What Is The Best Lap For Polishing Sapphire?


Polishing sapphire gemstones can have amazing results. It can also be tough on laps. “Star Sapphire” by Sheila Sund is licensed under CC By 2.0
Polishing sapphire gemstones can have amazing results. It can also be tough on laps. “Star Sapphire” by Sheila Sund is licensed under CC By 2.0

Question

I’ve been working on a fair amount of sapphire lately. I’m finding that polishing sapphire wears out laps fast. I’ve received many recommendations: cast iron, ceramic, tin/lead, Fast and Last Laps, BATT, etc. (I learned to facet with Ultra Laps). I’m just a hobbyist, so speed isn’t important. I just want good results without spending too much money. Can someone explain the difference between these laps? What’s my best option for polishing sapphire?

Thanks,

Howard

An Overview Of Laps And Recommendations For Polishing Sapphire

All the laps you mention have their advocates. There are people who will swear “x” is the best. First, here are some of the differences between these laps.

  • For a cheap cutting or pre-polish lap, a hand-charged copper lap is excellent. Grits from about 3,000 up can be charged on the lap by rubbing them with a CLEAN finger dipped in oil or even lipstick. If you can get some thin copper scraps, you can use them on a master lap. Be careful not to gouge the copper with a facet edge. The copper layer might strip off. You might try polishing with a copper lap, too.
  • Ceramic and cast iron laps work about the same. However, some people can’t get them charged right and get nothing but scratches. The secret seems to be spraying the lap lightly and then getting most of the diamond and medium off the lap before polishing. Unless you just happen to find a piece of cast iron somewhere, both of these are somewhat expensive. Ceramic and cast iron aren’t recommended for stones below around 8 in hardness.
  • Fast Lap and Last Lap are made of a pelletized resin. They can be charged like ceramic laps but are evidently less prone to scratching. You can use them for polishing more gem species, too.
  • The same is true of tin laps. However, they’re also a bit pricey, tend to creep and get out of true, and develop soft spots. A tin/lead lap is a better option.
  • The BATT lap is the ultimate development in this series. It’s a great deal harder than tin or tin/lead and gives you flatter facets. Some very good faceters swear by BATT laps and diamond for most stones, even quartz. Again, it’s a little pricey, but you’re getting a practically universal lap in one purchase.

As for polishing sapphire, some people polish directly from 600 grit cutting. Others will pre-polish with 3,000 to 8,000 to even 14,000 and then go on to either 50K or 100K. (One ceramic user I know swears by a Falcon ceramic lap and Graves 200K spray). I would advise you to try it both ways. If you run into orange peel which won’t go away, it could be you have damage under the surface from coarse grits that wasn’t removed by the finer ones.

Here are three inexpensive options for polishing sapphire.

  • Use a copper lap with 8,000 and 14,000 diamond spray. Copper holds the diamond well.
  • Try using a specially prepared compact disc. This is a technique used by Gerald Wykoff, CSM GG. For polishing sapphire, you’ll need an old CD, some PVC adhesive, and diamond powder. Spread the diamond powder on a flat, hard surface. Get some PVC adhesive on its ball-like applicator and roll it on the diamond powder quickly. Next, roller paint either surface of the CD. You can makes hundreds of these diamond polishing laps very cheaply.
  • Use a Corian lap. If you know someone who has an old countertop who will cut one for you, great. It may not need to be resurfaced flat. (However, if you have access to a lathe, surface it flat). A Corian lap works like a ceramic lap but it’s not as finicky (and not quite as flat).

One of these inexpensive methods will probably produce a pretty good polish on sapphire. You’ll probably get the best polish from BATT, ceramic, or cast iron laps. However, there might not be enough difference in results or time expended to justify buying one.

Hope this is helpful,

Roy Kersey

“Untitled,” yellow sapphire, by Christina Rutz is licensed under CC By 2.0
“Untitled,” yellow sapphire, by Christina Rutz is licensed under CC By 2.0

Tips For Polishing Sapphire

I’m a single stone custom cutter. Turnaround time is an important factor for me but not as important as quality. I have some advice that might help you resolve some of your polishing issues.

Pre-polish technique is most important. Starting with 1600 diamond and 3000 to 8000 diamond for sapphire will speed up the polishing process and yield a better final polish.

Due to the nature of the beast (directional hardness, orange peeling effect, cutting orientation, etc.), the two most important laps to have for polishing sapphire are a tin/lead and a ceramic lap. Don’t charge either one of them with any diamond bort. Instead, use the diamond Ultra Laps 14,000, 50,000, 100,000 placed on the tin/lead or ceramic lap. This way, you save money, the chance of contamination is nil, and you’ll still be able use these two laps on softer materials without scratching them.

Ron Campbell, Central Coast Gem Lab

Use The Right Amount Of Bort For Polishing Sapphire

I start out with crystallite diamond-impregnated steel laps in 100, 320, 600, and 1,200 grits as appropriate for the specific stone. Once I get to the pre-polish stage, I use 8,000 grit bort on copper. This is the MAGIC lap for me. I absolutely love the thing. As with all bort, the key is not to use too much diamond. How much is too much? I charge the lap by using as much bort as will stick to my finger when I upend the vial and then turn it back upright. I use common olive oil as the lubricant, just enough to “swirl” the bort evenly over the lap. When I have done this properly, the copper looks black from the bort and oil.

This is a fantastic pre-polish lap for stones from peridot to corundum. I use 50,000 grit diamond spray on a ceramic lap for polishing sapphire. Ceramic takes some getting used to. The secret to that is simple: not too much diamond and go S-L-O-W. Nothing else produces a finish like diamond on ceramic with the harder stones.

Good luck!

J. Sean Keane

“2009 04 19 - 4690 - Washington DC - Natural History Museum - Logan Sapphire” by thisisbossi CC By-SA 2.0
“2009 04 19 – 4690 – Washington DC – Natural History Museum – Logan Sapphire” by thisisbossi is licensed under CC By-SA 2.0