After a lot of trials and experiments with no conclusive result I just 
throw away my Mayer's Hämalaun and used fresh one. Because I was in a big 
hurry I forgot to add the recommended drops of ammonia - and no more 
"blueing trees"! So I made one more experiment and added a couple of 
drops of ammonia : and there they are again. So, I conclude from this, 
that the concentration of the ammonia is critical regarding precipitates 
on tissue sections and that probably my understanding of adding "some 
drops" as recommended was incorrect and I added too many drops?

Thank you all for your help and especially John for the comments on 
"Haema-what".

Heike Grabsch, Germany