Note : This publication serves as an extension of my exhibition titled Ceramic Rock Glazes : Developing a Geological Language of Alternative Ceramic Materials. The current series of blogs further expand the project by offering details related to each individual rocks. 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.

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Green Aventurine

Mineral Composition and Considerations for Glaze Development

Note : The rocks in this series only went through a basic identification process using hardness scale and a magnifying glass. Since they have been through any thorough assessment (eg., an XRF), the information I’ve written bellow are speculative in nature. That being said, I believe that the melt test and glazes produced can help confirm some of the initial hypotheses of the minerals within this rock.

As per usual, let’s first look at the mineralogy of the green aventurine. Based on the information I was given by my friend (who’s a geologist), the minerals composing this green aventurine are :

• Aventurine (Silica)
• Muscovite (Potassium)
• Calcite (Whiting / Calcium Carbonate)

Hypothetically, it shouldn’t be too difficult to make a ceramic glaze out of this rock, especially considering it’s high silica level (which is the primary glass former in glazes) as well as the presence of two fluxes : potassium and calcium. What this rock needs to become a viable glaze might be an increase in flux, but more importantly, a higher amount of alumina (eg., clay) to balance the equation.

Firing Temperature and its Effects

Before proceeding to the glaze testing phase, I made several melt tests using the grog form (<40 mesh) and powder form (>40 mesh) of the rock. I believe this step to be crucial in the process as it will give you valuable insights as to the way it will react in your kiln (in terms of safety) and how much work will be needed to figure out a good glaze recipe (eg., a rock that melts easily by itself is a much better contender than one that doesn’t AND a rock that doesn’t melt but stay together is easier to work than one that crumbles and disintegrateafter the firing).

As for the melt test, I used flat tiles, brushed a thin layer of slip and pressed the crushed rock powder onto the surface. If I were to do this step again, I would opt for a small walled recipient as they are safe for the kiln shelf in the event of a rock that would melt too much. As for the in-clay body testing, I simply added 5% of crushed rocks to one of my clay bodies – you can try lower or higher, simply be aware that the higher you go that you are increasing the chance of altering the integrity of the clay.

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 result : nothing spectacular. There’s a little bit of green, but it’s hardly noticeably at a distance of more than 1 meter.

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 coarser sample, this one almost completely loses its green colouration.

At last, as I’ve mentioned earlier, I also added the crushed red jasper into one of my clay bodies. The images bellow follows the same firing order from those above. Nothing worthy in my opinion.

Glaze Recipes

As I have said elsewhere, the sheer volume of rocks within this project combined with the shared community kiln situation made it unrealistic to use traditional line-blending and triaxial blending methods. Consequently, 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). On a positive note, 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.

Green Aventurine Glazes : An Overview

The glazes produced using a green aventurine rock as the primary ingredient offered a relatively small array of colours and textures. The most interesting were, in my opinion, the recipe using gerstley borate (GB) and zinc oxide (Z). That being said, what stands out is the way the glaze reacts drastically different on a black clay body (one that is really high in iron), especially the recipe using the same fluxes as mentioned before.

Glazes using EPK on PSH 540i

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.

Green Aventurine : A Closer Look

Macro photographs offer the possibility to look at the unique surface terrain of glazes. Here’s a few 🙂

Going Beyond Testing

All ceramicists know that glaze testing is just one part of the process. The true “test” is actually applying the glazes on some of our work. This step can be filled with surprised, happy moments and disappointment. Below are two moon jars are made to commemorate the last phase of this project.

Thank you for making it this far 🙂 I hope you have enjoyed this publication 🙂