Re: potters clay, composition

In the previous post, Steve said I mentioned:
>> I basically wonder about the similarities/differences among potters clay and
>> laterite. [1] I have heard that there are many types of potters clay,
each with
>> different ions (incl. metals). [2] Do you have any information on how to
>> a particular clay and what treatment if any is needed (e.g. short duration,
>> low temperature kiln firing)?

Steve made the point regarding [2], that firing would destroy one desireable
property of clay.

>For an existing tank, I'd mix the clay into small balls and dry them.
>You could assist the drying in an ordinary oven but definitely don't
>fire them in a kiln.<<

Since ultra fine clay particles have more surface area, they have great CEC.
But, this may not be the only property of clay/laterite. As Kelly suggested,
clay/laterite also may bind phosphate if the water is allowed to circulate
thru it. This may in fact be the MOST important property of laterite and
might not be eliminated with some modification to the particle size.Larger
particles would not have the same tendancy to cloud the water if disturbed.
Dupla laterite does not consist of ultrafine particles, as if it was
modified somehow; it settles out quickly , unlike untreated potters clay and
other 'commercial' laterites. (But I have never inspected clay/laterite from
the tropics first hand, so I do not really know what the original stuff
looks like)

>I am skeptical about the supposed superiority of laterite over other
>cheaper substrate schemes. 
>I should think that clay is clay except for Fe content.

While I (too) am not a big user of laterite (I tend to use soil or peat), I
am still curious about laterite and packaged alternatives, like potters
clay. Regarding my point [1] above, does anyone know how much chemical
variation there is in these potters clays (besides Fe). I am more concerned
about high concentrations of Cu, Zn, heavy metals and other elements that
might be in excess and even potentially toxic concentrations for plants/fish.


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