Obsidian & Apache Tears
(Applies to most soft stones and glass--Mohs 4 and 5)
When you receive your obsidian rough, it is usually in chunks too large for tumbling, and because it doesn’t have cracks or fissures, it cannot be broken up effectively with a hammer and chisel. So just get rid of the chisel, cover the rock with a towel and go after it with the hammer. I always wear long sleeves, gloves, safety glasses, and a face screen or face shield to protect myself from the flying particles. These “flyers” which accompany the breaking process come out at supersonic speeds and can cut to the bone. Cover up everything you value before you even start, and effectively protect your children from these flying particles.
As you break up the material, try to avoid using the small slivers and really thin pieces. They will disappear in a rotary tumbler, although you might try a few pieces in your vibratory tumbler to see if they survive Stage 1. I prefer to select the fatter “chunks” of obsidian so they can survive the tumbling process.
Note: If you are starting with “Apache Tears” (small round pieces of obsidian), you probably can skip the sizing process altogether.
Once you have enough sized material, rinse it well and start loading your tumbler.
A. Using a Rotary Tumbler
Loading a rotary barrel with obsidian is a little tricky. First, put a cup or two of obsidian or Apache Tear pieces in a salad bowl (yes you heard correctly!), then an equal amount of ceramic shapes (forget the plastic pellets--ceramic shapes are the “high-tech” media to tumble obsidian). Mix everything in the salad bowl and pour enough of the 50/50 mix into the barrel so it is exactly ¾ full (3/4 of the way between the bottom of barrel and the bottom of the seated lid)--Little Red Store sells AccuFill Templates to make sure you get it to the same level every time. It’s important to start out each new week with the barrel filled to the 75% mark--see the TechTip on making perfect slurries.
Now add the 80-Grit Silicon Carbide powder and finally, add water until it touches the bottom of the top layer of rocks. Stated another way, add water to about 1/8 inch below the top of the rocks. Fill the barrel the same way every time. Now put the top on the barrel, making sure that there is no grit on any of the mating parts which would cause it to leak. Immediately set the barrel on the rotating tumbler and leave it for a full week.
At the end of 7 days (168 hours) of uninterrupted tumbling, remove the barrel and pour the slurry and contents into a plastic colander suspended over a paint bucket--blast the slurry contents with water until the rocks, shapes and barrel are cleaned of all slurry.
Inspect your rocks--they will probably need at least 3 weeks in the 80-grit to get them nice & rounded, if you started with sharp obsidian. However, if you started with rounded Apache Tears, a week or two in Stage 1 may be all that is required.
Now refill your barrel with the clean washed rocks--add more ceramic shapes to bring the level to the ¾ point--then the appropriate grit--then water--finally the lid, as before, and fire it up for another week.
When the rocks are sufficiently rounded, look at the 80-grit scratches in the rocks with a 10-power magnifier so you know what they look like. Now proceed to Stage 2 (220-grit) for a week, and clean up as before. Now look at the rocks with the magnifier again--if you still see the 80-grit scratches, refill the barrel and proceed for another week in Stage 2. If you see only the smaller (220-grit) scratches, you can move on to Stage 3 (600-grit).
After a week in Stage 3, use your magnifier to view the scratch pattern again--if you still see the 220-grit scratches, plan to spend another week in Stage 3. If the only scratches you see are Stage 3 scratches, proceed to Stage 4 (1000-grit) for a week--then follow this same inspection procedure. When all you see are Stage 4 scratches, it’s time to proceed to the polish stage. Wash up really well, load the barrel properly, and run the load for 2 weeks continuously in Cerium Oxide polish. I always recommend a second 2-week continuous polish stage with Tin Oxide to get the very best polish the rocks were designed to give--you can tell you are done polishing when a dried rock looks just as shiny as a wet rock. Another method is to take a cleaned piece of obsidian from the batch and rub it vigorously on a Polish Stick which has been moistened and sprinkled with Cerium Oxide polish. When the un-rubbed portion of the obsidian looks as good as the rubbed portion, you have obtained the very best polish you can from the rock.
In summary, you let the rocks tell you when it is time to move from one stage to the next--by observing the scratch patterns in the rocks. Being human, most of us want to move on to the next stage before the rocks are really ready. We have to resist this urge to move on too soon!
Here’s a brief summary of the soft stone (and glass) rotary tumbling process:
Stage 1 (80-Grit) Purpose: To round edges & remove sharp corners
Stage 2 (220-grit) Purpose: To remove Stage 1 scratches
Stage 3 (600-grit) Purpose: To remove Stage 2 scratches
Stage 4 (1000-grit) Purpose: To remove Stage 3 scratches
Stage 5A (Cerium Oxide) Purpose: To remove Stage 4 scratches and achieve a high polish.
Stage 5B (Tin Oxide) Purpose: To improve the Polish and make sure the Polish is all the Rock can give.
Locating Problems
When someone calls the Little Red Store to tell me they can’t achieve a high polish on a rock known to take a high polish, I ask the following questions to help find the problem:
1. How many weeks did you spend in each stage? (This is an indication of overall operating procedures)
2. Did you load properly--3/4 full every time? (Indicates the efficiency of the tumbling process)
3. What did your “spent” slurries look like? (Similar to #2--helps identify how efficiently the rocks were tumbled)
4. How did you determine when to go to the next stage--did you look at the scratch pattern after every wash-up or did you “guess”? (Most people guess…)
5. Is it possible that your powders are contaminated? (Very common problem)
6. Is it possible that small holes in the rocks carried grit into the following stage? (Another common problem which causes no end of frustration)
7. What color is your Cerium Oxide polish? (If it’s not pink, they probably do not have Cerium Oxide Rock Polish.
Once the problem is located, the general rule is to go backward to the Stage before the problem occurred and go forward from there.
B. Using a Vibratory Tumbler
Start by placing your clean, rinsed, wet rocks in the empty, clean vibratory bowl (no additional water is added)--the level should be anywhere from ½ to ¾ full--you should have a 33-50% mixture of ceramic shapes & rocks in the bowl--you do not need to be exact in the loading of vibratory tumbling machines.
Creating the Required "Toroidal Motion"
Leaving the top of the bowl off, now start the tumbler and watch for the famous “toroidal” (or “donut”) motions which are peculiar to a vibratory tumbler. The rocks should be (1) rotating around the bowl horizontally while they are (2) also rotating down to the bottom on one side and back to the top on the other side. If this dual motion is not present, adjust the rock level higher or lower until you are able to observe these two energy-motions occurring simultaneously.
Now Create a Perfect Slurry
Once you have observed that the toroidal motion is present in your batch, it is time to sprinkle in a tablespoon of grit or polish. Wait 10-15 seconds and see if the rocks become coated with the slurry. If not, sprinkle in another tablespoon of powder and observe again. Keep following this procedure until the rocks become coated with a “thick pancake batter” of slurry. If you are using cerium oxide polish, the rocks will suddenly turn pink--when using silicon carbide grit, the rocks will suddenly all turn gray. You have achieved the perfect vibratory slurry. There should be NO standing water on the bottom of the bowl, and the rocks & shapes should be thickly coated with the slurry. Now you can fasten the top on the bowl and begin the Stage 1 (220 grit) process.
This Perfect Slurry Must be Re-created Often
Recheck the slurry in 4 hours--it will probably be pretty dry, due to the quantity of rock dust which has been created by the 220-grit powder and added to the slurry. As above, leave the top off, turn on the machine, verify for toroidal motion and add 2 or 3 squirts of clean water from a household sprayer. Watching the slurry carefully, add squirts of water sparingly until the slurry is perfect again. Now continue tumbling for another 4 hours (I use a 24-hour timer to stop the load at 4 hours--this prevents the slurry from drying out really bad). Keep checking the slurry every 4 hours and readjusting it until the rocks are round enough for your purposes.
Note: Sometimes, so much rock dust is created that the slurry cannot be readjusted properly. When this occurs, add a cup of water to the thick slurry and tumble for 5 or 10 minutes--then remove the bowl and pour the watery slurry and contents into a plastic colander suspended over a bucket--blast the colander contents with water until the rocks, shapes and bowl are cleaned of all slurry.
Inspect your rocks--they will probably need at least 8 hours in the 220-grit to get them nice & rounded, if you started with sharp obsidian. However, if you started with rounded Apache Tears (or if the rocks are coming from a rotary tumbler after the 80-grit stage), 4 hours in 220-grit might be all that is required.
Now refill your bowl with the clean washed rocks and ceramic shapes--add a handful of ceramic shapes for good measure--then make up another perfect slurry to finish Stage 1.
A Real Slick Tip
When you’re happy with your Stage 1 rounding results (the 220-grit stage), add a cup of water to the thick slurry and tumble for 5 or 10 minutes--then remove the bowl and pour the watery slurry and contents into a colander suspended over a bucket--blast the colander contents with water until the rocks, shapes and bowl are cleaned of all slurry. Now look at the 220-grit scratches in them with a 10-power magnifier so you know what they look like.
Stage One Surface Enhancement (Optional)
Inspect your rocks carefully--I like to grind out all the spalls (make sure there is a water spray during the grinding process), chipped areas, and holes at this point. Then I make up a new Stage 1 slurry and let them go another 2 hours, or until the grind marks disappear. This repair process will really make your end product look quite a lot better. Many people skip this process because it may take several extra hours--I suggest you try it and then decide if you should continue to do it.
Now refill your bowl with the clean washed rocks and ceramic shapes--add another handful of ceramic shapes for good measure--then make up a perfect slurry and proceed into Stage 2 (600-grit). Because the 600-grit powder is not making as much rock dust, you may find that you can go 6 or even 8 hours before you have to adjust the slurry.
Do your first scratch inspection at 4 hours, then again at 6 or 8 hours, depending on what you observe. If you can still see the 220-grit scratches, proceed with the 600-grit tumbling process. If the only scratches you see are Stage 2 scratches (600-grit), proceed to Stage 3 (1000-grit) for at least 4 hours--then follow this same inspection procedure. When all you see are Stage 4 scratches, it’s time to proceed to the polish stage. Wash up really well, load the bowl properly (this time with Cerium Oxide rock polish), and run the load for at least 8 hours before you perform your first inspection. I will generally run the Cerium Oxide polish stage for 24 hours and follow with a 24-hour Tin Oxide polish stage. You can tell you are done polishing when a dried rock looks just as shiny as a wet rock. Another method is to take a cleaned piece of obsidian from the batch and rub it vigorously on a Polish Stick which has been moistened and sprinkled with Cerium Oxide polish. When the un-rubbed portion of the obsidian looks as good as the rubbed portion, you have obtained the very best polish you can from the rock.
In summary, you let the rocks tell you when it is time to move from one stage to the next--by observing the scratch patterns in the rocks. Being human, most of us want to move on to the next stage before they are really ready. We have to resist this urge to move on too soon!
Here’s a brief summary of the soft stone (and glass) vibratory tumbling process:
Stage 1 (220-Grit) Purpose: To round edges & remove sharp corners
Stage 2 (600-grit) Purpose: To remove Stage 1 scratches
Stage 3 (1000-grit) Purpose: To remove Stage 2 scratches
Stage 4A (Cerium Oxide) Purpose: To remove Stage 3 scratches and achieve a high polish.
Stage 4B (Tin Oxide) Purpose: To Improve the Polish and Make Sure the Polish is all the Rock can give
The efficiency of the vibratory tumbling process to remove rock dust is pretty remarkable. Because each rock is fully coated with slurry, every one of the 3600 rubs/minute will cause the desired abrasion or polishing effect, whether the rock is at the top or the bottom, or somewhere in-between. The vibratory process is very fast ONLY when the slurry is perfect! Therefore, it is important to check the slurry often for proper consistency--check more often with coarse grit than with finer grits and polish.
If you would like more information about key procedures for rock tumbling, we advise that you pick up a copy of our book, Modern Rock Tumbling, which will make you the neighborhood expert on how to do it and why!