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.

Quartz

As with all previously published articles in this rock glaze series, this text follows a consistent structure: an overview of the rock’s mineralogical components; images documenting the crushed rock at different stages of firing; photographs of glazes made with the rock as the primary ingredient; and, finally, the presentation of two moon jars that highlight two of the resulting glazes.

From Rock to Dust

As with most of the rocks in this series, I found it in a ditch near by my house. It was particularly interested in its sort of ferrous look – the interior has the typical glimmering quartz aesthetic. By the end of the crushing process, it became a light pink colour. To crush this rock, I have used some of the tools listed in the previous blog post : Tools for processing rocks into fine powder for ceramic glazes : an introduction.

Mineral Composition and Considerations for Glaze Development

Note : Unfortunately, I was unable to conduct 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, the identification process relied on a relatively quick and rudimentary methodology, including visual observation, comparison with similar rock types, hardness scale testing, and melt tests. This work was carried out with the help of a friend whose observational skills were invaluable, and to whom I am very grateful. Without XRF data, all conclusions regarding mineral composition remain provisional. Even now, I cannot state with certainty that every interpretation presented below is entirely accurate; however, these findings provide a solid starting point for understanding the material and guiding further investigation.

This quartz sample is slightly different from a previous quartz rock I have used before. It is mainly composed of :

Quartz (Silica)
Calcite (Calcium)
Sulphides

Based on the list of minerals, I assumed that it would either melt nicely and evenly by itself, or not at all. While the latter is not as desirable as the former, because this rock is rich in silica (ie., quartz), I am assuming that the addition of various fluxes will make it melt at cone 6, although this might not be the case. The pink colour might be due to some of the iron seen on the surface on the rock.

Firing Temperature and its Effects

Before using the rock in any glaze formulations, I carried out a series of melt tests to examine its behavior at cone 6. This initial phase had two main objectives: to develop a general understanding of how the material responds to high temperatures, including its melting behavior and potential color range, and to verify that it was safe to fire in the kiln (for example, that it did not splatter or melt excessively). I strongly recommend this step when working with raw or unfamiliar materials. For those undertaking similar tests, using small bowls or containers with raised sides rather than flat tiles is advisable, as this can help protect kiln shelves if the material becomes unexpectedly fluid.

In addition, I mixed crushed rock directly into one of my clay bodies, using 5% of the clay’s weight. This allowed me to see how the material behaves within the clay.

The first series of photos below shows the rock used in its grog form (i.e., relatively coarse particles), progressing from raw to bisque-fired (cone 04), and finally glaze-fired (cone 6). In the final stage, the sample is completely unmelted, giving a granulated look. What surprised me most was how, after firing, the pink colouration was actually stronger than in its raw form.

The second set of photographs shows the rock in its powdered form (40 mesh and finer), following the same sequence: raw, bisque-fired (cone 04), and glaze-fired (cone 6). Similarly to the test using the grog form, this sample doesn’t show any signs of melting. Typically, when in a powder form, I have noticed that most rock will lose its colouration; however, this rock, the colour was greatly accentuated through each firing with the bisque ending up with a pale orange-sand, while after glaze firing it was a light pink.

At last, as mentioned earlier, I also added some of the grog into one of my clay bodies. The images bellow follows the same firing order from those above. The grog, barely visible at first, remains that way throughout the firing process.

Glaze Recipes

Because I was working with a large number of rock samples, I decided not to use conventional testing approaches such as line blends or triaxial blends. Instead, I established a single base formula for all glaze experiments, consisting of 85 percent crushed rock, 10 percent flux, and 5 percent clay. The clay component alternated between EP Kaolin (EPK) and Redart (R), while the fluxes tested included Gerstley Borate (GB), Dolomite (D), Whiting or Calcium Carbonate (W), Zinc Oxide (Z), Nepheline Syenite (NS), and Soda Ash (SA). Although varying the proportions might have generated a wider range of glaze surfaces, maintaining a constant recipe made it easier to assess the specific influence of each flux and clay on the final outcomes.

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

Quartz Glazes : An Overview

The glazes produced with this quartz sample as the primary ingredient yielded a relatively limited color range. When EPK was used, only slight shifts in coloration were observed. As is typical in this project when Redart is introduced, the glazes tend to move subtly toward warmer tones. Some fitting issues are visible in the final test tile combining Redart and Soda Ash. Among the results, I am particularly drawn to the purplish hue achieved with Redart and Nepheline Syenite, as well as the clean white produced by the combination of Gerstley Borate and EPK.

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.

Raw Melt Test

Gerstley Borate

Dolomite

Whiting

Zinc Oxide

Nepheline Syenite

Soda Ash

Overview

Quartz Glazes : A Closer Look

The idea of using macro photography emerged after examining the glazes with a magnifying lens. The close-up view revealed details that are not visible to the naked eye, and I found it particularly compelling how these images allowed such subtle surface characteristics to come into focus.