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How to Make an Acidic Liquid to Polish a Stone

In general, acids are very poor agents for stone polishing. This is because decorative stones such as granite, marble and limestone consist primarily of calcium carbonate (CaCO3), which readily dissolves in acid to form carbon dioxide (CO2), water (H20) and a calcium salt. Worse still, the acid doesn't eat away at the stone uniformly; rather, it leaves surface with a swath of microscopic pockets. This process of altering the surface of a stone with acid is known as "etching". However, there is one commercially-acid available that is useful in polishing stone: oxalic acid. When applied to the CaCO3, the result is CO2, H20, a calcium salt and thousands of microscopic pockets. The difference is that the calcium salt formed, calcium oxalate, happens to be a strong yet extremely fine-grained abrasive. Once this abrasive salt has been formed, simply polish with a camel hair pad, either by hand or SLOWLY and GENTLY by machine. The fine grains of calcium oxalate will wear down the edges of the acid-etched micropockets, resulting in an excellent surface shine.
This article outlines a step-by-step process for preparing oxalic acid for stone polishing.

Mixing

  1. Buy some oxalate crystals to prepare an oxalic acid solution. Online Home & Garden distributors usually sell them in bulk but if you'd prefer a slightly less-expensive option, go to your local independent (or big box) hardware store and ask for wood bleaching agents. Read the back of each product. Some will contain trisodium phosphate or sodium percarbonate; ignore these. What you want is one that lists "oxalic acid" as the main active ingredient.

  2. Taking a 12 oz. cup, transfer about 2 cupfuls of crystals into an empty 2- to 5- gallon bucket.

  3. Slowly add five (5) cupfuls of water to the crystals and mix with a garden trowel. Do not use a pointed grout or masonry trowel; you'll be using the same tool to apply the oxalic acid to the stone and you don't want to risk scratching the surface.

  4. The ideal consistency for this oxalate polish is a wet-paste, similar to that of a milkshake. Depending on the crystals you obtained, you may need to add more water. Either way, make note of the crystal-water ratio that works best for you -- you may have to mix new batches multiple times, depending on the size of the area.

Polishing

  1. After achieving a milkshake-like consistency, scoop up a trowel's worth of oxalic acid and spread across the stone, creating a thin layer (~1mm) over the area to be polished.

  2. Allow it to sit for 5 minutes

  3. Polish the surface with a camel hair cloth until glossy. You can use a machine but use a slow speed and check the glossiness of the floor frequently to avoid etching.

  4. Once you've completed the entire area, rinse all the surfaces thoroughly with water.

  5. Mop up excess water.

  6. Let the surface air-dry for at least 12 hours, making sure that no one walks on it.

  7. Apply the appropriate stone sealer (NOTE: granite, marble, limestone and other decorative stones generally use different sealers. Consult your local hardware store or marble fabricator for details).