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Why are my brass castings changing colour once tumbled?

Hi, I’m casting pieces of lace applique in yellow brass and love the rich warm yellow colour of the castings when they’re freshly casted. However, the colour changes to a lighter, not-so-rich tone when I tumble them with steel shot. I am only guessing that it is the tumbling process which affects the colour. Any suggestions of how I should approach finishing the cast lace pieces and retaining the colour? I seal the cast pieces with a clear lacquer to prevent/delay tarnishing but have been advised to have them gold plated. Any thoughts and advice will be appreciated.
Warm wishes from South Africa where summer is finally here!

Desiree- With brass or bronze, I’d use rock (ceramic) for your first round of tumbling, and oak for your finish round of tumbling. NOT steel shot. I don’t know the chemistry of the steel shot with bronze and brass, but I find it more effective to use oak.

Thank you so much for the advice. I was wondering if the steel shot was the reason for the change in metal colour. Are you saying I should tumble with wood oak? I haven’t seen wooden pellets in my jewellery supply shop in Cape Town. I’ll have to search online for a company which will shop to South Africa.