Tuesday, January 31, 2017

How I Choose Slabs

 The Good, the Bad, and the Shattered: How I Choose Slabs

 .              You know that feeling when an almost finished cabochon shatters while you're finishing the polish stage? Me either . . . 

          Okay, maybe more than a few. And I've noticed that it usually happens late in the process and only to stones I'm fond of. The not-so-exciting stones are as strong as steel. Not every slab will make a beautiful cabochon, but every great cab starts with a good to excellent quality slab. Here are some ways I determine whether a particular slab is worth my time and effort.

1.     Check it Wet If it doesn’t have much of a  shine or luster wet, it won’t have it after it's finished (unless you want to shellac it--you can make even a sidewalk shine if you put enough shellac on it).

2.     Check it Dry - This is where the rock is going to tells me how easy or difficult it will be to work. I look for:
a.     Cracks Sometimes cracks are called ‘healed’ because the material has fused permanently where the rock originally split. These will be fine to work with and can offer some interesting design elements when I'm considering how to shape the cab. Other times they are still broken and I can usually feel them by sliding my fingernail across the break. In all likelihood, this slab will split along the crack and ruin the cab that had the crack going through it. If it’s not too bad a crack and you really like the damaged part for your cab, consider stabilizing it. Check out the stabilizing technique at http://www.mindat.org/article.php/1129/Methods+to+Stabilize+Material+for+Cutting  
    Otherwise, it is small material for my tumbler.😬 
b.     Cracks with fillings - Sometimes a crack will appear healed, but when I look at it closer, it has tiny bits of debris in it. I avoid using the cracked areas unless they are completely confined inside the cab shape and I'm using them for a design element. Otherwise, there’s a pretty good chance the cab will break very easily; probably while I'm still working on it.

Conglomerate with Pits

c.      Pits - It took me a few bad slab purchases to realize that I won't see the pits on a wet stone unless they're huge. So now I'll dry the slab before making a decision about giving it a good home (mine).  Holding it up by my eyes and looking across it will usually provide a good view of how sound the surface of the slab is. If there is pitting, assume the whole slab is pitted through and through—they can’t be polished out, because you’ll just get into another one!

3.     Check the texture - When I really want a cab to shine or have a great luster, the rule of thumb is the finer the crystal/grain structure, the better the polish. Of course, you can always take the easy way and wet it . . .

  The Wave

      Check for overall surface blemishes - I've found that a slab that has an area where the rock is a different texture is likely to mean that the two portions of the surfaces will not take a grinding or polish at the same rate. If the difference is significant, the softer portion will grind down faster than the harder portion. This makes it impossible to get a flawless smooth finish (at least at my skill level). If the blemish is an area of noticeably soft material say in an agate or jasper, I will usually be best to work the harder areas and discard the soft.  This little beauty keeps frustrating me. The blue coloring is surrounded by tiny quartz crystals--something I love to work with. However the brown band is a compacted mud and not nearly hard enough to do any significant work on. Any suggestions? And yes, that's my Snoopy ruler! 

5.     Check the edges - Sometimes a slab may have a different texture and/or color around the edges that were part of the outside of the rock. There have been times I've wanted to include that in the design, and nearly every time I have, the different-textured edges have flaked off while I'm working on it.Grrrrr . . .   

     Fatal Flaws - If I have doubts about the soundness of a slab, I tap it with a bit more than 'gentle' force. If it holds together for that, it will probably hold together when I cut, grind, and polish it. Although, even the best-looking slab will sometimes have a fatal flaw I couldn't see in advance. 
I          If a slab shatters into two or more pieces when it is tapped, that's where it would have failed while I'm working on it. On the other hand, I now know which parts are likely to be strong enough to cab!  

          TIP: Do not hit the slab on the table before you buy it unless you have the seller’s permission.  As the old saying goes, if you break it, you bought it!

     And, Check the back! It's incredibly frustrating to be listening to a chatty slab, and when you turn it over you see it has a bad undercut, missing material, or a fatal flaw on the back. It's more than frustrating to get home before you look at the back and discover the back of the portion of the slab you love, won't let you make the cabochon you planned. If the part of the slab you love is damaged goods, I've been able to negotiate a lower price based on the portion of the stone that's useless. 

          If you like what you see here, please go to the top of the right side column and subscribe. You'll get my blog automatically delivered to you every two weeks! Next time: VUGS, DRUZY AND LOST OPPORTUNITIES!

Your Lapidary Whisperer