Sulfamic acid is a weak organic acid typically used to clean limescale from ceramic tiles. It can be purchased at Home Depot but the cost can be as high as $6 per pound. Wausau Chemical Company in Wausau sells industrial-grade sulfamic for $72 for a 55 lb bag. Although it is nontoxic the dust can irritate the eyes and nose but will not cause tissue damage. These properties make it ideal for removing calcite deposits from copper specimens without damaging the copper. Vinegar is commonly used for this purpose but sulfamic acid is much faster. Hydrochloric acid (muriatic acid) can also be used and dissolves calcite much faster than sulfamic but it is more hazardous to use and if the copper contains arsenic (Baltic Mine Coppers) it will produce a black tarnish on the copper that is very difficult to remove.
This document provides a process that I like to use on my copper specimens.
I recommend to first wash your specimen with water, brush with a soft bristle brush and soak overnight in clean water.
Carefully examine the piece to determine if an acid soak will improve its appeal. If there is a significant amount of copper that’s covered with calcite the piece may be a cleaning candidate.
If the calcite is in a pretty crystal form and there is no sign of crystallized copper, it may be better not to acid clean it.
Be aware that copper specimens cleaned in sulfamic acid will come out with a bright copper surface. This can be transformed into a dull grey patina over time.
Sulfamic acid dissolves slowly in cold water but the hotter the water the faster it dissolves. For cleaning copper I like to mix 2 cups of sulfamic in 1 gallon of near-boiling water.
Try to avoid breathing fumes from the mixture as it will sting the eyes and nose.
Place the specimens in a clean plastic bucket and pour the sulfamic acid mixture into the bucket.
The calcite will immediately react with the acid forming bubbles of carbon dioxide. Allow the reaction to work overnight. The more calcite on the specimen, the longer it will take to remove.
Check the solution for bubble generation. If the solution is still generating bubbles there is still calcite being removed. Wait until bubble generation is complete.
Remove the pieces with a slotted spoon or wear rubber gloves to remove them. Rinse with clean water and examine them.
If there is still calcite on the sample and gas bubbles are no longer being produced, the acid has been exhausted and a new batch of sulfamic acid should be prepared to complete the cleaning. If the samples are free from calcite, the solution is still active and can be used for additional cleaning.
If reusing the solution it can be used cold but if you can reheat it, the process will be faster.
Once the specimen is clean, the residual acid must be removed. I like to thoroughly rinse with tap water and then soak it in tap water for 24 hrs. Repeat this process at least three times. If an ugly coating appears on the specimen when it dries, soak it in tap water a couple of times more.
Disposal of spent acid;
Although the spent acid solution is nontoxic, there is dissolved copper in the solution which is toxic to plants and aquatic species. If you are on a municipal sewer system the solution can be poured down the drain.
Do NOT pour the spent solution into a septic tank system. The copper will kill the bacteria that decompose the waste material in the tank. If you pour it on the ground, any plants in the area will be killed.
You may also consider taking the spent solution to a hazardous materials disposal site if your community provides one.