Reid Harvey on wed 14 apr 99
Here in West Africa there is a big problem with the low strength of
traditionally made, ceramic containers. For example, in villages
throughout the region water storage jars tend to be a porous, low fire
earthenware. (People love these because there is a cooling effect when
the breeze blows against the the water that has seeped through.) The
water jars are normally made by women, using techniques that have been
handed down over the generations. I am hoping that techniques for
strengthening these containers can explored
On several occasions I have seen women arrive at the weekly market, some
distance from their villages, with nothing but a pile of shards to show
for their hard work. The roads tend to be too rough, the packing
insufficient.
It's my understanding that as a high temperature flux CaO can be added
to an earthenware body, in amounts upto about 8.0%, as a way of greatly
improving strength. This may be of interest as a non-intrusive way of
assisting these potters, something they could possibly add to their body
without changing their techniques of forming, drying, firing, etc.
My fear is that in many cases the firing may be done at too low a
temperature for the CaO to have this positive effect. Can anyone
enlighten me as to the correct temperature, or any other ways of making
this idea workable?
Reid Harvey
Ceramiques d'Afrique d'Abidjan
la Cote d'Ivoire
Michael Banks on thu 15 apr 99
Hi Reid,
You've encountered a common problem with tropical earthenware clays. Clays
which produce weak earthenware are common in the wet tropics because of
acidic ground waters removing lime and other fluxing cations. But lime-rich
clays are quite common in temperate climate countries and are consequently
much stronger when fired. In SE and E Asia where limestone is abundant,
crushed lime is added to earthenware clays for improved strength.
Fine-ground limestone added to earthenware bodies does significantly improve
vitrification. The particle size should not exceed 100 microns (0.1mm), if
lime-popping is to be avoided. Earthenware bodies can be formulated with up
to 20% limestone or marl. A relevant eutectic composition is 20.9% CaO,
2.1% MgO, 14.0% Al2O3 and 63.0% SiO2 which fuses at 1185 degrees C. The
presence of feldspar lowers the fusion temperature further.
I suggest you try a few tests. You will note (from the above eutectic) that
a slightly dolomitic limestone is best.
Off the subject: I have a question for you. How do you test your
silver-impregnated water filters to find the optimum balance between high
flow rate and safe filtering?
Cheers,
Michael
Michael Banks,
Nelson,
New Zealand
>----------------------------Original message----------------------------
>Here in West Africa there is a big problem with the low strength of
>traditionally made, ceramic containers. (Snip)....It's my understanding
that as a high temperature flux CaO can be added to an earthenware body, in
amounts upto about 8.0%, as a way of greatly
>improving strength.
>Reid Harvey
>Ceramiques d'Afrique d'Abidjan
>la Cote d'Ivoire
>
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