🚀 go-pugleaf

HOW KNOWS METAFONT PRETTY WELL ?

I want to use a non-italic mu for the unit mykro-meter

I copied cmmi12.mf to mmmr12.mf  (my modern math roman)
in mmmr12.mf I changed slant:=0.25; to slant:=0;

I called metafont and typed
**\mode=localfont; mag=1; input mmmr12

I got the files
mmmr12.300gf  (generic font)
mmmr12.log    (...)
mmmr12.tfm    (tex font metric)
I ask myself *why* this worked. From where came all the
character-description ?

I called gftopk ./mmmr12.300gf and got
mmmr12.300pk

In Latex I wrote
  %\font\greekr = ./mmmr12 at 12pt
  \newfont{\greekr}{./mmmr12}  % greek roman
  \def \rmy {{\greekr \mu}} % roman mu

  This is very short: only 1 $\rmu$m
But it is still italic... :-(


What did I wrong ?
What can I do

thank you for *your* help,
Markus