Experiments with mineral inclusions within fused glass

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In fused glass, lots of materials, both organic and inorganic, can be used as inclusions. These add texture, colour and lustre to sheet- glass when fused. I wanted to experiment with using minerals, that I had collected, as inclusions. 
I planned to create 'samples' in the style of microscopy slides (7.5 X 2.5cm), each with a mineral sample. I experimented with six minerals that fit into four categories. The first of these are the sulphides and sulphates which include : galena (lead (II) sulphide), barytes (barium sulphate) and sphalerite (zinc sulphide). Also included was an oxide: haematite (iron oxide) and a carbonate mineral: malachite (copper carbonate). In addition to these there is mica (both biotite and muscovite). While most of the minerals were collected locally e.g. haematite from a spoil heap above Ennerdale, the mica sample was from further afield- collected in Sikkim, in the Indian Himalayas. 

Crushing the mineral sample to a powder/granules (galena, mica and malachite).

These minerals first crushed to a powder, although this was only done roughly with a hammer, and as a result weren't particularly fine, and then were sandwiched between two pieces of 3mm glass. This was then heated to between 700-820 degrees centigrade, fusing the glass together, enveloping the mineral and forming an inclusion. Generally only the contact area between the inclusion and the glass fused together and this meant that any 'excess' mineral would not be fused and would be loose in a pocket between the glass. This contrasts to organic inclusions (such as leaves) that would burn away.

Left top to bottom: barytes, haematite, galena, right top to bottom: sphalerite, mica, malachite.

 THE PREDICTIONS: 

Virtually all minerals are thermochromic (change colour when heated), however this varies massively in its extent. Below are my predictions of what the effect of heating to glass fusing temperatures would be on the crushed mineral samples (I'm not a metallurgist, nor have I studied chemistry extensively so these are mainly hunches based on a little research).

Sphalerite: sphalerite is mainly composed of zinc sulphide which is white in colour and is thermochromatic, changing from white at room temperature but changing to yellow when heated. However, sphalerite is not pure zinc sulphide and contains iron (in varying amounts) creating a darker and darker appearance. The sphalerite I collected was a dark brown-black colour and as a result likley had a high iron content. This will likely greatly alter the resultant colour. As for galena and barytes, I struggled to find any information on their heating, reduction or use in fused glass.

Haematite: RD Glass has used haematite from Florence Mine, I myself have used this pigment before -click here-, and apparently there is little colour change. I suspect this is due to the temperatures achieved for fused glass are not high enough for reduction.

Malachite: when heated malachite decomposes, and forms copper (II) oxide, as well as carbon dioxide and water vapour. The decomposition of malachite occurs below 200 degrees centegrade and therefore this will occur in the fused glass sample. Additionally the carbon dioxide will likely cause bubbling within the inclusion, unless it escapes. 

Biotite and muscovite micas: mica is commonly used in fused glass [1], and do change colour when fired but keep the metallic lustre. They also typically loose their colour (or fade), though keep their colour best at low temperatures [2]. 


THE RESULTS:


On collecting the slides I was pleased to see that there had been no blow-outs where gases had escaped, or disasters in the kiln. There were however some surprises in terms of the resultant colours and lustres.


The first set of photographs were taken on a light-pad, which really showed off the translucency in some of the samples, as well as the effects of heating the minerals such as bubbling as a result of gases being released and some precipitates.

The slides on a light-pad (barytes, haematite, mica, galena, malachite and sphalerite).

Barytes- this changed very little in terms of colour, and released no gas, resulting in a completely flat slide. 

Haematite- this too changed little in colour, however it did produce a large amount of gas creating a bubble. This bubble inclusion held some loose haematite which had not fused with the glass and moved freely. 

Mica- this changed considerably in terms of colour from a grey to a brassy gold colour. As well as the mica sheets, transparent grains were present in the inclusion, which appeared very quartz like- potentially also found the original mica-schist. 

Galena- this lost its characteristic 'lead' blue-grey colour, dulled to a dark grey, although retained its cubic habit. One interesting thing about this sample was the released gas forming distinct lines, not unlike a topographical map- an interesting effect.

Malachite- the malachite didn't change much, although the colour generally darkened, though it is still distinctly green. There was also a significant bubble as a result of gas buildup. 

Sphalerite- this was probably the most interesting sample, as the colour changed dramatically from a dark brown/black to a yellow/brown colour with an unusual lustre (this is more evident in later photographs). This partially agrees with my prediction.

When photographing the slides, the light pad struggled to show the true colours and textures of the pieces, due to backlighting. The following set of photographs are all naturally lit.

The full set of slides.

Mica- the metallic lustre is spectacular in the light- it is unsurprising it is commonly used as an inclusion in fused glass.

Haematite- showing it's distinctive colour.

Malachite.

Galena.

Sphalerite.


I was happily surprised by my predictions, sphalerite did indeed turn to a yellow hue, haematite retained its colour and the mica kept it's lustre. I was, however, wrong about the malachite which clearly did not reach a high enough temperature to decompose to copper (II) oxide as there was no colour change from green to black. There was however, interestingly, a large release of gas (potentially carbon dioxide).

Overall I was pleased with the samples, and while I'd like to use the different mineral inclusions in a larger piece, I'm more keen to experiment with other minerals such as limonite (yellow ochre) or some organic inclusions.

A big thanks to RD glass [3] for firing the fused glass, and use of her workshop.

[1] Experiments with mica and further information- http://www.bullseyeglass.com/blog/2014/03/12/messing-around-with-mica/

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