I recently dug up some nice big pieces of obsidian and I’d like to polish them. Thing is they have a coating of dirt and grime on the outside that won’t wash off with just water and a sponge. Thought about using sandpaper and/or steel wool but don’t want to make things hard down the line for getting a fine polish. Searched the internet but everything was about the final polish and I didn’t find anything that addressed this early cleaning part of the process. Any suggestions out there?
Someone who knows how to work with obsidian wsill have to address your main points, but a suggestion: use nylon scrubbing devices such as Scrunge (a brand name nylon-faced sponge) to remove the caked-on dirt and grit. It is abrasive enough to cut through dirt/grime but of a lower hardness than glass or obsidian, so will not scratch the obsidian.
I wouldn’t worry too much about scratching obsidian. I would worry about cutting yourself on the edges.
Depending on the size of the stones, and their shape, a wet tumbler is a good bet. Be prepared to be patient, it can take a while.
Are you looking to end up with a display piece (jewelry) or a cutting instrument?
I sometimes work with obsidian, but have never tried making cabochons or polishing the stone unworked. I can ask some of my friends, but I’m guessing using a rock tumbler is going to be your best bed. If the stones are too large to fit in a tumbler I don’t think you will ever be able to polish them by hand.
do you have experience with a lapidary? first you take your lump of obsidian, your work, and cut a smooth plane on one side using a diamond or tungsten saw. then you smoothen that plane on the lapidary wheel using medium and then fine grit to remove the saw marks. then you glue that smooth side either with epoxy or canada balsam onto a glass disk or some other holding tool.
and then you can start cutting it further with a saw and smooth / shape it on the wheel. just my experience, obsidian is a very brittle glass, with lots of micro-cracks. don’t try to shape it too thin or slim. better into something rounded and continuous.