Phooey! My Stones Didn't Take a Nice Polish!
Gritty Rocks: This is probably the number one cause of a poor polish. Rocks such as sandstone, incompletely silicified jasper, weathered material, - and many others - will shed particles as they tumble. These hard particles will be like adding coarse pieces of "grit" to the fine grit and polishing steps. Those pieces of "grit" will ruin the polish of every stone in the barrel. If you are tumbling a barrel of "mixed rough" or rough that was collected and is now being tumbled for the first time, suspect "gritty rocks" as a probable cause of a poor polish.
The Ingredients of a Great Polish
Tumbled stones with a fantastic polish do not happen by chance. They are produced by very careful and deliberate actions by the person who does the tumbling. Producing a great polish requires the five ingredients below. If any one of these is missing, a poor polish will probably be produced.
- Quality rough materials
- Proper rough preparation
- Cleanliness in tumbling
- Proper sizing and cushioning
- Quality abrasives and media
Quality Rough Materials
Most rocks are not suitable for tumbling. Rocks that work well in a rock tumbler usually have a smooth, uniform texture that is free of fractures, voids and soft spots. They are also hard enough to accept a bright polish and tough enough to survive the tumbling process undamged..
One of the most common problems encountered with tumbling is a person trying to tumble rocks that are impossible or very difficult to polish. Here are some of the problem rocks.
Some rocks have voids or cavities that cause problems. When polished, these voids are unsightly blemishes on the rock. The voids and cavities can also trap pieces of coarse grit that are difficult to wash out, and they can contribute gritty particles to the polishing step. This piece of agatized coral has grit trapped in the voids.
This piece of red garnet has a large number of soft inclusions. The included mineral wears away, leaving a polished surface with numerous ugly pits. Large garnets often have an enormous amount of included material.
Rocks that shed particles are a huge problem in a rock tumbler.
Add one gritty rock to a tumbler barrel and it is unlikely that you will get one nicely-polished stone out of that entire batch. As that gritty rock is tumbled, the impacts in the barrel will dislodge small particles. Those particles will be like pieces of coarse grit in the polishing step. Inspect your rocks before tumbling and remove any that have a "gritty" texture. Discard them.
Gneiss, schist, and quartzite that are not highly metamorphosed are also problem rocks in a tumbler. Weathered rocks always perform poorly.
Tip: If you can rub two rocks together and produce tiny grains, then the rock that's shedding the grains should not be placed in a tumbler. It will ruin the polish of every rock in the barrel! Don't be wimpy. Rub these rocks together vigorously to test them. Look for tiny particles being dislodged.
Tip: When you finish the polishing step, remove the rocks from the barrel by hand and allow the polish slurry to remain in the barrel. Allow it to settle for a few minutes. Then probe the bottom of the barrel to see if any "gritty" particles settled out.
ROCKS WITH VOIDS:
Rocks with small cavities or pores usually do not perform well in a tumbler. Pieces of coarse grit will enter those voids and not make their way out until you have them in the polishing step. An inspection by eye or with a hand lens can reveal these voids and the presence of contamination.
ROCKS WITH INCLUSIONS:
Inclusions are tiny pieces of foreign mineral material in a rock. If these inclusions have a different hardness than the rock, a poor polish will usually result. If the inclusions are softer they will wear away in the tumbler, producing pits and cavities in the polished stone. If the inclusions are harder than the surrounding rock they will be liberated during the tumble and act like coarse grit, scratching up every other rock in the barrel.
Garnet and corundum are frequently heavily included and as a result can be very difficult to polish. A lot of "rough ruby" (red gem-quality corundum) and "rough sapphire" (gem-quality corundum of any color except ruby red) is sold for tumbling. We have tumbled these several times and have found that they are usually so heavily included that nice tumbled stones are impossible to produce. They are also prone to "parting" (a phenomenon similar to cleavage) and break up or develop separation fractures that cause a "stepped finish". (When these rocks are called "ruby" or "sapphire" that is generally a misnomer because they don't have the quality needed to merit the name. They are simply common corundum.)
HIGHLY FRACTURED ROCKS
Rocks with a lot of fractures are problematic in a tumbler. The tumbling action in the barrel often breaks them into tiny pieces. If they break during the fine grit or polishing step, those sharp edges can scratch up every other rock in the barrel. The fractures can also be places for coarse grit to hide and then escape during the polishing step to produce scratches.
MIXED BATCHES OF ROUGH
One of the most common pieces of advice about tumbling is "tumble one type of rock at a time". People often write to us after they have tried to tumble a batch of rocks that contains stones that they collected from many different places and complain about getting a poor polish. In that situation, any one of the rocks in the barrel could be crumbling or shedding particles and those particles are scratching every other rock in the barrel.
For that reason, it is best to tumble one type of rock at a time - then if you have problems, you don't have to test every rock in the barrel to see if it was the one shedding particles. If you don't have enough of one type of rock for a full barrel, then try a smaller barrel or use media (or some proven material) to bring the barrel up to good operating capacity.
Rocks with fractures often take a nice polish, but the fractures cause them to be unsightly. Rocks that are broken by a crusher or by hand with a chisel generally have fewer fractures than rocks that are smashed with a hammer. (Always wear long sleeves, long pants, gloves and eye protection when breaking rocks. Anyone nearby should also wear protection.)
Proper Rough Preparation
Some people think that a tumbler is a great place for the rejected rocks from all of their other lapidary activities. The rule to remember is: "Garbage in means garbage out."
The best tool for preparing rough rock for your tumbler is a garbage can. Before you tumble a batch of rock, look through the rocks and remove any that are going to cause problems. Remove rocks that might shed particles and throw them away. Remove pieces with fractures, voids and other cavities that might offer hiding places for grit particles. These should be thrown away or broken to remove the parts that can cause problems. It's better to throw a few rocks away than ruin a month of work and the cost of grit and polish.
The secret to producing nice tumbled stones is being "picky."
Cleanliness in Tumbling
After you finish tumbling a batch of rocks in coarse grit your most important job is cleaning the rocks, media, barrel and lid. Every particle of coarse grit must be removed or it will cause scratching in the next tumbling step.
Tip: We have a toothbrush to scrub the edges and corners of tumbler barrels and rocks that might have attached grit.
It's also important to keep your grit and polish free from contamination. Be sure to wash and dry your spoon before using it to scoop out grit. Coarse particles can get into fine grit or polish if you don't keep your tools clean. Replace the lid on grit and polish as soon as you are finished with them to avoid contamination.
Tip: Before you open the container of polish it's not a bad idea to wipe off the lid or the bag to reduce the chance of contamination. (Steve Hart, the author of Modern Rock Tumbling, stores his grit and polish on two shelves. The grit is kept on the bottom shelf so particles of grit will not fall onto the polish containers.) Think about how you can reduce contamination problems where you do your work.
You need a variety of particle sizes in the barrel to get a great polish.
When using ceramic media, always be sure that it has been rinsed clean. Also, when running the polishing step, be sure that the media is extra clean and its most recent use was in fine grit or polish. New media must be run in fine grit before it is used in polish to remove any sharp edges that might remain from the manufacturing process.
Proper Sizing and Cushioning
A properly-loaded rock tumbler will contain a range of particle sizes. Large pieces of rock will have small pieces of rock in the spaces between them. This is very important for grit delivery and cushioning.
If you have a tumbler barrel filled with a few large rocks (like Figure 1A) there will be very few points of contact between the rocks. In that situation, all of the impact forces produced in the tumbler must be absorbed by a small number of rock-to-rock contacts. All of that concentrated force can lead to chipping and breaking.
If you place small rocks in the barrel with large rocks (like Figure 1B) then there will be many more rock-to-rock contacts to share the impact force. The small rocks absorb impact forces and cushion the tumble. If you don't have small rocks you can use small ceramic cylinders to fill those spaces.
Some materials such as obsidian, Apache tears and glass are easily "bruised" in the tumbler. Bruises are small fractures or chips produced by rock-to-rock impacts in the barrel. Some people add plastic pellets to their barrel to cushion the impacts. We have great success polishing glass, obsidian and Apache tears when they are "swimming" in small ceramic cylinders. "Swimming" means that at least 50% of the load is small cylinders. A lot of small particles in the barrel with your delicate materials supports them as they tumble and softens impacts. We have also found that a vibratory tumbler is gentler on these types of materials than a rotary tumbler.
Obsidian and quartz often reveal when they are not being cushioned properly in the tumbler. They will develop white marks on their tips and edges. These white marks are actually a concentration of microfractures in the rock that develop as the rocks are hammered against one another in the tumbler.
Adding more small material or ceramic media will often solve this problem. In a rotary tumbler a barrel that does not have enough rocks can be the culprit. The rocks get tossed around when your barrel is not up to proper capacity. In a vibratory tumbler, having the barrel too full can prevent a smooth circulation in the bowl. When the circulation is sluggish the rocks at the top of the bowl can hammer against one another at a rate of 3600 strikes per minute!
Using quality abrasives (grit and polish) of the proper size and in the proper sequence is one of the secrets to great tumbling.
A large number of rock-to-rock contacts also produces more grinding and polishing. Each of those contact points is a location where grit can be caught between the rocks to produce a grinding or polishing action. Very little grinding and polishing will be done in a barrel with a few large rocks (such as Figure 1A). Much more grinding and polishing will occur when the large rocks have small particles filling the spaces between them (such as Figure 1B).
Quality Abrasives and Media
Using quality abrasives of the proper size is very important for success in tumbling. We use three grit sizes followed by a polish and have great results. We begin with coarse grit silicon carbide (60/90), followed by medium grit (150/220), followed by fine grit (500), and finally a polish (TXP aluminum oxide). In a rotary tumbler we run each of these grit steps for seven days. The only exception is that we often run coarse grit for two, three or four weeks to obtain rocks with a nice rounded shape. The polish is always run for seven days and often followed by a short tumble in soapy water, known as burnishing, to remove any film that built up on the rocks during the polishing step.
Most of the people who sell rock tumbling grit and polish provide quality products. However, there are some sub-standard products being sold. They could be poorly separated, contaminated or mislabeled, at any point between the factory and end consumer. They might have been imported from a company that is cutting corners or using malfunctioning equipment. They could have been inadvertently contaminated by someone who placed a spoon with particles of medium grit into the polish container. Everyone makes occasional mistakes.
Media can be a source of contamination if it is not cleaned thoroughly, kept in an open container, or used without being propertly broken-in. We use an abrasive-free media that is rinsed clean before use. And, we never use media in the polishing step unless it was previously used in 500 or 600 grit followed by a thorough cleaning. Brand new media may have sharp edges that can produce scratches during the polishing step.
Few things are more rewarding than producing a batch of beautifully polished stones!
One Last Tip: Don't Overpolish
Sometimes folks think that if a week in polish is good then a month will be even better. That is absolutely not the case. When you run rocks in the polishing step there will probably be a few contaminating grains in there no matter how clean and tidy you work. The longer you run the polishing step the more scratches these grains will make. There might also be a few grains liberated from the rocks in the barrel. So, run the polish long enough to bring out a luster and stop. If you have doubts, pick out a few stones, wash them and save them for reference. Then run a couple more days, open the barrel, pull out a few stones and compare.
Generally, one week of polishing in a rotary tumbler is adequate. Two days in a vibratory is usually more than enough. The only exception to these is when you are tumbling really hard rocks. Rocks with a Mohs hardness over seven can require more than one week. Then a little more time makes a difference. A lot of time will make a difference in the wrong direction. So, resist trying to do too good of a job.
Correcting a Bad Polish
If you have a batch of rocks that receive a bad polish, the best thing to do is to examine them one-by-one to see if you can identify the problem. Are gritty stones shedding grains during the polishing step? Are broken stones scratching every other stone in the barrel? If these are happening you should be able to see scratches on the surface of your tumbled stones (use a magnifying glass for a good look). If this has occurred you can probably go back to the medium grit step (about 150/220) and rerun for a few days, then move on to fine grit and polish as normal.
If your stones are just not shiny, there might be a fog on your stones caused by hard water or a tenacious polish. Check out our article on burnishing tumbled stones to reveal the brilliant polish.
If your stones don't have a bright polish and you can see tiny scratches with a magnifying glass then you might have contaminated grit or rocks that are shedding tiny particles during the polishing step. Any rocks that will produce grains when you hold one in each hand and rub them vigorously together are not good rocks for rock tumbling. They will shed particles during the polishing step. Toss them out or use them on your driveway or landscaping. If you obtained an open bag of grit or polish at a yard sale, or have an open bag that has been sitting around your house for years, toss it out because it might be contaminated. Spilling a few grains of grit into your polish or dipping into it with an unwashed spoon can contaminate the entire container. Do everything possible to keep your grit and polish uncontaminated.
If you follow the tips above you should be rewarded with superbly polished stones. Try to diagnose any problems that you see. Most problems can be solved by simply being careful and not tumbling rocks that might produce questionable results.