Metal Casting Zone

Hi to all
Wounder if someone could put some light on a small problem.??
Did some more spin casting last week and when I was casting some items in a RTV mould that were very small in size , I wasn't getting the usual nice silver frosted surface on my metal castings as I normally do. Lead 42% tin 55% Antimony 3% mix ,
The items were masters for a new product line ,so wanted the best surface possible. Not to be . Temp up ,Temp down try this try that . No improvement. good enough castings but poor surface.(pit holes )Cleaned alloy, and removed dross from melting pot still not too good.

Have no fresh new alloy to add to my melting pot at present, so am woundering if all the runners that I return to the pot constantly has altered the alloy composition. Have read in my text book on spin casting that this can be a problem. Cleaning of the alloy and removing the doss can have an effort on the composition of a alloy, as the tin element can be removed with the dross when cleaning. If new metal is not added with the remelted sprues and defect castings, diffculties can arise.
Haven't had this problem before in 19 years of casting. could anyone enlighten me more on this area. If new metal has to be added to my 10KG melting pot ,what percentage of new alloy should be add to help the 6KG of metal already in the pot.??
Thanks JOHNCAST

I use type metal and have a machine called a Ludlow. The metal alloy for it and most other casters are listed around 12% Antimony, 5% tin and the rest is Lead. I never had any trouble with my machine and then one day it all went bad, bad surface, incomplete fill and what seemed like a lot of shrinkage. I was told the metal was depleated or was contaminated. I cast slugs of type and print with them and then melt them down again. I had to clean the pot out and use all new metal and the problem vanished.
The Wiki article has some info on depleation.

en.wikipedia.org/wiki/Type_metal

Thanks Jammer for your input . The link was interesting reading.