The first step is to get rid of the clay. For a few prize specimens, tools such as an old toothbrush and bamboo shish kebab skewers will do the job. If you have many to clean, however -- particularly if they are intricate formations -- leave them to dry in the shade until the clay cracks. Then hose them off hard with your spray nozzle set for as much pressure as your system will provide. Often you will need to repeat this step several times, allowing the clay to completely dry again between hosings.
Tip: Don't do this in your sink or bathtub! Clay will clog your pipes.
If your sample has organic material on it, like algae, use household bleach to clean it. Be sure to let it dry for a day before using any acid cleaning method.
Acid and Other Cleaning Methods
Here are some general safety rules for cleaning minerals with acid and other solutions:
- Only use chemicals that are clearly labeled and store them in a secure area away from pets and children.
- Keep a large container of clear water handy to wash off any chemical spills.
- ALWAYS wear safety goggles & rubber gloves.
- Don't work alone.
- Avoid splattering.
- Never pour water into acid -- always add acid to water.
- Keep an ample supply of baking soda handy for acid spills.
- Keep an ample supply of vinegar for alkali spills.
- Rinse specimens well.
Getting Rid of Iron Stains
A common disfiguration of quartz crystals is a rusty staining caused by iron. Soaking the crystals in oxalic acid is the usual way to remedy this. Oxalic acid powder -- sometimes called wood bleach -- is available at rock shops, drug stores (though this may be an expensive way to go), and cleaning supply stores.
Start by putting your specimens in a plastic bucket after cleaning all the clay off them (clay keeps the acid from doing its job). Cover them with distilled water and add the oxalic acid powder. Follow the directions on the package if available. If not, you may have to try several different concentrations of solution for different soaking periods, depending on how stained your specimens are. You can start by adding 1/2 to 1 cup of oxalic acid crystals to 1/2 gallon of water in the bucket. For larger quantities, use about 16 ounces of oxalic acid to 2 gallons or more of water. Too much oxalic acid can turn quartz yellow, so if you have light staining, only use about 2 tablespoons of oxalic acid per 1/2 gallon of water. Always remember to add acid to water, not water to acid! Let your mineral specimens soak for one to several days, or, if you're in a hurry (or the staining is very heavy), you can heat the solution by standing the bucket in the hot sun. Some people use an old crock pot instead of a plastic bucket, and heat their acid solution that way. Don't heat this solution on your kitchen stove, and don't use any kind of metal container. Do this procedure outside, particularly if you heat (never boil) the acid, as it puts off poisonous fumes. Oxalic acid is mild as acids go, but rubber gloves are a good idea when handling either the acid solution or your specimens until they are well rinsed.
If the crystals start to develop a powdery coating as they dry, soak them in a baking soda solution -- about 1/3 cup to a gallon of water -- and then rinse them well. If they get a yellowish stain or coating during the process, soak the specimens in water for up to a week. For your next batch, try using a weaker solution of acid and stir the solution frequently.
The oxalic acid solution can be reused many times by just adding a little water and powdered acid each time. When it turns a deep green, you need a new batch. Neutralize the worn out solution by adding baking soda or agricultural lime (not unslaked lime from a building supply store!) until the mixture stops fizzing. Then you can pour it down the drain or out on the ground.
Other methods of dealing with iron stains use Iron-out or Naval Jelly. Iron-out is a commercial bisulfate cleaner meant for getting iron stains out of kitchen and bathroom fixtures. Add about 1 tablespoon of Iron Out to a pint of warm water in a plastic bucket or container. Soak your crystal clusters in the Iron Out solution for 5-20 minutes, up to a maximum of 1-2 hours. When done, pour the liquid down the drain and thoroughly rinse your mineral specimens. Naval Jelly, which contains phosphoric acid, is sold for getting rust off metal, but it works on quartz crystals as well. Just paint it on and then wash it off.
Other Cleaning Challenges
If your quartz crystals are encrusted with calcite, barite, or lime carbonates, you can try cleaning them with ordinary household vinegar and washing ammonia. You'll want to soak them for 8-12 hours in full-strength vinegar. Wash the crystals well, and then soak them for the same amount of time in washing ammonia. Rinse them well and wipe dry. If this helps remove the coating, but doesn't finish the job, you can try repeating the process a couple of times.
For very resistant calcite encrustations on quartz, or for limonite or some other tough mineral coatings, you may have to resort to washing your samples in a muriatic (dilute hydrochloric) acid solution. CAUTION: If a specimen contains pyrite crystals, try some other method! The muriatic acid will damage pyrite (and of course calcite crystals should never be treated by this process).
Muriatic acid is sold as a concrete and pool cleaner, so it isn't hard to get. It is, however, much stronger than simple vinegar, or even oxalic acid, and must be used with great care. You'll need rubber gloves, goggles, large boxes of baking soda, several buckets, and a secure outdoor space to work in.
In the first bucket put your crystals that need cleaning. Fill a second bucket with water for rinsing, a third and fourth bucket with a strong baking soda solution, and set a fifth, empty bucket handy for draining the muriatic solution into when you need to check the progress of the cleaning.
Whether you dilute the muriatic acid is your choice. Some people use it straight from the container and get good, quick results. If you DO dilute it, however, remember to add the acid to the water and not the other way around.
Make sure that the specimens in your cleaning bucket are dry. Using your gloves and goggles, pour enough muriatic acid over the crystals to completely cover them. Be careful not to breathe the fumes from the acid! If you are cleaning calcite or another alkaline mineral from the specimens, the solution will fizz wildly. In most cases you can leave the solution to work until the fizzing stops. If you are cleaning clusters of quartz crystals, however, you may want to check the specimens after five minutes. Sometimes quartz clusters are held together by calcite, and you might end up with a bunch of loose crystals.