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 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.

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Red Jasper

Mineral Composition and Considerations for Glaze Development

Note : 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.

Before I started making and testing the glazes made of the red jasper, I found it useful to spend a little bit of time research its mineralogy. This allowed me to get a clearer understanding of the way it would/could work as a glaze, more specifically how it would interact with the various fluxes I used in this series. Based on the information I was given by my friend (who’s a geologist), the minerals composing this specimen of red jasper are :

• Quartz (Silica)
• Calcite (Whiting / Calcium Carbonate)
• Hematite (Iron)
• Pyrite (Iron)

As a whole, the minerals forming the red jasper are perfect, at least theoretically speaking, to produce a glaze. It has sufficient amount of silica coming in the form of quartz, which is the primary glass former for the majority of glazes found in pottery. Calcite provides one of the most common and well-known flux : whiting / calcium carbonate. This has the effect of reducing the melting temperature of a glaze, something that is usually needed for cone 6 firing. Finally, the presence of iron (from hematite and pyrite) both influences the colouration of the base glaze (which I am expecting to be quite similar to a iron oxide wash : a deep red-brown) and the fluidity of the glaze. What is interesting is that iron can act both as a flux (promotes melt and fluidity) and a refractory (promotes rigidity). In so far, the red jasper looks quite promising.

Firing Temperature and its Effects

Before mixing the rocks with other fluxes and clay, I proceeded in the making of melt test using the grog form (<40 mesh) and powder form (>40 mesh) of the rock. If you are thinking of incorporating foraged, collected or unknown materials in your glazes or clay bodies, I highly recommend to do these tests first. Not only will they offer you valuable insights as to the way these materials are behaving across different temperatures (bisque and glaze firing), but will also give you confidence in regard to the safety of them in the kiln, as some can melt too much, make the clay brittle, or splatter.

As for the way I proceeded, it is rather simple. For the melt test, I made little tiles — although I would recommend making small walled recipients instead of flat tiles — and added the crushed rock by itself. 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). As it can be observed, the majority of the sample fully melted, leaving only a small portion not melted or partially melted. To me, this was an excellent sign of the viability of the red jasper as the primary ingredient in my glaze recipes.

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 is interesting, although not much of a surprise, is that a finer grain size increases the melt quality on the surface of the tile. I say that this is not surprising as it has been documented for a long time how finer particles more easily enter the melt matrix during the firing.

At last, like 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 and used 5% of crushed red jasper in the form of grog.

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. 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.

Greenschist Glazes : An Overview

The glazes produced using a red jasper rock as the primary ingredient offered a rather wide variety of colours. Randing from some sort of khaki green, to some dark and light browns, and finally some nice oil-spot-like glazes with notes of orange. What is most noticeable, specifically in regard to gerstley borate (GB), is that by using redart instead of EP Kaolin, we get more fluidity in the glaze.

Glazes using EPK on PSH 540i

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.

Greenschist Glazes : A Closer Look

I particularly loved taking macro photographs of the glazes. They offered a totally new perspective into the landscape of the glaze that tends to go unnoticed to the naked eyes.

Going Beyond Testing

Testing glazes is really fun. In and of itself, all the test tiles are a piece of art. That being said, I always wondered what they would look like on actual pieces. To illustrate and showcase them, I made a series of moon jars. Two moon jars were made for each rock, which were then used to display two glazes among the six I had made.

Thank you for your interest in my projects ! I hope you have enjoyed this publication 🙂