Note : This publication extends my exhibition Ceramic Rock Glazes : Developing a Geological Language of Alternative Ceramic Materials. It brings together all of my previous publications (on Instagram) into a single document in order to improve accessibility while also offering a more detailed and comprehensive exploration of the subject.
Violet Lepidolite
Each publication in this series follows a similar structure: a return to core principles of mineralogy to develop a more comprehensive understanding of the chemical reactions that occur during firing. The process is documented through a range of images, showing both the raw materials and selected finished results from glaze testing. I hope these writings prove useful, informative, and potentially inspiring for your future projects.
From Rock to Dust
As with the majority of the rocks in this project, I found this one on the side of the road during on my walk in the summer of 2023. It was most definitely the green that caught my attention at first. Splitting it revealed beautiful green crystal of quartz. At the end of the crushing process, I was left with a pale green powder. If you’re interested about some of the tools that I have used in this project to process the rocks, I wrote a short blog on the subject : Tools for processing rocks into fine powder for ceramic glazes : an introduction.
Mineral Composition and Considerations for Glaze Development
Note : XRF analysis could not be conducted for this project due to cost constraints, the large number of samples, and limited access to suitable laboratories. Consequently, identification relied on a rapid visual observation, comparison with similar rock types, hardness testing, and melt tests. This work was carried out with the invaluable assistance of a friend, to whom I am deeply grateful. In the absence of XRF data, all conclusions regarding mineral composition remain provisional; while not definitive, they provide a solid foundation for understanding the material and guiding further investigation.
The general composition of this rock :
- Lepidolite (Lithium-bearing Mica)
- Quartz (Silica)
- Calcite (Whiting / Calcium Carbonate)
Firing Temperature and its Effects
Before incorporating the rock into glaze recipes, I conducted melt tests at cone 6 to evaluate its high-temperature behavior, melting characteristics, and potential color development, and to confirm that it could be fired safely without excessive splattering or overmelting. I strongly recommend similar tests when working with raw or unfamiliar materials, using small bowls or containers with raised walls rather than flat tiles to protect kiln shelves in case of unexpected fluidity.
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). I was rather surprised by how bright this rock ended up being after firing.
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).
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.
Glaze Recipes
Given the large number of rock samples, I chose not to use conventional line-blend or triaxial methods. Instead, all glaze tests were based on a single recipe: 85% crushed rock, 10% flux, and 5% clay. The clay component alternated between EP Kaolin (EPK) and Redart (R), while the fluxes tested included Gerstley Borate (GB), Dolomite (D), Whiting (W), Zinc Oxide (Z), Nepheline Syenite (NS), and Soda Ash (SA). Although varying these ratios could have produced a wider range of surfaces, a fixed formula allowed for a clearer analysis of the specific effects of the fluxes and clays.
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.
Violet Lepidolite : An Overview
The glazes from this rock offers different kind of whites. My assumption is the that the transparency we are seeing is mostly coming from the flux themselves rather than the rock becoming translucent.
Glazes using EPK on PSH 540i
Glazes using EPK on PSH 519
Below is a short series of videos, recorded in early 2024 (approximately one minute each), in which I share my on-the-spot observations, including reflections on the raw melt test.
Violet Lepidolite : A Closer Look
The idea of using macro photography emerged after examining the glazes with a magnifying lens. The close-up images revealed details that would otherwise be imperceptible to the naked eye, a quality I found particularly compelling.
Going Beyond Testing
The true test of a glaze lies in its application on finished pieces. For each rock in this project, I selected two of my favourite glazes and applied them to moon jars to present them in a functional and aesthetic context. Below are the two jars produced using glazes derived from this violet lepidolite.
Moon Jar [É23B – 1]
Moon Jar [É23B – 2]
As always, thank you for your interest in my art ! I hope this publication can be useful and inspiring 🙂
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