Recently I’ve been wondering if there’s a reliable way to test glazes for transparency. A method that would allow one to compare results from different firings and glaze types.
Paint manufacturers have a system for testing paint opacity that uses a black and white card from which a contrast ratio can be calculated. The primary manufacturer is Leneta.
I couldn’t find any parallels in the ceramics industry.
I wanted to try a similar method using porcelain (white) and stain/colored porcelain (black), adjusting the results to account for the fact that our whites and blacks are not pure.
Paint Opacity Chart from Leneta
Using my whitest porcelain, I created a colored slip adding 8% of a local black stain. (Ideally one would use a standard mason stain.) Adding Darvan, I made a thin slipcast slab that I then cut into small squares.
Cutting square slabs of stained porcelain
Using the same casting porcelain I made a thicker slipcast slab which was then cut into square test tiles. The black stained squares were then applied to each test tile and rubbed flat. Finally, the tiles were bisque fired in the hopes of minimizing contamination of the glaze when dipping.
Test tiles after adding black-stained squares.
Two 100 gram batches of glaze were prepared: Pinnell Clear and Pinnell Clear with added 10% Zircopax. Using volumetric blending I created tests in 2% increments. The tiles were dipped in the test glazes. Ideally, steps would be taken to ensure even thicknesses of glaze.
Test tiles after firing in reduction to Orton cone 10.
The fired tests display a nice opacity gradation as zircopax is added to the glaze.
Unsure of the best way to measure transparency (or opacity) using these test tiles, I tried the simplest approach I could think of. Adjusting the image to greyscale, I averaged the colors of the white test tiles as well as each black-stained square. Below are the Brightness levels measured in Photoshop using the HSB scale. If these tests were going to be made consistent across firings, I suppose one could normalize the photos based on the color of the unfired white porcelain body.
For the opacifying power of Zircopax relative to this specific test, I created an opacity scale in Photoshop using the 0% glaze as a baseline and then matched the tests to this scale. According to the scale, a 4% addition of Zircopax opacifies the glaze by 30%, while a 10% addition of Zircopax opacifies the glaze by 70%. I’m probably vastly over-simplifying things. For instance, I didn’t take into account the fact that the entire test whitens as Zircopax is added. Also, there will probably be few times in ceramics where there is a neat linear relationship, for instance adding 14% Zircopax to the glaze won’t necessarily get me to 100% opacity.
Below is a closeup of the black squares. If I had made these tiles more consistently, with a crisp, straight border between the black and white porcelain, it might also be possible to compare diffusion.