Quartz Crystals:
How to clean them

Some Basics

Mineral Identification
Collecting Tools, and How to Use Them


Introduction to Geology
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The Geologic Time Scale
Stories Fossils Tell
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The Ouachita Mountains
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Quartz Crystals

Introduction to Quartz
Digging Quartz Crystal
Cleaning Quartz Crystal
What's it Worth?
Types of Quartz
Geology and Mineralogy
Quartz as Gems
Experiments You Can Do

Other Collectable Minerals


Managing a Collection

Making Your Collection the Best
Cleaning Minerals
What to do...

Minerals Special to Arkansas

Some are New to Science


No Gold in Arkansas


Cleaning Quartz Crystals

Quartz cleaning tips and custom-made quartz cookers

by J. Michael Howard

How to start Now that you have collected some nice specimens, and because they are still dirty, you need to know how to clean them. If you have just a few pieces, use an old toothbrush to get the clay off. Otherwise, start by building a 2 by 2 foot framed 0.25 inch mesh screen. You can get this type of screen, called hardware cloth, at your local hardware or farm and garden store. Remove the newspaper wrapping and let the specimens dry on the screen for a couple of days. Keep everything in the shade to prevent the crystal from heating up too rapidly in direct sunlight. When the clay is well cracked and dried, rinse with a garden hose. Let dry a couple of days and repeat the cycle.

Removing the iron
     Then you are ready for the next step. If the crystal only has a very light iron staining, then a few days soaking in a weak oxalic acid solution in a covered plastic bucket will remove it. If the iron staining is heavy, then we must cook the quartz in an acid solution. People have many ways to clean quartz, all involving basically the same scenario. Your specimens may be coated by iron or manganese oxides, with or without clay and you wish to remove the staining so the specimens are as clean as possible.

Use with care     
The most commonly used, readily accessible chemical for cleaning quartz is oxalic acid, which may be bought as a white crystalline powder. It may be purchased from many mineral dealers in Arkansas, especially those who specialize in quartz. When mixed with water at a measure of a few ounces per gallon and then heated to just below a boil, oxalic acid is capable of removing all but the most stubborn iron staining. It is a weak organic acid, but don't kid yourself, it will hurt you especially if you breath the fumes. So use it only outside in a protected and well vented area, where no children may gain access.   Some key points     
There are several key points to preparing quartz to be cleaned. First, you need to remove the clay. This is accomplished by cycling the specimens through the several wet and dry periods to loosen and wash the clay away. You may have to use a pressure washer to remove the last of the clay. Remember: this first step is critical.
     You do not want to have to clean the material several times, and that is exactly what you will have to do if the clay is not completely removed before the first acid cleaning. Trim your specimens to the size and shape you want before cleaning in acid. Remove the dinged or broken portions. This action will save the time of re-cleaning after trimming and the cost of the additional acid used to clean what you trimmed off and threw away.
     Once you have the specimens prepared for acid treatment, then you must consider the situation. Do you have small specimens and just a gallon or two of crystals or do you have some big pieces? Maybe you got lucky and have one piece that would fill a 5-gallon plastic bucket!

Tips for using oxalic acid
     Don't use expensive reagent or chemical grade oxalic from the pharmacy. Instead, when you visit Arkansas, ask at the crystal shops around Mount Ida or Jessieville. Many dealers provide written instructions and sell it by the pound or multi-pound package for around $3 per pound or $2.50 per pound for 5 pounds. How much do you need to buy? It depends. Did you get a 5 gallon bucket of quartz specimens or a 3/4 ton truck bed full? A five gallon bucket might take 1 to 1.5 pounds of acid to 5 gallons of water if the crystal is really dirty.
     Start with a weaker solution first and build its strength if you need to. Also, the acid solution will not be used up until it turns dark emerald green by becoming saturated with the removed iron, so you can reuse it by adding a very small amount of fresh water and powdered acid to the old solution. Remember: it is an acid, though a relatively weak one. Do not leave this stuff where kids or animals might get into it. I usually wear dish washing gloves when working around it even though it will not burn skin, it will let you know if you have a scratch or cut, by burning you. Take care not to get any acid in your eyes. I always keep a garden hose handy so when I get a splash, I can rinse it off immediately. Accidents happen!
Experiences with Cookers
     To clean small pieces, you need to search for cookers at yard/garage sales. Whenever you find a crock pot (the slow cooker ceramic-lined type) for $4 or less, buy it!.You can get some 10 to 12 processing cycles before the acid finds its way through a hairline crack in the ceramic inner glaze and corrodes the heating element. But that's OK, if you got 10 gallons of small quartz specimens cleaned, then it's worth it.
     Place the specimens in the crock pot, add cold water, then a couple of ounces of dry oxalic acid and top off with cool water. Be sure the water is above the crystals because any crystal sticking out will not get cleaned. Cover with the glass or plastic lid, plug in and set the temperature control to low. Check this every two days and add a little warm water as needed to keep the crystals submerged. DO NOT DO THIS IN THE HOUSE. ACID VAPORS ARE POISONOUS.

     After about a week, turn the crock pot off and let it cool down overnight. Do not get too anxious to pull the crystals out while they are hot or they will shatter from the thermal shock. Then remove the specimens and rinse them thoroughly. If your specimens begins to grow a white powder as they dry, place them back in a clean crock pot, add water and a 1/3 a cup of baking soda, and cook overnight. This will neutralize the remaining acid as it comes out of the nooks and crannies of the specimens. If this does not work to get rid of the white powder problem, then you will need to cook them again in clean water with baking soda as a neutralizer.

Disposing of acid
     To dispose of a volume of spent oxalic acid (it will be a dark emerald green color from the dissolved iron it contains), add lime (CaO) like you use in the garden to the liquid until you get no reaction. Then it will be neutralized due to the formation of harmless calcium oxalate. You can just dump it on the ground like I do where I wash my crystals with a garden hose. That way, the next time I wash rocks or it rains the material is diluted. If you lived in town or in an apartment, just take a funnel, pour it in a 1-gallon milk jug and put it in the trash or in a dumpster. Since its neutralized, it is not considered a hazardous material and since it is water-based it is not flammable.

Cooking big pieces
     Now, how about your big specimens or your 5-gallon bucket of hand-sized pieces that would take too long to clean in the crock pot 2 or 3 at a time. There are many ways to do this. By years of experimentation with different types of cookers, I think I have found some methods that work satisfactorily.
     First though, I should tell you that I have cooked large specimens in several different types of tanks over direct heat (a wood fire). I got the idea for my first one by observing Sonny Stanley's cleaning operation.
     I had a cooker made at a metal shop from sheet steel. It had 18-inch tall legs and the tank measured 3 X 3 feet and 2.5 feet deep. Two expanded metal screens fit inside this cooker, one stacked on the other for 2 layers of quartz. It held about 10 gallons of crystal at a time and once hot stayed hot in the spring, summer, and fall for about two days with no fire underneath. I had a cover for the top to prevent too much evaporation loss and set the entire cooker on a small concrete slab I poured for a level base.
     I fired this cooker with pine to start and would stoke it with oak firewood overnight. Scrap pine lumber was saved from cooking to cooking and free oak palettes were cut apart for the oak firewood. It worked very well for 2 years, probably 15 cooking cycles, but then a weld seam sprung a slow leak. I had it rewelded, but knew that the acid was slowly eating it up from the inside out so sold it while still water-tight. It cost around a $125 to have made and I sold it for $75. At 150 gallons of cleaned crystal for $75 final container cost (the wood was free), it was pretty economical to operate. It did require some periodic tending several times a day. After selling it, I came across another style of cooker. The idea came from Gene Newsom.
     Take a steel drum, like you would use for a burn barrel, and have someone cut the thing off 18 to 24 inches up the side from the bottom. Place this on a brick or steel stand. I had a friend who had some old kiln-liner brick which can withstand very high temperatures. Or you could take the cutoff barrel to a welding shop and have them make a free stand with legs about 15 inches in length. Fire with wood. Again you need a top to prevent rapid evaporation. This unit worked well for a couple of years and the barrel only cost $8 and $2 to get it cut off. But it finally rotted out from the acid. Another kind of cooker
     My most recent type of homemade cooker may be the best. It is simple and is built on a double boiler principle. This idea I adapted from Meredith York. The crystal is placed in a 5-gallon plastic bucket with water and acid and a lid set on top (not snapped down!). One person I know uses an old bathtub, which sits on concrete blocks. He plumbed the drain with a pipe which extends out the side of the unit and ends with a gate valve.
     Three to four 5-gallon buckets at a time are cooked in this setup for up to 10 days at a time with wood. One time he used propane, but it was very expensive. You must add water to the bath tub as needed to prevent evaporation. He used bricks to set his buckets on to prevent melting the plastic bucket. Since no acid is in the tub, it will last for years (and has!). My setup is a bit more modest, but since it is not heated with wood, takes less space and is less messy.
     My latest cooker uses a two-burner hot plate for the heat source. I bought the hot plate at a junk shop for $8. On the burners, I set a small galvanized wash tub (smaller than a # 10, but not a bucket size). You could substitute a Gene Newsom-style cutoff barrel. I place several pieces of brick in the bottom, then set a covered 5-gallon plastic bucket containing specimens, water and acid on the bricks. Fill the tub with water and turn both burners on to medium. After several hours, I check the temperature. I want it hot, but not boiling. Then comes the real sneaky trick!
     I don't want to have to add water every day or twice a day to the boiler so I take a 20-gallon trash bag and slide it over the 5-gallon bucket. Where the plastic reaches the water I spread it as wide as it will go. In an hour it will blow up like a balloon and is make a condensing tower over my setup. Steam that would have escaped contacts the side of the cold bag, condenses and flows back into the washtub! It works well and I only have to add water to the tub every other day. I expect my wife will have something to say about the electric bill next month, but when my quartz is cleaned, I won't mind. So long as the electricity is on I have got it made.
     Since I first printed this article in the Hot Springs Geology Club Newsletter, I had two experiences I must relate concerning the electric double-boiler cooker. Placement of the brick is critical as you must have a piece under the middle of the bottom of the plastic bucket or it will split out from the weight and heat. Secondly, the electric bill was way too high for the value of the specimen cleaned so if you plan on using electricity to heat a large container, be ready to pay the piper. But this method does work well and if you do not have the time to keep a wood fire burning, especially in the dead of winter, then this may be what you wish to use.
     I like the double boiler idea, but I think the cut-off barrel, heated by wood, with the specimens inside a plastic bucket and the bucket setting on bricks will be my next experiment. It has the advantage of low initial cost and low fuel cost, but the disadvantage of having to check it several times a day while cooking. If I could just get someone to pay my electric bills!!

A yellow crust problem
If you use oxalic acid, sooner or later you will have a batch that gets a yellow crust coating it. I do not know exactly what causes this problem, but I have an idea and I will tell you how to remedy it if it does happen. I have encountered this problem most often when I used a white plastic bucket, almost never with a colored plastic bucket (like red or black). I think the sunlight causes some type of reaction which allows this yellow iron compound to precipitate out of the acid solution. When this happens you will think "all my crystals are ruined!" Don't get too upset. Remove the crystal and place in another bucket. Fill near the top with water and add some muriatic acid (Be careful to always add acid to water, not water to acid!). Let set over night and the yellow staining should be gone. About a pint of muriatic should be enough. If the yellow stain is not gone, then add another pint and wait another day. Be careful as this acid will burn your skin. Muriatic is a commercial grade of hydrochloric acid and is available at most hardware stores. Always keep kids away from this stuff. When working with it, keep a garden hose nearby and turned on at the faucet. Wear safety glasses and rubber gloves when pouring and only work in the open air. The fumes from the full strength acid in the bottle will damage your lungs! I occasionally work with this acid and because I follow the guidelines above, I have had no problems from it. By the way, the water hose is always close, so if you get any splashed on you, you can immediately rinse it off.

When you buy oxalic acid for cleaning your crystals, get a receipt that plainly states what this stuff is and who you bought it from, because it looks a lot like drugs to a policeman!!