rock gives a very viscous melt, full of tiny bubbles, giving a smoky quality to the pale blue grey glaze. Where it gets a lot of heat the glaze rapidly fluxes becoming a glassy transparent of striking vivid blues. It is one of my very favourite rocks that I have found on my travels.



As soon as I saw the new rock I could see that it looked very different and has obviously crystallised out in quite a different manner. It was macrocrystalline, which means large crystals, and therefore slower cooling deeper within the earth's crust. The pink is there in the large feldspar crystals and in some of the ground mass surrounding them, but the main difference is in the grey sparkly crystals that are so abundant. These are flat plate like glittering crystals of mica and are the most variable of all the components of granite. They contain iron and sodium or potassium but also other minerals such as magnesium, manganese and possibly things like chrome and titanium.

These 'impurities' turn the pale blue colour into something else, depending on what is actually in there. More iron, and manganese give deepening greens and titanium is the kryptonite to superman's blue. As you can see from these initial tests the colours are much darker and very definitely green. The rock is much more fusible (easy to melt) and so produce quite clear runny glazes. 

I have about 4 initial tests that I do on a new rock, mostly progressive additions of one or two extra ingredients. Occasionally, when the sun is shining on me,  I find something stunning right here at this early stage but, as in this case, mostly a lot of extra work is needed. It's easy to get a rock to melt, but often quite hard to find that exact balance of additions that will coax its most beautiful colours out and transform it nto something sublime.



Winter in the Cumbrian Mountains

What better way to start off the new year and shake off that post-Christmas torpor than a few days walking in the Cumbrian National Park? The previous week’s snow held off, as did the rain. The barmaid in the pub I stayed in told me that the area had been unusually lacking in rain, however the moss covering absolutely everything was as thick, bright and moist as ever, and water seemed to be draining into every gully and dip in the landscape.

It was cold  with low sweeping clouds but once I got into the higher ground of the craggy glaciated valleys, where sheer rock faces protrude through the thining soil, towering above enormous scree slopes, I scarcely noticed. It is a beautiful an elemental place, carved out of an enormously complex variety of volcanic rocks, formed when the landmass of England and Wales collided with and joined that of Scotland.

Walking these lonely paths I found lumps of ‘frothy’ lava, huge slabs of laminated tuffs made from layers of compacted volcanic ash and huge ragged seams of pure white quartz. The brook and bridge in this image have become a tourist attraction after appearing in the ‘fishing with a shotgun’ scene in Withnail and I. The bridge itself is made from slabs of nearby limestone and is now hung with thousands of tiny white stalactites.

As I wound down this smaller valley I was approaching the edge of the National Park. It is hard to get all the permissions necessary to collect rocks from Parks as they are, rightly, protected areas. I had already secured the permission needed to collect from the site that I was heading to, which had been pretty easy, but the main reason for my interest in this spot was its geological interest. I had previously collected a similar rock from a site on the other side of the Park, a place over 40 miles drive away. Here, making up a small peak on one side of the end of this valley, is apparently the same rock. The two outcrops being exposed peaks of an absolutely enormous underground batholith that stretches the whole way beneath the mountains and lakes of Cumbria.

And here it is. Pink granite. But it doesn’t look the same as the rocks I collected from the other exposure. The pink feldspar crystals are much bigger and the crystalline ground mass surrounding them appears much darker with mica. One of the striking things about the other rock is the pale smoky blues and whites I can coax out of the other rock in my wood kiln, as it is so low in iron and manganese and other impurities commonly found in mica. But, as with books, I have learnt never to judge a rock by its colour/cover, and am very interested to see how this new version of geologically-the-same rock responds in my tests.