Frederick M. Hueston, PhD

I had just returned from my afternoon ride on the Harley. Got to love the early Spring here in Florida. There’s plenty to look at, so I try not to look at my phone while I’m riding. I have a rule of never talking on it while riding, either, even with a fancy headset.

I could tell it had been ringing, though. When I got off the bike, I noticed I had seven missed calls! I opened up my recent calls and noted that all the calls were from the same number, so  I figured this must be important. I also noticed that there was only one voice mail from that same number.  I took off my helmet and headed into my office to listen to the voice mail and heard to the following:

“Mr. Stone detective, this is very important. I need you to call me as soon as you get this message. I have a problem with some stone that was installed on the façade of our building and it is getting darker and darker. Please call me.”  

The caller was female, with a smoker’s gravely voice. She sounded a lot like Nina Blackwood from the old VH1 channel. I image only you older readers will know who I mean. Wow, now I’m starting to feel old –LOL!  I returned the call and discovered that the building was only a few hours away, so I arranged to do an inspection of the building the next day.

The next day arrived and it looked like it was going to be another warm, sunny day so I decided to take the Harley. Of course, I stopped to have a cup of Joe and flirt with Flo before heading off.

I arrived at the address in the late morning. The building was only a three-story structure, but was clad in a beautiful, grey granite. I immediately noticed that 90 percent of the stone had a dark, wet appearance, as if it had just rained. The odd thing was that it hadn’t rained here in over a week.  

I headed toward the entrance and before I reached the door, a tall, thin blonde exited the building, headed right for me. She had one of those long cigarettes hanging out of the corner of her mouth and I knew the minute she spoke that she was the lady on the phone with the gravely voice.

“You must be the Stone Detective. I looked you up on your website.” She extended her right hand and at the same time she removed the cigarette from her mouth. She tilted her head back and released a huge plume of smoke. No wonder her voice sounded so rough.  

She told me that the stone was recently cleaned, and several months after the cleaning the dark areas started to appear.  I told her I needed to perform some nondestructive testing and that I would need to talk to the contractor who did the cleaning.  She said OK as she coughed on another puff and walked back inside.  

I took out my moisture meter and placed it on the stone, and it was soaking wet! So, my next question was, why was it still wet after several months? It should have dried by now! This was going to require some detective work, I thought. Ms. Blackwood returned just as was about to take some photos and handed me a business card with the contractor who cleaned the building. I thanked her and called the contractor.

The contractor told me they used a mild neutral cleaner and allowed the stone to dry for several days before they sealed it. “Wait a minute,” I said, “you sealed it?”

I asked what they used, and he told me a standard impregnating sealer.  I knew right away what the problem was. The following is what I wrote in my report to Mrs. Blackwood.

When stone is exposed to unregulated humidity and temperature fluctuations, like it would in an outdoor environment or in a shower, the air contains vapor in what we know as humidity. Temperature along with humidity can result in condensation as well.

Most of the impregnators on the market today are breathable. This simply means that the stone will be protected from water entering the pores of the stone in liquid form, but will allow water vapor to pass through.

In wet environments, vapor can be present for several reasons: rain, high humidity, temperature fluctuations, steam, etcetera. Since these impregnators are breathable, this vapor can easily penetrate into the stone.  One would think that this is a positive feature. The fact is that once the vapor enters the stone it can condense and become a liquid. 

Since impregnators protect against water in its liquid phase it becomes trapped within the stones pores and will not escape until it evaporates, or in other words, turns into a vapor.

Once this water becomes trapped it can result in all kinds of problems. Stones with iron content can begin to oxidize, natural salts within the stone can become dissolved and cause pitting and spalling. Aesthetically, the stone will appear darker since it is constantly wet.

I have seen this problem numerous times when stone is exposed to this condition. I have even seen this problem occur in showers, especially shower floors.

I told her it would eventually dry out but was going to take a while. I also told her the next time the building is cleaned not to seal it. In this case no protection should be necessary.

Stay tuned for a detailed article on this subject and the results of some testing being conducted on sealers in wet environments.

The Stone Detective is a fictional character created by Dr. Frederick M. Hueston, PhD, written to entertain and educate. Dr. Fred 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. Send your email comments to