The best all round book I have found on working with pewter is 'The
Techniques of Pewtersmithing' by Charles Hull & Jack Murrell,
The techniques of pewtersmithing
By Charles Hull
Manufacturer : B.T. Batsford
Release data : 1984
In the UK we have a Worshipful Company of Pewterers,
www.pewterers.org.uk They are a sort of guild and may be some help.
I do have some recipes for colouring pewter: For black patina, add
some copper filings or thin copper wire to around 10% nitric acid 90%
water. For dark gray patina, use around 10% sulphuric acid 2% nitric
and 88% water. Always add acid to water, (I'm sure you knew that)
and you can use the nitric-copper mix for etching the pewter too.
Hope this helps, Adrian