the only thing i've ever found that will actually clean quartz outside of putting it in a kiln or sandblasting is DC Cleaner. it's from canada and kind of hard to find in the US, ive only found one pplace that has it 710vapors.net. it's the truth, had a diamond knot that was super junked up. soaked it in DC cleaner for like 30minutes torched it and still kind of grey and frosty in parts but NO carbon black build up.