Frederick M. Hueston, PhD

Spalling is not normally associated with interior stone installation, but crank up the humidity high enough and you will get condensation. This case is a rare example of what can happen on unsealed, unprotected stone.

Spalling is not normally associated with interior stone installation, but crank up the humidity high enough and you will get condensation. This case is a rare example of what can happen on unsealed, unprotected stone.

It was one of those rare cold mornings in Florida. OK  – so, not as cold as Chicago or the Midwest back at the end of January. But in Florida when it gets below 60 degrees Fahrenheit, that’s considered cold! I guess I’ll have to wear socks with my flip-flops. (Just kidding – I don’t wear flip-flops. They don’t go with my fedora.)

It was so cold, the heat came on in my office for the first time EVER. I was starving, in the mood for a hot cup of Joe. So, off I went to visit Flo at the local greasy spoon.  

I walked outside and just as I was getting into the ole Woody, my phone rang. “Stone Detective here,” I answered, my teeth chattering. The caller had a high-pitched voice sounded like they were doing a cartoon voice impression. I couldn’t tell if they were a guy or  gal. When a name was mentioned (Chris or Kris?) that didn’t help, either. Anyway, the voice was squeaky and difficult to understand, so I listened carefully as the caller described their problem. 

I got the picture that the business this person worked for had a slate wall that was getting these odd white blooms on the surface. I asked all the standard questions, such as how old was the wall? How long had the problem existed? Squeaky really didn’t have any answers, but suggested that I come take a look at it. I made an appointment to call them back shortly and shivered my way over to the Diner. 

The project was in a little town outside of Boston, and the Weather Channel said it was really cold there. So I guessed I would be shivering even more, as they needed me there ASAP. I made arrangements for an inspection, and asked them to gather as much information for me as possible before I arrived.  I got into the ole woody and headed back to the office to chill (literally) and make arrangements for my flight to Iceland, Massachusetts  (LOL).

Well, the day came for the big chill. I headed out the door with all my winter gear, wondering if the airline was going to charge me extra for all the coats I was wearing. Have you noticed how they charge for everything, nowadays? 

I got to the airport and found my seat on the plane.  Once we were in the air I had to use the restroom, so I asked the flight attendant how much it was to use it. She laughed and kiddingly said, “They’re not charging… yet.” At least I hoped she was kidding.

I arrived in Boston and headed for my rental car. I was hoping they would have it running with the heat on! To my surprise, they did. I put the address in the GPS and off I went. It was cold but the roads were clear. It looked like they had pushed all the icebergs to the side of the road.

I arrived at the office building, parked my car, and when I got out, I could hardly see for the fog my breath was putting out. That’s cold, folks, especially for this Florida Boy.  

I entered the building lobby and Squeaky was waiting for me. Squeaky was a slightly-built man and when he spoke I thought he was imitating Mickey Mouse. No offense to all you Disney fans, but that’s what he sounded like to me.  

He introduced himself and took me right over to a partition on the wall that was clad in slate. It was a small wall about four feet high and maybe 10 feet wide.  As I walked closer I immediately noticed that there were numerous spalls with a white haze to them.  Normally, when you see this condition you would suspect a moisture problem. However, this wall was in the middle of the room, so it wasn’t an exterior wall, or a wall containing any plumbing.

I pulled out my moisture meter to see if the wall was wet, and it was dry as the Sahara. (Not the casino, but the desert.)  Now I was scratching my head.  I thought I was going to have to pull a tile off the wall or take a sample to send out for lab analysis, but I thought I would investigate a little more.

By this time the building’s engineer had come by, and he appeared to know more than Mickey did.  He told me that the wall was about 10 years old, and they first noticed the problem about six months ago. That’s odd, I thought. I asked him if there had been any extreme temperature or humidity changes in that period. He snorted a laugh, rolled his eyes and said, “Not that I know of. It shouldn’t – our entire HVAC system is computerized. But if you really want to, we can look at the old data.”

 “Great. Let’s take a look,” I said. Well, to make a long story short, I discovered there was a two-week period where the air conditioning had failed, and the humidity level skyrocketed. BINGO, we have a huge clue!

Now I knew where the moisture was coming from: it’s called humidity. Apparently, moisture had accumulated on the slate surface. It penetrated into the slate and caused several areas within the slate to break down and spall. This is a typical problem with certain slates when exposed to moisture. 

I told him those spots could be removed and filled, and the spalling shouldn’t come back as long as the HVAC system is working.

Another baffling case solved –this was worth a trip to the frozen North.  I’ll thaw out, eventually!

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 him at