Frederick M. Hueston, PhD  

Stone Care Consultant 

Well, I’m sure my readers are scratching their heads wondering what was causing the mystery etch I had mentioned last month. For those of you who didn’t catch last month’s problem, the following is a recap.

I stopped what I was doing and immediately opened the package to start my testing. I felt like a little kid at Christmastime! 

The stone was called Brown Antique and was dark brown to almost purple in color. I placed the stone on my kitchen table and starting foraging to find some vinegar and muriatic acid. 

I placed a few drops of vinegar on the granite and waited about one minute. I wiped off the excess vinegar and picked up the stone to examine it under better lighting. 

Lo and behold, it etched. The etch was consistent and not spotty. I next took some muriatic acid and placed it on the granite and let it sit for one minute. I wiped it off and, lo and behold, it did not etch. Let me shout this out…IT DID NOT ETCH!  

Holy cow dung, I thought, this can’t be. So the next thing I did was…

I asked everyone to try and guess why a mild acid such as vinegar etched the granite and not the stronger acid. Here are some of the guesses I have received:

The stone is not granite.

The muriatic acid used was not really acid; someone put water in the acid bottle.

The granite really didn’t etch. It removed a wax coating.

And my favorite was that I need new glasses.

These were good guesses, but wrong.  

This is how I unraveled this mystery. First, I checked the Ph of both the vinegar and the muriatic acid to make sure it was truly acidic. Both tested below a Ph of 4. 

Next, I tried etching another type of marble and granite and the results were what I expected. Both acids etched the marble and both acids didn’t etch the other granite. So I sat down and was wondering if I was losing my touch. 

What was I missing? I thought. Just as I was about to just give up and head out for a ride, I looked over to my bookshelf and noticed my old chemistry book. Well, I thought I knew how acids reacted but I didn’t write the book on chemistry and I certainly wasn’t Robert Boyle or Linus Pauling. (Google them – they were famous chemists).

I opened up the old textbook and looked up acetic acid, which is what vinegar is and Hydrochloric acid, which is muriatic acid.  Boy, did this bring back memories of my college days.  

I put on my new glasses, just to make sure I wasn’t seeing things and, lo and behold, I found the answer. Now, I will try to keep this simple. 

Acetic acid (vinegar) is an organic acid and Hydrochloric acid (muriatic) is an inorganic acid. This simply means that something in the granite is organic in nature and is reacting with the organic acid. This is also why the inorganic components are not reacting with the muriatic acid. Now I had to find a way of explaining all this to the contractor installing the etched countertop. 

“OK,” I said, “if this is confusing, you can go out and take some chemistry and geology courses or you can just accept the Reader’s Digest version: Organic matter has somehow incorporated itself in this granite and is reacting with the organic acid, the vinegar. It is very, very rare for this to occur. The fix is to repolish the granite but the problem will continue unless a sealer is used that will prevent etching.” Another case solved... sort of.

The Stone Detective is a fictional character created by Fred Hueston, written to be entertaining and educational. He has written over 33 books on stone and tile installations, fabrication and restoration and also serves as an expert for many legal cases across the world. You can send any email comments to him at fhueston@stoneforensics.com.