Wednesday, 30 March 2016

Organic and Mineral Inclusions

Encasing organic materials adds a new level of complexity to inclusions.  In addition to bubble formation, you need to consider how to eliminate the combustion gases from the vegetable matter.  On the other hand you don’t need to worry about expansion differences.

Some examples of metal and glass inclusions

The requirement is to burn out all of the vegetable matter to avoid creating big bubbles from the burn off of the material.  There are two elements to this burnout.  One is the amount of moisture contained in the object and the second is the volume of dry material that has to burn out.

Unless you have dried the material before including it, you will need to leave enough soak time before the glass begins to move to ensure all the water is removed.  It is also advisable to place small shards of glass at the corners of the piece, to allow easy ventilation both for the moisture to evaporate and the vegetation to burn easily.  You can estimate the time required and then put a witness piece of glass or better mirror above the vent or peep hole to see if there is any fogging on the glass from escaping moisture.  You need to continue soaking until there is no fogging.

The second element is to give the vegetable matter enough time to burn out.  The burn out should occur at about 400°C.  This is high enough to ignite carbon based materials, but not so high that an extended soak will allow the glass to sufficiently deform to seal the un-burnt material inside.  If you have a really good sense of smell you can tell when the carbon has burned away by the absence of the smell.  For the rest of us, we need to open the peep hole and use a strong light to tell how much is left to burn away.  The burning is much more like a smouldering with very little light coming from it.

An incompletely burned out leaf in a large trivet with felt feet at the corners. 150mm square

The length of time you need to soak below the softening point of the glass is directly related to both the water content and the amount of combustible material you have included.  The burning will not begin until everything is dry.  If the material is not dry, the time for this needs to be added to the burnout time.  The length of soak for burnout is much more difficult to determine and needs periodic observation beyond the time when the smoke stops coming out of the kiln ports.

Bubble squeeze
Once the drying and burnout are completed, you need to advance to the bubble squeeze.  This will need to be longer or slower than usual to ensure all the combustion gasses are out for organic materials.  Minerals will normally be thicker than the organic materials and so need long bubble squeezes. These can be at or just below slumping temperature, or a slow rate of rise, taking an hour or more, from about 50C below the slumping point.

It is possible to include other minerals such as bone, or ash, or other inert particles that will not stick to the glass. Materials that contain silica are not suitable, as they stick to the glass and cause breakages.  So most stone, which contain silica, however thinly sliced will not be suitable as an inclusion.