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.
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.
Drying
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.
Burnout
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.
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.
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