All that glitters

....is not gold. It's probably mica!
I was being optimistic thinking that the rock I collected a couple of weeks ago from the same parent body but 40 miles away from the place I visited before would be the same. This rock is a beautiful varied mass of rose pink and white crystals. It is very high in silica and surprisingly free from impurities.

The colours it gives in the wood kiln can be seen best in this image of one of my disc forms. The pure, high silica, low iron, high potassium feldspar rock gives a very viscous melt, full of tiny bubbles, giving a smoky quality to the pale blue grey glaze. Where it gets a lot of heat the glaze rapidly fluxes becoming a glassy transparent of striking vivid blues. It is one of my very favourite rocks that I have found on my travels.

 

 

As soon as I saw the new rock I could see that it looked very different and has obviously crystallised out in quite a different manner. It was macrocrystalline, which means large crystals, and therefore slower cooling deeper within the earth's crust. The pink is there in the large feldspar crystals and in some of the ground mass surrounding them, but the main difference is in the grey sparkly crystals that are so abundant. These are flat plate like glittering crystals of mica and are the most variable of all the components of granite. They contain iron and sodium or potassium but also other minerals such as magnesium, manganese and possibly things like chrome and titanium.

These 'impurities' turn the pale blue colour into something else, depending on what is actually in there. More iron, and manganese give deepening greens and titanium is the kryptonite to superman's blue. As you can see from these initial tests the colours are much darker and very definitely green. The rock is much more fusible (easy to melt) and so produce quite clear runny glazes. 

I have about 4 initial tests that I do on a new rock, mostly progressive additions of one or two extra ingredients. Occasionally, when the sun is shining on me,  I find something stunning right here at this early stage but, as in this case, mostly a lot of extra work is needed. It's easy to get a rock to melt, but often quite hard to find that exact balance of additions that will coax its most beautiful colours out and transform it nto something sublime.