Basalt in Flux

All the materials we use in ceramics come from the earth, formed by geological processes over thousands or millions of years. It is easy to forget this when we buy pre-processed powders and clays by the kilogram. The majority of the materials used to make glazes are powdered rocks that have been chosen because of their particular chemical composition and are sourced from all over the world. Historically materials would have been sourced locally, informing both the type of ware made and its aesthetic.

Rangitoto basalt feild, Photography by Grace Ryder

I wanted to create a glaze from a readily available, local material, that not only utilises the chemistry of the local geology but also reflects it aesthetically in some way. Most of Tāmaki Makaurau Auckland lies on basalt lava flows, the numerous maunga dotted around the city are the remains of past eruptions. The most recent and iconic being Rangitoto, where chunks of basalt still litter the surface of the dormant volcanic cone. Basalt has not only shaped the topology of the city but also the built environment itself, it has been utilised in buildings, walls, roads and footpaths for many years. Its abundance, accessibility and distinct visual characteristics made it an obvious choice for this experiment.

To understand how basalt could be used to create glazes, I’ll first break down what a glaze is, its different components and why they are necessary. All glazes are predominantly made of silica (SiO₂). By heating silica, deposited on the outside of pots as a powder, crystalline silica melts to form a new amorphous solid–Glass.

On its own, silica has a very high melting point: around 1700°C. To do this at temperatures achievable in kilns, it is necessary to lower the melting point by adding a flux. Fluxes are certain metal oxides that help break silica’s crystalline bonds. There are two main groups of fluxes, the alkaline metals; lithium, sodium, potassium; and the alkaline earth metals; magnesium, calcium, barium, strontium and zinc. Zinc is technically not an alkaline earth but functions very similarly to one.

Alumina (Al₂O₃) is the last key component of glazes and functions mainly as a stabilizer. It affects the melting temperature and fluidity of the glaze, and in certain concentrations forms microscopic crystals in the glaze turning it matte. Alumina is mostly sourced from clay but feldspars also contribute alumina to the glaze. The most common type of clay included in glazes is kaolin or ‘China clay’. The addition of clay to glazes is also essential to suspend the glaze and stop hard panning. Basalt is a rock that forms when molten lava cools at the earth’s surface and is rich in calcium and magnesium. Other volcanic rocks like rhyolite, andesite and dacite, are formed the same way but basalt has the highest concentrations of fluxes, making it more useful as a substitute for these chemicals. Basalt also has a high concentration of iron oxide, which is what gives it its dark color. Iron is one of the most common colorants used in ceramics and has a wide variety of appearances, from blue and green to yellow, red, brown, and black. Glazes colored with iron can vary greatly between oxidation and reduction firings.

Tāmaki Makaurau basalt stone wall

I obtained a suitable material for mixing glazes by crushing up waste debris from a Tāmaki Makaurau basalt stone wall. The large chips were smashed up then ground into as fine a powder as possible with a small mill. I then sieved all the material through a 100 mesh sieve to obtain a powder of similar fineness to the other materials.

I formulated four recipes each with the same ingredients in different proportions using Basalt as the main source of fluxes. Below is a biaxial blend of the four different glaze recipes. From left to right is increasing silica in proportion to the fluxes. Bottom to top is increasing alumina in proportion to the fluxes. Rather than just mixing a few glazes to test, using volumetric blending in this way is a very efficient method to test materials and capture the full potential of what glazes they can create. This is particularly helpful when the exact chemistry of your materials isn’t known.

Basalt glaze biaxel blend, Increasing SiO₂(up) and Al₂O₃(right)

The glazes near the top left-hand corner have enough alumina to matte the surface. The variation of colour you can see, from amber to black, is due to changes in the amount of iron in the glaze.

The glazes near the bottom of this blend fall in the crystalline region. The crystalline region is where the flux levels are high enough that when cooled slowly or held at specific temperatures visible crystals can start growing with the glaze.

From this initial series of tests I was most interested in the matte glazes as they reflect the textural qualities and colour of basalt.

Normal firing (left) and crystaline firing (right)

As a further experiment, I took the lower left-hand corner glaze (lowest silica and alumina) fired in a normal firing (left) and fired with a two hour soak at 1100°C (Right). A network of crystals has entirely covered the surface. This example shows the importance of testing broadly as the firing schedule can have a dramatic effect on some glazes.

Matte basalt glazes

In this final series of tests (below) I added more alumina to some recipes to make them even more matte and applied some to a black clay body to give a darker appearance overall. By understanding the chemistry of both glazes and geology it's possible to create ceramic ware specific to its geographical context. Taking what once was liquid lava and remelting it into a functional glaze, these tests begin to form a visual language informed both aesthetically and materially by the geology of Tāmaki Makaurau

Glossary

Amorphous Solid: A solid state where the molecules do not have a repeating crystalline structure.

Biaxial blend: A volumetric blend between four different glaze recipes.

Crystalline solid: A type of solid whose structure consists of a highly regular pattern of atoms or molecules, forming a crystal lattice.

Feldspar: An abundant rock-forming mineral typically occurring as colourless or pale-coloured crystals.

Hard panning: When a mixed glaze settles into a hard, nearly unusable mixture at the bottom of a container.

Forming Extruder, Extruding Forms

Extruded Forms

My motivation to make an extruder began with the desire to make the “perfect” test tile. I decided the most economical method of making these was extrusion. Extruding is a common method of forming in ceramics often used to make handles, tubes and tiles. It involves pushing clay through a specifically shaped hole called a die, resulting in a continuous form with a consistent cross-section. From here the extrusion can be cut to the desired length, bent, or generally shaped as desired. After looking at the extruders available on the market in New Zealand, I found they weren’t very well made and weren’t big enough to extrude a test tile with the desired width so I decided to make my own.

Making the extruder

After a significant research period of looking up models available overseas and reading about their pros and cons, I came up with my own design and had it fabricated. The extruder works by trapping the die between a 100mm diameter stainless steel tube and a large modified nut that screws onto the end. This assembly sits in a “cradle” that is screwed to that wall. A plastic plunger with a long levered handle is used to force the clay down through the die. The tube is detachable to make it easy to clean. All the parts that come into contact with clay are either stainless steel or plastic so white clays won't be contaminated with rust.

Extuder components

Designing Dies

I make dies by first drawing a vector file on 3D modeling software then I get them laser cut in 6-12mm thick acrylic. Although acrylic may not be as strong as steel or aluminum I’ve found that it’s cheaper, easier to get cut, does not rust and leaves a very smooth edge which is translated to the final extrusion.

Die vector file

Multiple smaller shapes can be put in one disc and a second disc can be put over top to select the desired hole, blocking the rest. However, holes that are not in the centre of the die tend to curve the extrusion more as it comes out.

Aluminium dies

Hollow extrusions require another level of intricacy. The interior part of the die, which defines the hole in the extrusion, must be supported from within and with enough room around the die opening for the clay to be squeezed back together and not leave a seam. Often this is done by fabricating individual dies with fixed supports. My solution was to create a support that fits into the tube so different interior sections of the dies can be used. This allows almost infinite variations of exterior and interior shape profiles.

Extruding

I find clay on the softer side easier to use as quite a lot of force is required to push the clay through the die. The extrusion also tends to curve as it first exits the die but as it gets heavier gravity pulls it out straighter. Sometimes these curved sections can be interesting outcomes in and of themselves. Softer clay is also easier to bend and shape afterwards and less likely to crack.

Extuded Vases

When extruding long round forms like these lamp components I either lay them on foam or make a cradle that has the exact same diameter as the extrusion to avoid getting flat spots when the clay is still soft.

Extrusions in wooden craddles

Once the extrusion is leather hard I cut it to length, any sooner or softer and it tends to deform the extrusion too much. To cut very straight, square cuts I made a jig out of an aluminium box section lined with hardboard. Hardboard is very smooth, stable, and has a slightly porous surface so the clay does not stick to it. I made a cutting wire tool from a coping saw handle with tungsten wire stretched very tightly in the frame. Tungsten doesn’t corrode and has a much higher tensile strength than steel so the wire can be very thin, resulting in less resistance when cutting and cleaner straighter cuts.

Cutting jig

Greenware coat hooks cut to length

Some more complex forms that have delicate flanges like these vases, must be supported from the bottom. For this, I have small plaster bats that I extrude directly onto. There is a small hole in the middle, this lets air into the middle as the extrusion grows, otherwise, a vacuum forms sucking the hollow extrusion in and distorting it.

Extruded forms in red stoneware

Test tile

I wanted to make a test tile that could be displayed on the wall, be big enough to show a decent representation of the glaze surface, and have enough space on the reverse to write critical information like recipe number, firing temperature, clay body, etc. I found a suitable profile of aluminium extrusion to fix to the wall then designed the test tile to fit around that. The small “Z” shaped profile allows me to hang tiles on the wall for display and storage, with no visible fixing on the face of the tile. The exact width of the tile also lets me calculate the dry and fired shrinkage of different clay bodies.

Test Tile Evolution

Hanging Test Tiles

The extruder and the customisable dies allow me to push the traditional outcomes of clay into new directions like hooks, lamps, handles and sculptural pieces. The tools I use heavily inform the aesthetic of my work and can often be the catalyst in the creative process.

Approaching Red Rocks: Additions of Titania in Chrome Tin Red Glazes

Rocks and their infinite subtle variations in colour, texture and form are a constant source of inspiration for my Ceramic practice. I often come home from walks with pockets full of small pebbles or bits of brick worn smooth. This article discusses a series of tests that emerged from the desire to create a red matte glaze that would resemble the well-known outcrop of Te Kopahou (Red Rocks) in Owhiro Bay on Wellington’s South Coast. To do this I will briefly explain the basics of the Unity Molecular Formula (UMF)—a key concept in understanding glaze chemistry, outlining the specific chemistry of matte glazes and how it relates to chrome tin red glazes.

Te Kopahou, Red Rocks

The UMF is a method of expressing the chemical analysis of glazes. It breaks down the recipe’s materials into their molecular components, totals them, and puts them in proportion to the fluxes. There are numerous online UMF calculators, including Glazy.org and Digitalfire’s Insight-live.com.

UMF Analysis from glazy.org

In this UMF analysis taken from glazy.org, you can see all the fluxes on the left add to 1 (0.23+0.07+0.7=1) and the amounts of Silica and Alumina are put into proportion. So for every mole of flux in this recipe, there are 0.45mol of alumina (Al₂O₃) and 3.37mol of silica (SiO₂).

This analysis also gives us two important ratios, the first on the left is the ratio of the two different types of fluxes, alkaline metals (R₂O) to alkaline earth metals (RO). The second ratio is the amount of silica in proportion to the amount of alumina in the glaze. In this glaze for every 1mol of alumina, there is 7.41mol of silica.

True matte glazes are formed when the melted glaze is saturated with alumina, so as it cools it precipitates out, forming small crystals at the surface. In most cases, this occurs roughly between 3:1 and 5:1 moles of silica to alumina. For comparison, a normal glossy glaze is between 6:1 and 12:1. In most glaze recipes the majority of the alumina is sourced from kaolin clay. Feldspars and some frits can also add alumina, but kaolin is the most concentrated form. Silica is sourced from a variety of materials including kaolin but can also be added in its pure form.

In my first attempt to create a red matte glaze, I took a previously formulated matte recipe and added certain colourant oxides to create the desired red. One of the few ways to create a bold red at cone 10 oxidation (there are a few more options in reduction), is a combination of chrome and tin oxide. Often referred to as pinks, chrome tin glazes can be red in the right concentrations, however, when I added 5% Tin oxide and 0.1% chrome oxide to the previous matte glaze the result was a purplish-grey, not the intended deep red.

Matte Glaze with additions of 5% Tin oxide and 0.1% Chrome oxide by weight

After some further research, it became clear that chrome tin reds require a relatively low alumina level. I formulated a glossy chrome tin red glaze from recipes found online, making some substitutions and adjustments for local materials. This glaze has additions of 5% tin oxide and 0.1% chrome oxide by weight.

UMF Analysis of chrome tin red from glazy.org

This line blend shows the transition from glossy to matte (left to right) through increasing the alumina level via UMF and the resulting colour change.

Line Blend Increasing Alumina via UMF

As increasing the alumina level killed the desired red colour, it was necessary to try something different to create a matte glaze. Titania (either as pure titanium dioxide (TiO₂), rutile or ilmenite) has long been documented to crystalize and phase separate glazes in interesting ways. To determine whether titania would have the desired effect, I performed another series of line blends. The first blend shows the base chrome tin red recipe, on the left, and the same glaze with the addition of 10% titania by weight on the right. The second blend shows the transition between the base red glaze and another chrome tin glaze that contains no alumina, instead, containing the equivalent amount of titania via UMF.

0-10% Additions of Titania to Chrome Tin Red

Line Blend Substituting Alumina for Titania Via UMF

These tests show additions of titania do matte the glaze, both the glazes on the right have a fine matte finish and in some cases there is even a visible network of crystals growing at the surface. There is still a colour change, but much less drastic compared to additions of alumina.

Some final tweaks were made to achieve the darker, slightly browner hue seen in the rocks. This was done through adding a small amount of black stain to the existing recipes and creating new recipes that sourced the titania from rutile (instead of pure titania) to add more iron. These experiments resulted in a collection of glazes that approach the subtleties and range of colours; reds, browns and pinks, of Te Kopahou (Red Rocks).

Rutile Recipes (left three) and Additions of Black Stain (right three)

Glossary

Alumina(Al₂O₃): The oxide of Aluminium. Pure Alumina has a melting point of 2,072°C.

Atom: The smallest particle of an element that can exist.

Flux: Certain metal oxides that lower the melting point of silica and alumina, allowing us to form glazes at achievable temperatures. The main fluxes are lithium oxide, sodium oxide, potassium oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, and zinc oxide.

Kaolin (China Clay): A clay mineral consisting of Silica, alumina, and chemically bound water.

Molecule: A group of atoms bonded together.

Mole(mol): A unit of measurement used to express large numbers of things, like atoms and molecules. 1 mole is equal too 6.02214076×10²³. So 1 mol of pure Silica is 6.02214076×10²³ silica molecules.

Molecular weight: How much one mole of a given element or molecule weighs. Every element, and molecule, has a different weight. For example, 1 mole of alumina will weigh a different amount to 1 mole of Silica.

Phase Separation: Two different glass formations within the same glaze.

Silica(SiO₂): The oxide of silicon consisting of one silicon atom and two oxygen atoms. Silica makes up the majority of our glazes. Pure silica has a melting point of 1,710°C.