Note : This publication serves as an extension of my exhibition titled Ceramic Rock Glazes : Developing a Geological Language of Alternative Ceramic Materials. Whilst both the in-person and online version of the exhibition offered an overview of the process, the current series of blogs further expand the project by offering details related to each individual rocks. I will be delving deeper into the gritty details of each rock by going over various aspects such as the theoretical (and hypothetical) chemistry of the rock, the way this has (or could have) influenced the glazes, all while showcasing many unseen footage of both the rocks and the final glazes. Furthermore, I have taken the opportunity, at times, to share my own commentaries and notes, which I hope will provide additional insight if you ever intend to pursue such project.

---

Greenschist

Mineral Composition and Considerations for Glaze Development

Note : I believe that it’s important, before getting in the nitty gritty of the topic, to clarify the intent of this section. Since the rocks have not been through any thorough assessment – such as XRF which would give me a relatively accurate chemical understanding of the rock’s composition -, what I am presenting below is hypothetical in nature and is reflective of a rather rudimentary process of identification that was done mostly via observation and hardness testing. Furthermore, I am using this section to reflect on “what could have been” and not necessarily “what is”. In other words, this section reflects my initial thoughts before the rocks were used as the primary ingredient in a glaze. The next section, however, will include a short reflection on whether some of these hypotheses were true, and why that might or not be the case.

Let’s first look at the geological composition of the greenschist. By doing so, I believe that I might be able to formulate some hypothesis regarding its potential as a ceramic glaze. For example, this particular rock contains (what appears to be) the following minerals :

• Fuchsite (Chromium-rich Mica)
• Chlorite (Iron & Magnesium)
• Albite (Sodium)
• Pyrite (Iron)

In this list, the first mineral particularly interests to me. Fuchsite, a variety of muscovite, is a chromium-rich mica. As I read on the subject, I found very little information concerning the properties of mica in ceramic glazes. In some old forum entries, I read a few people using crushed mica in low-temperature firing (~1000°C), but none provided details as to its effect in mid-to-high temperature (~1200-1350°C). Unfortunately, to much of my disappointment, I was not able to get a proper understanding of this material, specifically for glaze making. That being said, I also read about some of its usage in clay (i.e., as clay additive), some saying that it could increase the strength of the clay. Consequently, upon reading that information, I was under the impression that mica might possibly stiffen the glaze and decrease its chances of melting. Finally, again in regard to fuchsite, what I thought could be interesting is the presence — even if barely any — of chrome in the rock. As I knew before from reading books about ceramic glazes, chrome is a powerful colorant which could, under the correct chemistry, produce shades of green and, in unbalanced mix, create tones of brown. Since I did not know the exact proportion of chromium available in this rock, I was not able to predict the outcome.

As for the rest, we can look at them either as colourant or flux agents. In regard to the former, chlorite and pyrite both provide iron. This metal usually, by itself, creates colours in the family of brown and black. That being said, I am uncertain about pyrite since, in ceramic at least, we typically speak about iron oxide, and the type of iron in pyrite is a sulfide. As for fluxes, chlorite should be a source of magnesium and albite — a sodium-rich feldspar — a source of sodium. These two minerals can help to lower the melting point of the glaze.

Now, I think it’s worth repeating : all of this is purely hypothetical. Although all of these materials will influence the durability, the fluidity and the colour of the glaze, it is difficult to provide an accurate prediction of the outcome without knowing the exact proportions of these minerals. In the next section, I will cover the recipes I used to create an array of glazes using this rock.

Firing Temperature and its Effects

Prior to the mixing and application of glazes to test tiles, I did various tests using on the raw material both in its grog form (<40 mesh) and powder form (>40 mesh). While this step is optional, I believe that it can offer you great insights and allow you to proceed with better judgment, whether that concerns aspects of aesthetics or safety. More specifically, I did melt tests and in-clay body tests.

Doing melt tests is a rather straightforward, simple tests that involves putting the raw materials in small recipients and then putting it through all the stages of firing. This procedure, least for me, were a way to gauge both the aesthetic possibilities of the rock, but more importantly its safety (especially since I am using a shared kiln). For example, colours and textures were something I was looking for; on the other hand, I also paid close attention to the fluidity of the melt test (e.g., does it melt, and if so, can it be quantified ?) as well as the durability of it (e.g., does it adhere well to the surface of the test tile). The following images shows all of these aspects.

This first set of pictures were done using the grog form of the rock (i.e., relatively large pieces). The order is from left to right (or top to bottom) is : raw, bisque firing (cone 04), glaze firing (cone 6). What I found particularly interesting is not necessarily the lost of colouration from raw to bisque (this is almost always expected), but more so the return of some hints of green after the glaze firing, as if the higher temperature allowed some of the minerals to regain some colour during the longer cooling process after reaching “melting temperature” for some of the minerals.

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 grog, the fully fired test tiles do not really regain any colouration.

Finally, I also did in-clay tests. For this part, I added 5% of the crushed rock inside a white-firing clay body (PSH 519). Although the previous melt tests gave me sufficient information in regard to colours, the clay additive tests allowed me to see whether the rock would be viable to use inside of clay. More precisely, I paid attention to the durability of the tile after firing as some of the tests ended up extremely brittle. I have decided to only share the test tiles using the grog as they were visually better at showing the effects – the powder, in most cases, only produced small speckles that did not differ much from those you find in speckled clay bodies. The images bellow follows the same firing order from those above.

Glaze Recipes

It’s important to note that, for this project, I tested 15 different rocks. Combined with the fact that I was doing all of this in a shared kiln, I could not, realistically speaking, pursue an in-depth experiment in glaze making. Consequently, 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 all rocks, I varied both the fluxes and the type of 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. This choice was made to avoid redundancy since many glazes this not change much between clay bodies or by altering the clay type; otherwise, extra images will be shared.

Greenschist Glazes : An Overview

The colour palette of these greenschist glazes hovers within the territory of browns and beiges. One thing that is worth noting is the warmer tones that Redart gives to the glazes compared to EPK – this is most likely due to the iron content found within Redart. Finally, although it was difficult for me to capture with my camera at the time, I found the glaze using Zinc Oxide (Z) to display very subtle hints of pink.

Glazes using EPK on Tucker’s Mid Cal 5

Glazes using Redart on Tucker’s Mid Cal 5

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.

Greenschist Glazes : A Closer Look

Macro-like photographs create totally new landscape that would otherwise be difficult to capture by the naked eyes. Once I started taking these pictures, I started to have a clearer understanding of what was happening. For example, although this greenschist is not a good example, other rocks that appeared “melted” and “smooth” were revealed to be rather coarse when given a closer look. I hope the following set of photos gives you a new and different perspective.

Going Beyond Testing

My final assignment for this project was to go beyond the phase of glaze testing. Sure, it is incredible to look at all of these test tiles, but it begs the question : what might they look like on actual pieces ? Surprisingly, this idea is not something I had entertained until the studio manager (at the time of the testing) generously offered me the opportunity to create a small exhibition for my work. Rising to the occasion, I used the few summer months of 2024 to improve my wheel throwing skills and completed 30 moon jars, all wheel thrown using one kilogram of clay. Finally, I selected two moon jars to accompany each rock, which can be found below.

If you made it this far, I generously thank you for your time and interest in my art.