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I went to Amazon and bought this three pound capacity rock tumbler by Thumlers. It came with polishing grits, but I judged them to be too fine for what I was wanting to accomplish. I just tossed in some sand and some rocks along with the buckles, added water, and tumbled it for 20 hours. It worked beautifully.
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To make the buckles look a bit beat up, I took a ball peen hammer to them (on a steel plate), giving them flat spots and dents and dings.
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Next, to darken the soft gold color, and to add a little green patina, I gave the buckles a vinegar bath which just speeds up their natural oxidation. I dunked them three times, allowing them to dry in between. Each time they developed more color.
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And here is the result. On the left is the original buckle as I bought it from Tandy's Leather. Next is after tumbling. The third one is after hammering. And on the right is the final look, after the vinegar bath.
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Here, side by side, I present the buckle transformation showing the before and after.