The way that you fire glass and other materials in your kiln affect the longevity of the kiln elements. Some things you can do and avoid are given here.
Venting
Even if you have the best aluminium
oxide coating, the fumes that emit from glazes, paints, organics, inclusions
and devitrification solutions can still attack the element through cracks in
the coating. Downdraft vents are your best defence against potentially harmful
fumes. Downdraft vents pull the fumes from the kiln chamber before they have a
chance to damage the elements.
If you do not have a downdraft
vent your next best option is to prop the lid a couple of inches until the kiln
reaches 540°C to allow the fumes out of the chamber. You should also consider leaving
at least one peephole out during the entire firing for the fumes that escape
above 540°C.
This presents a dilemma, as the
recommendation is to keep the kiln closed from 540°C upwards to protect the
glass from cold air drafts. Those who
rarely fire above 800°C do not have the same problem as those who regularly fire
at 850°C and above for casting, combing, and melts. The higher the temperature, the greater the
effect of fumes on the elements. At
fusing and below temperatures the effect on the elements is not as great. Thus, low temperature firings can follow the
standard practice of closing the kiln above 540°C. Those going higher, should consider venting
the kiln all the way to the top temperature to reduce the wear on the elements.
Maintain an Oxidising
Atmosphere
Elements need an oxidising atmosphere to provide a long dependable
service. Subjecting elements
to reducing atmospheres will age the elements quickly. This is be done by introducing organics or
oils into the kiln without venting.
Among the things that will attack the aluminium oxide coating of the
elements are
- ·
Carbon - this includes materials made
from carbon and plant-based inclusions.
- ·
wax burnout – it is best to steam wax out of
moulds to eliminate most of the wax before any burnout, as the fumes are
largely carbon.
- ·
halogens (such as chlorine or fluorine)
- ·
molten metals (such as zinc, aluminium). This is a more important reason for avoiding
the use of zinc and aluminium in kilnforming than the possibility of health
problems.
- ·
lead bearing paints and glazes – lead is a
common component of paints, enamels and glazes.
- ·
alkaline metals – the main one we come across
in kilnforming is magnesium which produces an amethyst colour of varying intensities. This has a melting point of 650°C and boils
at 1090°C, so some fumes can develop during firings and affect the elements.
- ·
borax compounds – used in enamel glazes and some
devitrification sprays.
If
you use these materials in the kiln, you need to ensure that the kiln is well
vented while these are in the kiln.
When
you do have to use these elements - even when you vent - it is good practice to
follow this firing by one without materials corrosive to the coating. This allows the coating to re-form around the
element surfaces after a corrosive firing.
Trying to do
reduction firings in your kiln will greatly limit their useful life and is
definitely not recommended.
Avoid Contaminants
Contaminants
such as silica which is contained in kiln wash and some glazes attack the
aluminium oxide coating of the wire.
Powders,
paints and kiln wash accidentally touching the elements cause rapid corrosion
of the elements if not cleaned off before firing.
Placing
Firing
close to the elements allows any fumes from materials being used to affect the
elements more than allowing some space between the glass and the elements. This provides another reason to keep the
glass away from the edges of the kiln in addition to the possible uneven
heating of the glass.
High Temperature Firings
High temperatures with very long soak times will accelerate an increase
in element resistance through the differential expansion of the inner wire and
the coating. The higher the temperature, the longer the soak, the sooner the
element will decrease in life. Usually short soaks work much better for the
longevity of the element. This is not
such a big factor for glass kilns as it is for ceramic kilns.
The next part in this series deals with the maintenance of the
elements.
Earlier relevant posts
Element Description
Earlier relevant posts
Element Description
