Ciment Fondue
Ciment fondue was a French discovery and so the French
name has become common in Europe. The name ciment fondu is used for the formal
name Calcium aluminate cement which is also called high alumina cement and
aluminous cement. It is composed mainly
of Aluminium oxide (alumina) and calcium oxide (quicklime) with varying amounts
of ferric oxide. The Aluminium Oxide varies from 40% to 80% for various
applications. The calcium oxide content varies from 40% to 20% and the ferric
oxide varies from 16% to none for refractory applications. For kilnformers, the general purpose
composition of 40% aluminium oxide, 40% calcium oxide and 16% ferric oxide is
sufficient (the rest is made up of minor amounts of incidental minerals and
metals).
It is costly in relation to Portland cement and is used
mainly where quick curing strength is required and at low temperatures; in
refractory concretes where strength at high temperatures is needed; and in
sewer piping and other applications to provide protection against biological
attack of the concrete.
It is also used in sculptural applications, both as the casting
material, and as a strengthening element in a non-metallic structure.
It is mixed with water to form a paste. The proportions are not required to be exact,
as the ciment fondu separates out of the water due to its weight and very low
water absorption. Slightly different
methods are needed to repair breaks, and to fill divots in the surface.
Breaks
To repair breaks or cracks in
ceramic moulds the ciment fondu needs to be used on its own. Mix the dry particles with water until a
stiff slurry is formed. Thoroughly wet
the edges of the broken pieces or the cracked area. Then apply the ciment fondu slurry to both
edges. Press the pieces together and
bind them if they would otherwise separate.
This can be with elastic bands or tape or any material that will
withstand moisture.
The internal surface must have all
the ciment fondue cleaned from it. It
cures so hard that it is not practical to sand it smooth without damaging the
ceramic surface. This clean up can be
with a lot of water and paper towels. Any tools you use need to be immediately
cleaned with water. Do not dispose of
this clean up water down your drains. It will harden and narrow your drains,
potentially blocking them so firmly that whole sections of the drain will need
to be replaced.
When fixed together put the mould in
plastic or other waterproof material for at least 24 hours to give a wet
cure. The ciment fondu is not completely
cured until it is given a heat cure.
This should be above the expected operating temperature. Although I have never fired any of my ceramic
moulds above 680°C, I fire my repairs to 800°C.
The firing is smelly, so ventilate the kiln and room well. Try to do the heat curing when the smell will
not disturb you or your neighbours.
Divots
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| This mould had glass stuck to it and was damaged in removing the glass. |
If there are scratches or divots in
the mould surface, you need to add some material that will absorb water into the
ciment fondue mix. Cured ciment fondue
rejects water and so does not get as well coated as the rest of the mould when
kiln wash is applied.
To prevent this
rejection of water, I add finely ground vermiculite to the mix. I use 3 parts or less vermiculite to 1 part
ciment fondue (measured by volume). This
provides a firm surface that absorbs some water. Although the absorption of
moisture is not as good as the ceramic, it is sufficient to get the kiln wash
coverage required.
Once the mix is prepared, you need
to thoroughly wet the area to be fixed. This prevents the ceramic absorbing the
water from the ciment fondue too quickly. Apply the ciment fondue mix with
whatever tools seem appropriate.
You must smooth the applied mixture
before it dries, as it is so hard when cured that it is not possible to sand it
smooth without damaging the ceramic surrounding the repair. Smoothing can be done with significant
amounts of water and a smoothing tool such as a ceramicist’s kidney or a palette
knife.
Once smoothed to achieve the surface
required, pour off the excess water.
Enclose the mould in a plastic bag for 24 hours for a wet cure. Once out
of the bag and dry you can further smooth with very fine sandpaper.
Then fire to 700°C to 800°C to complete
the cure. When cool it is ready to kiln
wash. If you warm the mould to around
100°C, the kiln wash will adhere to the repaired areas a little better than the
cold mould. Once the first kiln washing
of the mould is complete, further applications of kiln wash will be easier. Of
course, if you use boron nitride to coat the mould, there will be no difficulty
with the repaired areas.
All tools need to be cleaned
immediately of the ciment fondue and the cleaning water disposed of on the
garden or waste ground. It should never
be put down domestic or public drains.
It does no harm to the soil or plants, but it will certainly harm your
plumbing.
