Note : This publication serves as an extension of my exhibition titled Ceramic Rock Glazes : Developing a Geological Language of Alternative Ceramic Materials. My goal with these is to agglomerate all publications I have previously made into a single document to both facilitate the access to the information as well as to provide more in-depth details on the subject.

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Amphibolite

Mineral Composition and Considerations for Glaze Development

Note : Unfortunately, I was not able to get any XRF done for this project – both due to cost (because of the large volume of specimens) and access to labs in the nearby region. For this reason, I had to make due with a relatively quick and rudimentary identification process consisting of observation, comparison with similar rocks, and a hardness scale test. This was one by one of my friend whom I am grateful as, without this information, I would have not been able to provide such a rich narrative around the creation of these rock glazes.

The amphibolite is a dark, dense, metamorphic rock. The minerals identified were :

• Biotite (Iron & Potassium)
• Muscovite (Potassium)
• Plagioclase (Calcium & Sodium)

Based on the list of minerals, the amphibolite is mainly composed of important fluxes commonly found in ceramic. It is extremely rich in potassium, and has a decent amount of calcium and sodium in the form of feldspar (plagioclase). Additionally, the iron content both indicate that it is likely for this rock to produce a glaze with tones of brown and black. On the other hand, because of the low silica content, I was unsure whether this rock would melt or not.

Firing Temperature and its Effects

Melt tests were done prior to mixing the rock with other ingredients to create glazes. This step allowed me to, first, have a general understanding of how the rock behaves at cone 6 and an idea of the general colour palette and, second, make sure it was safe to use in the kiln (eg., looking for splatter or overmelting). This step is something I would highly recommend anyone to do. If this is something that you would like to do, I would also recommend using small bowls or a small recipient with walls instead of a flat tile as it cannot save your kiln shelf in case the rock melts too much.

Finally, I also added pieces of the rock inside one of my clay bodies to see what it would do. I added 5% of the weight of the clay in crushed rock.

The first set of pictures presented bellow were done using the grog form of the rock (i.e., relatively large pieces). The order is from raw, to bisque firing (cone 04), to glaze firing (cone 6). The sample is almost fully, but partially melted. This can be seen by the large bumps across the surface, indicating some of the grog did not melt. In terms of colouration, it’s a deep black-brown.

The second set of photographs were done using the powdered form of the rock (40 mesh and finer). It follows the same order : from raw, to bisque firing (cone 04), to glaze firing (cone 6). Compared to the previous set of melt test, we can see that the melt quality has increased. This is not surprising as finer materials generally melt easier. Although it is not as noticeable in the picture, the surface was spotted with dots of crimson-purple. This melt test is quite similar to the red jasper.

At last, as mentioned earlier, I also added the crushed amphibolite into one of my clay bodies. The images bellow follows the same firing order from those above. The grog, barely visible at first, quite literally “come through” after the glaze firing.

Glaze Recipes

Due to the sheer volume of rocks I had to work with, instead of following typical line-blending and triaxial blending methods, I decided to use a singular recipe (i.e., ratio of ingredients) for all my glazes : 85% crushed rock, 10% flux, and 5% clay. For example, I used, for clay content, both EP Kaolin (EPK) and Redart (R) and, as for fluxes, decided to test Gerstley Borate (GB), Dolomite (D), Whiting or Calcium Carbonate (W), Zinc Oxide (Z), Nepheline Syenite (NS), and Soda Ash (SA). And so, while varying the ratio could have given me a wider range of glazes, this method allowed me to further expand and better understand the role and effects of fluxes and clay content in glazes.

Finally, all of the glazes have been tested of different clay bodies (PSH 519, Tucker’s Mid Cal 5, PSH 540i). In the following section, I have decided to include a few selected test tiles instead of all of them.

Amphibolite Glazes : An Overview

The glazes produced using an amphibolite rock as the primary ingredient offered a rich landscape of colours and textures. Although many of the glazes could be viewed within the family of iron-brown colour, each had little particularities such as purple spots and orange-red veins. The primary outstanding glaze is the one using zinc, which created a sort of old fashioned mustard colour and look.

Glazes using EPK on Tucker’s Mid Cal 5

Glazes using Redart on PSH 540i

You will find below a short series of videos (recorded in early 2024 and about one minute each) sharing my on-the-spot thoughts about all of them, including the raw melt test.

Amphibolite Glazes : A Closer Look

The idea of macro photographies came after observing the glazes using a magnifying lense. I though it was really interesting the way the close-up shots allowed us to view details that would go missing to the naked eye.

Going Beyond Testing

The real test in “glaze testing” is actually applying them on actual pieces. For each rock of this project, I wanted to showcase two of my favourite glazes from each by applying them on moon jars. Below are the two that were made for this amphibolite rock.

As always, thank you for your interest in my art ! I hope this publication can be useful and inspiring 🙂