What is Concrete Acid Stain and how does it create those beautiful floors?

 
Although concrete stain floors have become "all the rage”, they really are not new. As far back as the 1920's these floors were used as a beautiful and long-term solution to flooring questions.

The L.M. Scofield Company started manufacturing acid stains in the 1920’s. The “Big Sur Inn” in Cal. was stained in the early 1900’s and the floor still looks great today.

The great architect, Frank Lloyd Wright, even used it in the early 1900’s with stained walls and floors.

Gilbert Stanley Underwood, another fine architect who worked with the National Park Service, used textured and concrete acid stain floors at the Jackson Lake Lodge in the Grand Teton National Park in the early 1950’s.

These are just a few of the examples of the pioneers of the use of acid stains on concrete floors and other surfaces.

What is acid stain and how does it work, is probably what you are thinking right now so let's address that question.

Concrete acid stain is a combination of mild hydrochloric and/or phosphoric acid and inorganic metal salts. These stains are not a paint or a coating, instead the acid opens up the pores in the concrete allowing the metal salts to react with the chemicals in the cement (free lime) and this is what produces color.

The normal appearance of the reaction is that of a mottled, variegated look resembling marble or polished stone. In the case of highly power troweled concrete the look can be vary dramatic, with some areas "taking" in the metal salts more readily than others and creating wide variations of color.

The wide variations and dramatic look are what make these incredible floors so popular. The mixing of colors can also add to the appeal, "new" colors can be created through this process as seen here. 


Please understand that with this process you are more than likely will not get a uniform color across the floor, rather a variation of color and tone. The floor you create will be your own though as no two slabs are ever the same. You might get close but expect differences as all concrete is finished and cured in different manners and conditions.

Concrete acid stain can be applied to walkways, driveways, pool decks, interior floors, basements etc. The most beautiful floors will be those that have a “smooth” or troweled finish. The reason for this is that when a concrete surface is troweled it closes the pores in the concrete. When the acid stain is then applied, the amount of stain that enters the concrete varies across the area where the acid stain is applied. This is where the variations of color occur.

In the case of rough or “broom finished” concrete, the pores in the concrete are already open thanks to the action of the broom being pulled across the concrete to create the skid resistant surface. The metal salts are able to react with the free lime in the concrete on a more consistent basis. This results in a more uniform color throughout the area, usually absent of the mottled and variegated look.

Is all concrete eligible for concrete acid staining, in a word, no? Extremely dirty, greasy or oily floors may not be great candidates. Floors that are covered in old tar based cutback (used as tile mastic in the past) may not be either. While there are specific soy-based mastic removers available, and they do a good job removing the cutback, the tar in the cutback may have actually stained the concrete. Only you can be the judge.

A word of caution here. There are a few contractors, I presume with limited experience in concrete acid stain that will tell you they can stain any floor. While this may be somewhat true, the quality of the concrete acid stained floor you end up with might be different from the beautiful floor you were expecting. Sealers, curing agents (used in the hydration process to retain moisture) and a host of others are normally not visible to the eye and can limit the amount of stain introduced to the slab. This will leave the concrete surface “spotty” and unevenly stained. We will address this and other similar issues on our “Troubleshooting” page.

Concrete acid staining can be done right and it can be done cheaply, it however cannot be done right cheaply. There is quite a bit of labor that goes into a properly acid stained concrete floor.

Although the age of the concrete is really not an issue (unless it is extremely old), some concrete that has been “spalled” or has the aggregate exposed may also not be a good candidate. The aggregate will not react to the stain only the concrete paste and the chemicals within it can be stained.

We hope this short introduction has helped you understand this process a little better. Please feel free to browse the rest of our site for a more in depth study of this fascinating subject. 

 

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