It's that latter part that is the rub. When 3M first started trying to
clone the wartime BASF tape, they tried several different grades of
gamma ferric oxide (the red kind of rust, ground up). They found that
the cheap grade sold for barn paint use, which had a wide mixture of
different particle sizes, gave best linearity.
The difference between that tape and, say, 3M type 111 from the 1960s
was very substantial. Much lower noise floor (partly because they learned
how to make smoother surfaces and partly because they got tight control
over particle size and shape) and much higher output level (again mostly
because of particle control but also because they learned to add some
additional oxides in there).
The HOLN tape that started coming out after 111 was a mixture of gamma
ferric oxide and other iron oxides, and that's when things started getting
sticky. You could easily make red-oxide tape slurry in your basement with
a ball mill but mixing HOLN types gets harder.
Modern tapes of the GP-9 variety are doped with all kinds of other magnetic
compounds, which might include chromium dioxide or cobalt, nickel, and
samarium oxides. There was some talk of using iron sulfides for a while.
It gets scarier.
If you need cheap red oxide tape, JAI in Bombay can ship you all you need.
But if you need Zonal 999 or BASF 911, you got trouble. The good news is
that all of this stuff is WAY easier than making quad videotape.
"C'est un Nagra. C'est suisse, et tres, tres precis."