How to Acid Wash Limestone

Limestone's surface is naturally pitted.

Limestone is a porous rock made from the compressed remains of sea life, particularly shellfish. It contains a lot of calcium because of this, and, because it's formed by compression, it can contain traces of other minerals as well. All of this makes it sensitive to acid. Even lemon juice or cola can burn into the rock and leave a mark. Using an acid wash on limestone must therefore be done with tremendous care.

Things You'll Need

• Heavy-duty latex gloves
• Eye protection
• Filtration mask
• Extra bucket
• Water
• Large glass measuring cup or 10-gallon metal bucket
• Paint brush
• Glass measuring cups
• Muriatic acid
• Limestone
• Pressure washer
• Baking soda
• Rags

Instructions

Acid Wash

1. 

   Safety is imperative when working with acid.

   Set up your materials in a well-ventilated area where the acid-water solution can safely drain or be soaked up. Put on your protective gear and fill a spare bucket with water.

2. 

   Muriatic acid will eat away plastic.

   Measure one part acid and pour it into nine parts water. Add the acid to the water. Do not add the water to the acid.

3. 

   It's safe to add more water if you need to.

   Test the solution on an inconspicuous area of the limestone, or on a spare piece, if it's available. Adjust the dilution as necessary.

4. 

   Discard brushes safely.

   Paint the diluted acid onto the limestone in a thin, even coat.

5. 

   Water is the best defense for the limestone.

   Rinse the limestone thoroughly with the power washer almost immediately. The longer you leave on the acid, the more it will wear away the stone. Leaving it on too long can permanently damage the limestone.

6. 

   Dispose of the used baking soda, and other materials, safely.

   Sprinkle baking soda over the entire surface to neutralize any remaining acid mixture. Alternatively, mix the baking soda with water to create a paste and apply it to the limestone. Allow the surface to dry, and then wipe away or rinse off the baking soda.

Tips & Warnings

• Because limestone is so sensitive to acid, you can try a milder acid, such as oxalic, rather than the stronger muriatic.

• If you aren't able to follow the necessary safety precautions and cleanup, find someone who is experienced in acid washing.

• Always add acid to water, never water to acid. Adding water to acid can cause the water to abruptly turn to steam, which can cause burns and splashing.

• Check with your local ordinances to safely discard any remaining acid-water mixture or materials such as rags and brushes.

• Wear clothing that covers all of your skin when using the acid.

References

• TrowelTrades.com: Brick Cleaning Techniques
• National Park Service: Preservation Brief 1: The Cleaning and Waterproof Coating of Masonry Buildings
• World Floor Covering Association: Stone Flooring Styles Review
• The Tile Doctor: Tile Maintenance, Cleaning Stone
• Rockhounds.com: Bob's Rock Shop: Mineral Cleaning