You can usually go to a stone yard (flagstone, concrete, etc.) and ask them if you can grab some small pieces of slate (I think it's typically called flagstone) they usually won't charge for it since it's just scrap pieces. Then wash them all down, and bake in an oven for 15 mins at 350F, let cool and rinse again. If you want to be sure it won't alter your PH
, soak in HOT water over night, take a reading of your tap water before you put the slate in, and again the next morning. if it doesn't change then it is perfect.
I have about 20 - 30 lbs of slate upstairs, I was trying to get some to stick together, but I couldn't find a good way of joining them, can safely rule out glue guns. perhaps silicone..
Stone yards are a great source for free/low cost rocks, just make sure you clean them all first.
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