Ouch Ozzy, those stones do look a bit beat up don't they.
I'll give you the best tips I know on getting chippy soft stuff like those quartzes to go from grind to shine without falling apart.
Always make sure you have a correctly filled barrel, meaning the correct proportions of smaller & larger stones filled to the correct level in the barrel. A typical batch going into prepolish may be about 50% pea or smaller sized stones and 50% cherry to chestnut sized and no more than one larger than a plumb.
Hmmm all of a sudden i'm feeling hungry now for some reason...?
Too many large stones and not enough smaller ones will lead to more aggressive interactions between the larger rocks as they can only hit each other and will have more "empty" space between them as they move. This is also why you don't want more than one larger size stone in a smaller barrel.
Tumbling should only impart enough energy to get the grit evenly distibuted through the load while it's running. Friction is the key element in tumbling, not force. The smaller stones are needed to fill in those gaps between larger stones and redirects some of that force gently.
Not having enough stones in the tumbler (less than 3/5 full in most cases) will give the stones a longer "run" as they tumble down, allowing them to build up more momentum which can also lead to impacts. Filling the barrel up between 3/5 to 2/3 for shaping and between 2/3 to 3/4 for polishing is the norm for most models. Over-filling the barrel is bad too because it reduces the tumbling space in the barrel to the point where it retards the abrasive action and will end up in a lot of extra rolling time and left-over grit.
Maintaining the proper rock load can be a bit tricky if your new to tumbling because even if you start with the correct amount of rock, grinding action will reduce the volume up to 65% of what you started with.
Keep all your "failures" in marked containers. Whenever you need some extra material to top up a load in progress you can always draw on your stash of "not quite good enough for stage 'X' " rocks to bail you out.
Barrel peed is another issue. Sometimes you have some amount of control over this and sometimes you don't. A stock tumbler and matching barrel "should" be turning at the correct RPM for the most part already but some can turn a bit too fast to be effective for blemish free polishing or a bit too slow to grind and shape effectively. It's a Goldilocks act unless you have a faster and a slower tumbler to transfer barrels between (like I do).
The type of rock you are tumbling also has to be considered. Fast formed crystaline quartzes will always tend to chip and fracture and make you wonder what you're doing wrong but they are also a good way to learn those better tumbling techniques.
When it comes down to investing your hard hours and dollars into making tumbled stone jewelery somewhere donw the line, it's good to have the confidence that comes with mastering chippy material like the infamous amethysts, citrines and semicrystaline quartzes and quartzites etc.
I am always recommending crushed bottle glass or beach glass (if you're coastal) as a good filler material that is cheap, easy to find, easy to sort out and to some degree is re-useable if you're not using it in carbide grit. Glass is usually always softer than the stones your tumbling and it's uniform in strength and consistency throughout (amorphous). Glass also sinks in water but it is not very dense so it tumbles fairly gently and cushions the rocks well.
In fact, I am polishing a batch of about a dozen pendants and preforms right now using pre-tumbled beach glass as the primary carrier and so far i haven't seen a mark on anything for the three weeks they've been rolling together.
It's still a good idea to use pellets even if you're using another filler material. Since pellets usually float they artificially increase the viscosity of the slurry which gets harder to do naturally after the carbide grit stages and they do work pretty good without compromising much abrasive action if used in the right proportion.
I hope all that rambling helps you somehow Ozzy.
Maybe you can tell us a bit more about exactly how you got the stones up to that point so we can offer you some more specific advice and ideas of what you can try to remedy the problem.
Cheers!
Rob
