From an engineering stand point I think Wally is correct about the
brass hardness.  But, I do know that annealed brass (soft temper) is
not suitable because the tongue will not keep a nice curve for very
long.  It will flatten out.  Hard brass is difficult to work with
because the correct amount of curve needs to be set into the tongue.
Since this is done by hand (there are less than a handful of machines
to do this), it is frustrating and taxing to get a repeatable correct
curve.

Therefore, the brass I have used is known as "half hard" brass.  The
material can be worked (curved) with a moderate amount of energy and is
reversible (correctable), if curved too much or incorrectly.  The
thickness of the material depends on the amount of air pressure to be
used.  Heavier pressure usually means thicker material and vice versa.
A .018 to .005 thickness has been used on a lot of reed pipes I have
worked on.  Pressures are between three and twelve inches water column.

Though I am very much interested in band organs and other mechanical
organs, the majority of my work has been with church-type reed pipes.
Regarding Rockwell hardness scales, I do not know how this translates
but there must be a cross-reference somewhere.  Are "temper" and
"hardness" interchangeable terms?  I don't think so, but cannot prove
it at this point.

Each tongue has to be curved for the particular pipe (pitch), which
takes into account the power desired from the pipe and somewhat the
timbre (dark or bright quality).  It is obvious that a pipe of the same
note name two octaves higher will not have as much curve as its lower
octave note.  If it were curved the same amount, the smaller pipe would
not speak, because the air pressure must first close the tongue against
the shallot to start the vibrating process.

It can be done so don't give up.  Checking the air pressure of a
particular organ and then checking the thickness and dimensions of the
reed tongues would be the best place to start.

George Cooper