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.

---

Biotite Schist (part 2)

Mineral Composition and Considerations for Glaze Development

Note : Unfortunately, I was unable to conduct any XRF analysis for this project, both because of the cost involved (given the large number of specimens) and the limited access to suitable laboratories in the surrounding region. As a result, I relied on a relatively quick and rudimentary identification process based on visual observation, comparison with similar rock types, and hardness scale testing. This work was carried out specimen by specimen with the help of a friend, to whom I am deeply grateful; without this information, it would not have been possible to construct such a rich narrative around the making of these rock-based glazes.

The minerals in this rock are :

• Biotite (Iron & Potassium)
• Muscovite (Potassium)
• Quartz (Silica)

Based on the identified mineral assemblage, this biotite schist is primarily composed of flux-bearing minerals that are commonly encountered in ceramic applications. The predominance of biotite points to a high content of potassium and iron, while the presence of muscovite further contributes potassium as a significant fluxing agent. Quartz is present but appears to provide only a limited source of silica in comparison to the overall mica content. Consequently, although the composition suggests a strong potential for melting and for producing iron-rich glazes ranging from brown to black tones, it remains uncertain whether the available silica is sufficient to sustain a stable melt without additional silica input.

Firing Temperature and its Effects

Melt tests were conducted prior to combining the rock with other materials in glaze formulations. This preliminary step served two purposes: first, to gain a general understanding of the rock’s behaviour at cone 6, including its melting characteristics and overall colour potential; and second, to ensure that it could be safely fired in the kiln (for example, by identifying risks such as splattering or excessive melting). This is a step I would strongly recommend, as it provides essential information before moving into full glaze tests. When conducting such tests, I would also advise using small bowls or contained vessels with walls rather than flat tiles, as they offer better protection for kiln shelves in the event of unexpected overmelting.

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 using grog is barely melting.

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). What I found surprising, is that, compared to the other biotite schist, this one does not fully melt. In fact, it’s quite granular to the touch.

At last, as mentioned earlier, I also added the crushed rock into one of my clay bodies. The images bellow follows the same firing order from those above. The grog starts showing after the glaze firing and reminds me of iron sand in a clay body.

Glaze Recipes

Given the large number of rock samples involved, I chose not to use conventional line-blending or triaxial blending methods. Instead, I developed a single, consistent base recipe for all glaze tests, defined by a fixed ratio of ingredients: 85% crushed rock, 10% flux, and 5% clay. For the clay component, I worked with both EP Kaolin (EPK) and Redart (R). The fluxes tested included Gerstley Borate (GB), Dolomite (D), Whiting or calcium carbonate (W), Zinc Oxide (Z), Nepheline Syenite (NS), and Soda Ash (SA). While varying these proportions could have generated a broader range of glaze outcomes, maintaining a stable recipe made it possible to more clearly isolate, extend, and understand the specific roles and effects of each flux and clay type within the glaze system.

All glazes were tested on different clay bodies (PSH 519, Tucker’s Mid Cal 5, PSH 540i). In the following section, I present a curated selection of test tiles rather than the complete set.

Biotite Schist Glazes : An Overview

The glazes produced using this biotite schist rock as the primary ingredient offered a rich landscape of colours and textures, which is quite similar to an amphibolite rock I tested before and the other biotite schist. Although many of the glazes fall broadly within the iron-brown family, each exhibits distinct characteristics, including purple spotting and orange-red veining. Among them, the most notable result is the zinc-based glaze, which drastically changed the colour.

Glazes using Redart on PSH 540i

Glazes using Kaolin on PSH 519

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.

Biotite Schist Glazes : A Closer Look

A few extra pictures of the glazes produced.

Going Beyond Testing

The final step in glaze testing lies in the application of the glazes on finished pieces. For each rock in this project, I selected two of my preferred glaze outcomes and applied them to moon jars in order to assess their behaviour in a functional and aesthetic context. Presented below are the two moon jars produced using glazes derived from this biotite schist.

As always, thank you for taking an interest in my work. I hope this publication can serve as both a useful resource and a source of inspiration.