Updated: Oct 1, 2020
Based on 'Rock glazes of NZ' Minna Bondy
Sample 84. Maruia River Sand
Mainly quartz and feldspars, minor chlorite, epidote and tremolite. Almost certainly derived from schist. Probably fairly close in composition to the original schist and thus greywacke.
This sample was gathered from directly behind the Mariua Hot Springs with permission from the staff. The rock is mostly Greywacke with the most beautiful green schist scattered amongst the pebbles. It is these places that I'm most interested in the test results as the sand is so varied, the only way of knowing the result will be to test.
There are two schools of testing, The Seger 'Unity Molecular Formula' (UMF) created by Hermann Seger, looks at the number of molecules in the raw materials. When applied this formula gives the ratios needed for a stable glaze. I am only just getting my head around this method, so I won't even attempt to explain, for more details follow this link www.digitalfire.com
The other method is a trial method using a series of tests, increasing quantities of the ingredients to assist the glaze melt and structure. This method is referred to as 'line testing'.
Bondy began her test following Seger formulas but with no success. I would assume that
due to the nature of the sand being a mix of many elements there was no way to correctly identify the molecular structure without testing each grain of sand.
She then tested all the samples with two different recipes.
Test A. 10% Whiting
90% Rock material
Test B. 10% Wood ash
90% Rock material
Bondy only tested using whiting and wood ash in a single quantity of 10%, her work can be extended upon by varying this quantity in lesser and greater quantities using line testing. Steve Harrison's book Rock glazes is a wonderful resource on these topics if you are interested. I spoke to Steve the other night and he has also told me once the specimens are fired using a line test it will be easier to identify the elements. The kiln process decomposes these materials back to their basic building blocks, revealing their mineral content.
I will be testing following Bondy's methods and then from those results, I will select the materials with the most promise to line test.