"Danny D." writes:

On Tue, 09 Apr 2013 07:54:14 -0400 dadiOH wrote:

The porcelain is glass. The only acid that will attack glass is
hydrofluoric

Is that really true?

Do others concur?

If so, that good news opens a whole realm of chemicals
we can use to clean the brown stains off the toilet.

Porcelain is mostly glass, but more important, the glaze
applied to porcelain is also mostly glass:

Ceramic glaze raw materials generally include silica, which will be
the main glass former. Various metal oxides, such as sodium, potassium
and calcium, act as a flux to lower the melting temperature. Alumina,
often derived from clay, stiffens the molten glaze to prevent it from
running off the piece. Colorants, such as iron oxide, copper carbonate
or cobalt carbonate, and sometimes opacifiers such as tin oxide or
zirconium oxide, are used to modify the visual appearance of the fired
glaze.

My experience with brown stains, is that regular toilet cleaners and
some scrubbing do the job. It took more than one session, but no heroic
actions were required.

Oh, also, yes, etching glass is done with hydrofluoric acid.

--
Dan Espen