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.

Pink Feldspar

As per all previously published publication in this rock glaze series, the structure of this article is as follows : an overview of the mineraology components of the rock, pictures showing the crushed rock at various stage of the firing, pictures of the glazes created using the rock as the primary ingredient in the glazes, and, finally, the presentation of two moon jars to showcase two of the glazes created.

From Rock to Dust

As with many of the rocks in this series, I found this specimen in a ditch near my home. Its unassuming exterior conceals a striking deep pink, marble-like interior. Once crushed, the material yields a fine, light pink powder. 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 carry out XRF analysis for this project, due both to the cost associated with the large number of specimens and the limited access to laboratory facilities in the surrounding region. As a result, I relied on a relatively quick and simplified identification process, including visual observation, comparison with similar rocks, as well as hardness and melt tests. This work was conducted with the help of a friend, whose observational skills were invaluable, and to whom I am very grateful. In the absence of XRF data, the proposed mineral composition remains hypothetical. Even now, I cannot state with certainty that all of the interpretations presented below are entirely accurate; however, they provide a useful starting point for further investigation.

This quartz feldspare is pink on the inside and appears to be primarily composed of:

Felspar (Potassium, Sodium)
Quartz (Silica)

Based on this mineral composition, I expected that the material would not melt on its own, as these components are generally refractory in their raw state. That said, it becomes a strong candidate for glaze development once combined with small additions of clay and fluxes.

Firing Temperature and its Effects

Before incorporating the rock into glaze formulations, I carried out a series of melt tests to observe its behavior on its own at cone 6. This preliminary step served two main purposes: first, it provided a general understanding of how the material responds at high temperatures, including its melting behavior and potential color development; second, it confirmed that the rock could be fired safely, without splattering or overmelting. I strongly recommend this step to anyone working with raw or unfamiliar materials. If you plan to conduct similar tests, using small bowls or containers with raised walls rather than flat tiles can help protect kiln shelves in case the material becomes excessively fluid.

In addition, I incorporated crushed rock directly into one of my clay bodies at 5% of the clay’s total weight. This allowed me to observe how the material behaves within a clay matrix.

The first series of images below presents the rock in its grog form (i.e., relatively coarse particles), progressing from the raw state to bisque firing (cone 04), and finally to glaze firing (cone 6). At the final stage, the sample remains completely unmelted, resulting in a granular texture. The coloration is predominantly white, with scattered black speckles, likely due to impurities within the rock.

The second set of images shows the rock in powdered form (40 mesh and finer), following the same sequence: raw, bisque-fired (cone 04), and glaze-fired (cone 6). As with the grog test, this sample shows no signs of melting.

Finally, as mentioned earlier, I also added grog to one of my clay bodies. The images below follow the same firing sequence as above. The grog, barely visible in the raw state, remains largely unchanged throughout the firing process.

Glaze Recipes

Given the large number of rock samples I was working with, I chose to move away from standard line-blend and triaxial blending methods. Instead, I established a consistent base recipe for all glaze tests: 85% crushed rock, 10% flux, and 5% clay. For the clay component, I alternated between 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). Although adjusting the proportions might have produced a wider range of glaze surfaces, maintaining a fixed formula allowed for a clearer assessment of how each flux and clay influenced the final results.

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

Pink Feldspare Glazes : An Overview

Some of the glazes produced remind me of those made with a biotite granite. Texture goes from smooth-ish glass to rough sand.

Glazes using Redart on PSH 519

Glazes using EPK 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.

Raw Melt Test

Gerstley Borate

Dolomite

Whiting

Zinc Oxide

Nepheline Syenite

Soda Ash

Overview

Pink Feldspare Glazes : A Closer Look

The idea of macro photographies came after observing the glazes using a magnifying lense. I though it was really interesting the way the close-up shots allowed us to view details that would go missing to the naked eye.