How To Clean Rock Landscaping Stones (removing unwanted iron stains & mineral deposits)

(IMPORTANT: When using any rock cleaning methods, safety comes first! Where applicable, always wear approved protective safety glasses and rubber gloves where appropriate. And of course, using common sense is a must).

Here are useful tips and guidelines on several simple enhancing techniques to clean unsightly stains or stubborn crust material from landscaping rocks before placement in your garden. (Note: Be sure to first thoroughly scrub and wash off all caked-on dirt, sand, and lose mineral deposits from stones using a brass bristle brush while continually rinsing with water using a garden hose with spray-nozzle to removing such debris by water pressure):

-- Using Oxalic Acid ( A slow but effective process)

Anything that has the word "acid" sounds ominous. But oxalic acid is easy to find, use and the safest cleaning agent. In fact it is found in many vegetables including spinach. It is used to dissolve the iron oxide (brown or red) stain on all types of stones. Rock specimens collected at Phoenixville, Ellenville, Case Quarry, NH smoky quartz and many others clean up beautifully with oxalic acid. However, some stones may not respond as well, so its best to test beforehand on small specimens to see how they react to this type of cleaning agent.

To make this as simple as possible here is an effective four step process - do not take any shortcuts or make substitutions.

1. Purchase a one pound box of Oxalic Acid (OA) powder at your local hardware store in the paint department or at a paint store. It is used as wood bleach and will be labeled as such. The most common commercial brand is "Rainbow".

2. Fill a plastic one gallon container 3/4 full with hot tap water. Pour in the OA crystals and stir for five minutes. Be careful not to inhale any powder when adding the OA crystals. Once they fully dissolved in the water top off the container with more hot water to a full gallon. For safety sake, label the container and put out of reach of children or pets.

3. When ready to use this stone cleaning agent, place your rock(s) in a plastic container and add enough OA solution to cover the rock(s). Set aside for several days.

4. After the stain colors have disappeared, with rubber gloves on remove the rock(s) and wash under running water for several minutes. Next soak in clean water for a day changing the water as often as possible.

To speed up the last process (step #4), using heat and/or agitation will help. If you have a hot plate that can be set up outdoors or in an area with good ventilation then repeat step #4 but heat the solution only to bath water hot - Never Boil!!! An hour in hot solution usually will do the trick. Best of all is an ultrasonic cleaner with built in heater. Sometimes only 30 minutes is necessary. But you should not put the OA directly into the stainless steel basin. Make a double boiler type of arrangement by partially filling the ultrasonic cleaner basin with water. Then place your rock(s) and OA solution in a plastic container or heavy duty plastic bag that is suspended in the water.

You can reuse this cleaning solution over and over. As it dissolves more and more iron from rocks it will get darker often taking on a green color. After the cleaning solution gets really dark discard it and mix a new batch. Safety is important. OA solution is highly toxic. It can absorb through the skin and builds up in your organs cumulatively. Same goes for the fumes, which is why you never boil the solution and always have proper ventilation when using the heated solution. Be careful not to spill the solution on porcelain and keep away from food preparation surfaces.

In spite of all its bothersome concerns, this is one of the best all around method for cleaning rocks.

-- Using Muriatic Acid

This rock enhancing cleaning technique is a more aggressive - but very effective. Hydrochloric Acid is available in most hardware stores as Muriatic Acid. It is usually sold in one gallon containers and is used to clean masonry as a rust remover. In spite of its easy availability, it is dangerous. Do not inhale the fumes or get any on your skin or in your eyes. Always wear appropriate eye protection, rubber gloves, and old clothes. Keep your arms covered even if it is a hot day. And always observe the safety precautions on the container.

There are two main uses for hydrochloric acid: removing carbonates like calcite that often are the last minerals to form on rocks, and of course, the more aggressive removal of iron oxide rusts stains (which acts faster as a rust stain remover than Oxalic Acid). The former use is the most common and often produces staggeringly beautiful results because the calcite layer being dissolved had protected the stone surface underneath. If hydrochloric is used to remove iron oxides from rocks be careful that there are no carbonates in it that you want to preserve in the stone's surface. The acid will dissolve them. Which is why, no matter what kind of stone you are cleaning, always test your cleaning agents on it in an inconspicuous spot to make sure you will not ruin its overall desired natural appearance.

The basic cleaning procedure is:

1. Again, wash the rock(s) in clean water to remove any remaining loose sand and dirt to make the acid last as long as possible (NOTE: sand and dirt contains iron oxide which will quickly exhaust the reactionary properties of acid).

2. Place rock(s) in a large plastic container with a lid that can be later tightly sealed such as a five gallon joint compound bucket often found at construction sites. Let rocks dry and move the container outdoors to an area with good ventilation. Pour in enough acid to cover the rocks. Always wear heavy rubber gloves and be very careful not to splash any acid on yourself.

3. Depending on what you are removing with the acid you will want to leave it in from 5 minutes to 5 days. If you are etching carbonates/calcite off a specimen then you should check it after five minutes. Be careful not to inhale any fumes when checking the progress. When removing calcite or marble from specimens the action is very fast and active. Your bucket should be large enough to prevent the bubbling foam from overflowing. If you are removing rust stains from quartz the action can take up to a day and is less energetic. When removing the "sphalerite" crust on quartz rocks it is not uncommon to repeat three day sessions removing any loose material between each session. For safety, place the sealing lid on the bucket to prevent children and animals from exposure (but provide a small vent hole for relieving eventual build-up of gas pressure). (Note: Do not leave the bucket in the sun light. This may results in a yellow stain developing on the rock's surface).

4. Neutralize the acid left on the rock acid wash can be achieved in many different ways. After a brief pre-rinse in clear water, some rock cleaning experts prefer to dissolve ordinary household baking soda in a bucket of warm water immersing the rock(s) in the baking soda solution for 15 minutes before proceeding with the final washing. However for expediency and as a matter of preference, some experts just skip this particular step.

5. Lastly, because the acid has penetrated the rock(s), it is best to wash very thoroughly for its final cleaning step. As a rule of thumb, three times immersion in clear, clean water for a five minute dip bath each time is best. Altogether a 15-20 minute wash will be adequate to complete the procedure.

6. If residual stains and/or mineral deposits remain, repeat these cleaning steps again.

As the acid is used up it will eventually turn yellow/green/brown. It can be used until it no longer is effective or until it starts to stain rocks when used. If you are using it to dissolve carbonates/calcite the acid will exhaust itself long before the color changes. You will see that it no longer actively dissolves the calcite. It should then be discarded. You can fully neutralize your old acid with crushed limestone or baking soda. When it no longer fizzes then you can dispose of the acid safely.

IMPORTANT NOTE: if removing calcite from a landscaping rock, do not dissolve all calcite. Often it may be the only thing holding the stone together. A little calcite can provide a nice contrast and make the stone more aesthetic and appealing. Some types of rocks, if immersed in a hydrochloric acid cleaning bath too long, will turn powdery and crumble apart.

-- Using Mechanical Methods

By mechanical we mean using force of some sort to remove unwanted stains or mineral encrustations. Obviously such method has the potential of damaging landscaping rocks by fracturing them causing unsightly breaks or surface cracks. As such, care should be taken to go slow and easy to minimize any potential damage and to stop the process altogether if need be.

A. Sand Blasting

Sand blasting sounds exotic but is more common than you would think. The sand blasting removes the softer material or deposits very quickly and leaves a more natural softer appearance than chisels or scrapers.

A sand blasting unit is not expensive. The basic setup can be purchased for around $50.00. However the air compressor to drive the sand blaster is expensive. Unless you already have access to a 3.5 HP air compressor, then sandblasting is not for you. There are many different media that you can use in a sandblaster. Anything the consistency of table salt can be blown through the gun, wet or dry. Glass beads are readily available and are the hardness of quartz. But you can purchase many different harnesses down to 3.5 on Moh's scale. The goal is to choose a media softer than the mineral you want to keep but harder than the mineral you want to remove.

B. Air Scribe

This is a miniature reciprocating impact chisel commonly used by fossil preparers to expose fossils. ARO air scribe Model 8315 costs $289.00 and comes with a medium carbide tip. Additional tips are available in fine to coarse for $31.00 each from Main Tool Supply, 55 Lafayette Ave., North White Plains, NY 914-949-0037. These tools reciprocate at 36,000 cpm and are very efficient at locally removing matrix.

B. Water Gun

Similar to a sand blaster, the water gun is used in the dry cleaning industry as a stain remover (they blast cleaning agent right through the fabric). The Krebs 5000 cost $350.00 and is available from Aurora Mineral Corp. (do web search for contact address).

The action is a combination of sandblasting and ultrasonic. The gun creates a fine, high powered jet of water that will loosen most clays or dirt. The advantage is that you can put solutions other than water in the gun. In theory you could shoot oxalic acid through it. But since oxalic acid requires time to work the value of this option is questionable. And remember the toxicity of oxalic acid - the last thing you should do is create a fine mist of oxalic acid that you could accidentally breathe.

In conclusion, you do not have spend lots of money to clean landscaping rocks. In many cases a brass scrubbing brush and plain old sand paper is all that is often need. Remember also that these mechanical methods are often the first in a many step process. You may simply only need to start with a brass brush and sand paper then use hydrochloric acid or oxalic acid to get the desired rock cleaning effect.