Whether you use a big kiln or a little microwave kiln you’ll have noticed that fibre paper and ceramic ropes change when fired, first by turning black and then miraculously turning white again as their organic components burn away leaving just the ceramic materials and a residual smell in the air. This observation led me to experiment with small pieces of plant material as inclusion.
You may be wondering about the fire risk when putting combustible materials into a kiln, to which I simply point out that small amounts of organic matter can not cause a big fire just as a piece of ceramic rope does not cause a big fire.
For my experiments I placed a dwarf Acer leaf and a small portion of a grass seed head onto pieces of 3mm clear glass and capped them with another piece of 3mm glass. The first picture shows the base glass with the plant materials on top. You will notice that they are fresh plant materials.
To give the plant materials a better opportunity to burn away and leave only inorganic ash I put a small piece of 2mm clear scrap between the glass layers at one end. The gap allows oxygen to get into the void to aid combustion and to allow the products of combustion like carbon dioxide to escape. I put those pieces of scrap at the stem-end of the Acer leaf and at the pointed end of the triangle that had the grass.
The two experiments were chosen to contrast smaller and larger fresh plant inclusions. Together they only just fit into a Hot Pot Maxi microwave kiln.
The smaller piece containing the grass seed head has produced a really nice ghostly shadow of the plant materials. All the organic matter has burned off, leaving only the inorganic ash. You can see that the 2mm spacer shard of glass did allow the pointed-end to bubble-squeeze the air out of the piece, but not enough time at the blunt end of the triangle – suggesting the possibility that combustion was note quite complete resulting in the liberation of some trapped gases.
The larger piece containing the dwarf Acer leaf has been less successful but does give us some clues about what worked and what did not work. You can just see the white ash remnants of the stem end of the leaf which had enough time to burn away the organic matter due to the proximity of the 2mm spacer shard of glass. However, they rest of the leaf has not completely burned away, leaving a mess of carbonised leaf. When the glass capping sagged over the incompletely combusted leaf it trapped the gases being produced (mainly carbon dioxide) to produce a rather large bubble which was big enough to even lift the base glass!
So, what have we learned from these first experiments with plant materials? What will I be trying for my second round of experiments?
Providing enough time to burn away all the organic matter seems to be the problem to resolve, particularly for larger pieces of plant materials, because the middle of the piece may get trapped before full combustion is complete. Using more shards of glass as spacers around the perimeter, at 3mm rather than 2mm thickness, should cause the capping glass to sag more slowly from the middle outwards and the greater number and thickness of the spacers should give more time to allow combustion to finish and produced less trapped gas.
As you are probably aware, fusing glass in a microwave kiln is a fast and furious event that is not so controllable or predictable when compared with fusing glass in a “proper” kiln. This leads me to speculate whether using the microwave at a less powerful setting will give the plant materials more time to dry out and then burn away before getting trapped between the glass layers. Maybe pre-drying the leaves would aid the process, for which nature’s autumnal processes will do this with no effort for me!
With such delicate and ghostly images of the original plant material I will need to choose a base glass that best suits these plant inclusions. Black strikes me as an obvious choice. Something to try once the basic technique has been refined.
But can these ghostly images be made more visible? It would certainly be possible to choose plant materials that have more inorganic content – such as thicker leaves, or leaves that have more robust veins, which in turn implies more inorganic content.
And what about making the ghostly images more colourful? I am reminded of a childhood experiment in which a cut flower was put into a vase containing ink – the objective being to make the plant veins more visible. This leads me to the possibility of dipping cut plant materials into chemical solutions, such as the copper sulphate (which we already use to patinate solder and lead), to introduce chemicals that result in more inorganic ashes that in turn can react with the glass to produce tinted colours.
Get out into your garden. Do try this yourself.