[ATM] Dog Biscuit

[mdholm at telerama.com (Mark Holm)]

>
>
>What causes dog-biscuit?
>
I am not an expert, but do know some causes:

    Uneven drag/velocity during polishing strokes.  The most common 
description of this is saying that the lap is "grabby".  I had a bad 
case of this on my last mirror.  Microfaceting with nylon net solved it 
that time.

    Polishing stroke velocity too high.  At least I have read this one 
as a cause.  For most of us, a deliberate polishing pace is best , at 
least after the pits are gone and we are trying to get a good surface.

    Pitch too hard or perhaps too soft.  I think this (too hard) was the 
root of the problem I solved with microfaceting.

    Some lap materials are more likely to cause dog biscuit than good 
optical pitch.  Texerau has a Foucaultgram of a mirror polished on 
honeycomb foundation (beeswax) that makes a dog biscuit look quite 
smooth by comparison!  Of course some have produced good mirrors on 
HCF.  The pads that are a modern way to speed up polishing are usually 
found to produce a pretty lumpy surface, though some users have managed 
to avoid this.  The major reason that high quality pitch is still the 
favorite lap material, despite it's unpleasant qualities, its that it 
(reasonably often) produces a nice smooth surface.  The new Acculap 
brand ptich substitute, available from Salem Distributing may aleviate 
some of pitch's bad points while retaining the good.  Anyway, that is 
what their advertising claims.

>  Could an etching action during pressing have 
>something to do with it?
>
I have seen patterns that looked like they were etched.  There is 
certainly some chemical action in all polishing.  Different polishing 
agents vary in the amount of chemical action.  Cerium oxide is supposed 
to be fairly high in this regard.  I saw the patterns I thought might be 
due to etching while using cerium oxide and after long pressing 
periods.  When I switched to zirconium oxide, they went away.  Rouge, 
either red or balck is supposed to be fairly low in chemical action .  
(Black rouge rivals carbon black for the amount of mess it produces.  
Red rouge is certainly messier than cerium oxide, but much better than 
the black.  Zirconium oxide is even less messy than cerium oxide.)

>  How about the slight pause at the end of each 
>stroke when figuring by hand?
>

Probably not, since nearly everybody has that pause and most manage to 
conquer at least the worst of the dog biscuit.

Mark Holm
mdholm@telerama.com