Frederick M. Hueston, PhD  

Stone Care Consultant 

At my age I thought I had seen it all. But, today would prove me wrong. This is what I love about the stone industry; there are always surprises and things to learn.          

I was headed back from my morning cup of jo and, you guessed it, my phone rang. The voice on the other end was a soft-spoken gentleman. I could hardly hear him, he was speaking so low. He told me that they had a granite lobby wall that had these spots that would show up every now and then and they would just disappear. 

“Are they present now?” I asked. 

He said, “Yes, that’s the reason I’m calling you.”  

I asked him how long the spots stay before they disappear and he said a day or two. Well, this is something I had to see, so I told him I would be there in about an hour or two.

I arrived at the building which was one of the major buildings downtown. The building wasn’t that old, and since it housed a major bank, there were several guards walking around. 

Parking was going to be difficult, so I drove around the block several times, hoping a spot would open up. Of course I was getting all kinds of looks since you don’t see that many 1947 Woodys. I felt like I was in a one car parade. A spot finally opened and it happened to be right in front. Perfect, I thought.   

I walked into the bank and was immediately greeted by one of the guards. He asked me who I was here to see. I told him who I was and that I was there to look at the granite walls. 

“Did someone call you?” he asked. 

“Yes, that would be Mr. Soft Talker.”   

“Oh, OK,” he said and went to the front desk to make a call.  

As I was waiting for Mr. Soft Talker to come down, I looked up at the wall. It was composed of a gray granite panel; each panel looked about 36 inches square. The wall went from the floor up to about the third story. I immediately saw the spots he was talking about. They were in a distinct, five dot pattern. One on each corner and one large spot in the center. 

I suspected right away what the spots were but was puzzled at them disappearing and reappearing. That didn’t make sense. Just as I was about to go up to the second floor, a short, stout gentleman dressed in a very expensive suit walked up to me.  

In that soft-spoken voice he introduced himself and then pointed to the wall and said,  “There ya go.”  

“How long has this been going on?” I asked. He told me he had only been there for about a year and it was happening since he got there.  

“The spots are blobs of mortar that were used to shim the panels to make them even,” I told him. “They are supposed to use white mortar since gray mortar can cause permanent staining on light-colored stones.” I also told him that I was puzzled why they would appear and disappear and that I would have to investigate further.  He told me to do whatever I needed to do and turned away and went back to his office.

I took out my moisture meter, and bingo! The areas where the spots appeared were wet. Interesting, I thought. I quickly surmised that the spots were getting wet somehow and then drying out. Now the challenge to this mystery was figuring out how they were getting wet. 

I walked back to the front desk and asked if I could talk to the building engineer. The guard said, “You’re in luck because he just walked outside to check on something on the front entrance.” He pointed out the door and said, “That’s him right there in the blue shirt.” 

“Thanks,” I said as I made a beeline out the door. I made my way outside just as he was setting up a step ladder. I told him who I was and why I was there. He looked at me and said, “Oh, wow, this is great. I’ve been trying to solve this mystery for years.” 

He stopped what he was doing and told the worker who was with him to put the ladder away. “Let’s head back inside,” he said. 

We walked back in the lobby and he went on and on about these spots. He told me they just seem to appear one day and might stay there for a few days and then disappear again. “This has been going on for years,” he said. 

I told him what the spots were and that they were somehow getting wet. 

“I don’t understand how,” he said, “since there is no plumbing or roof leaks in that area.”  

So, I started asking him some questions. I asked if they kept temperature reports and other data about the inside of the building. He told me that their HVAC system was completely computerized and, “it’s a pain in the donkey. We have had nothing but problems with it.”

 Now I thought I knew what was happening. I asked him if I could get a printout of the temperature and humidity data. He said, “Sure, but it will take me a while.”  

I gave him my card and told him he could fax or email the data to me. I thanked him and headed out the door.

When I got back to my office, their pages and pages of data were piled up on my fax machine. I had to load more paper in the fax because it kept on going. He must have sent me the data for every day since the building opened. 

Well, long story short… after spending hours going over the data, I discovered that in several days during the year, the dew point was exceeded. Now, for those of you who don’t know what this means, it’s simply that the dew point is the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water. In other words, water condensed on the surface…which is how the mortar spots were getting wet. 

I wrote up a report and emailed it over to both the building manager and the engineer.  I suggested they either fix the darn HVAC system or to just not worry about it. Another case solved.

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