Mineralogy Surrounds You

Mineralogy surrounds you. From mica flakes in your shimmery makeup, to granite countertops and slate floors, to the chalk in the classroom and graphite in pencils. You don't have to understand where it all came from or how it got to you as a consumer to make use of it, but these things are ever so easy to take for granted.  Someone had to recognize the value of the rocks and minerals and have the know-how to change them into something useful.

I mentioned in my last post, that there was a fire in my home. Never did I imagine that the fire in my house would lead me down a path to understand and appreciate the properties of gypsum.  But it did.

Gypsum is an evaporite mineral made of calcium sulfate dihydrate, (CaSO₄·2H₂O).  A Google search on gypsum will lead you to lots of online resources to learn more about the mineral. It has a multitude of uses, including Plaster of Paris and it is also a major constituent in a product of prime interest to me: Sheetrock.

So let's walk down the path that led to this interest in gypsum and see what we can learn.

It was a cold, late afternoon in January. We went grocery shopping, came home, hastily unloaded the car, then headed back out to take care of other errands before the shops closed for the evening. We came home from our errands, maybe 30 minutes later, to find the house on fire.

From the exterior there were no signs of problem until we opened the door and smoke came pouring out. My husband grabbed the emergency fire extinguisher we kept in our garage to try and help, but heavy smoke prevented him from getting anywhere close to the fire. He also noted that the floor was wet. Of course, we immediately called 911 and thanked God that everyone was safely out of harm's way. We waited across the street as five ladder trucks responded to the call, neighbors came out of their homes from blocks away, and my children clung onto my legs desperately wanting everything to be ok.

My mind raced back to our quick unloading of groceries just earlier. What was out of place?? What could have started this fire? We weren't cooking or burning candles. Was there bad wiring somewhere? Did we have an issue with our space heater in the bathroom - I thought it was turned off? And why is the floor all wet?

The fire department had to use the fire hose to put the fire out. I was impressed that they took care in the process to remove pictures from the walls, pull all the furniture into the center of the rooms, and cover them with a water and fire-proof tarp. Afterwards, they opened all the windows in the home and set up large fans to blow out the remnant heat and smoke.  The fire inspector then went in to analyze the scene.

My 7-year-old's letter to the Firemen

Turns out, large box placed on electric stove top = very bad news. Apparently, one of the burners was turned on by the box that was quickly placed and pushed on top. I hate that I now serve as the example for company safety moments. Lesson definitely learned!!

Our home was found to be structurally sound and was released back to us that night. The insurance company classifies it as a "partial loss".  We are very fortunate. However, the house was in no condition for us to live in.  Smoke and soot permeated into EVERYTHING.  The entire downstairs floor was completely saturated with water. The fire melted a water pipe in the second story subfloor, which poured water out (actually acting like a mini-sprinkler) and flooded the first floor, well before the firemen came in with the water hose.  Upstairs, even though the fire had not actually spread that far, the heat that was trapped up there melted all the feaux wood blinds and the plastic AC registers in the ceiling.  The AC control box, doorbell, and even the smoke detectors were warped and melted.  My daughters' dolls and plastic toys were sticky and soot-covered. The walls in all the rooms, save the far-end of the downstairs, had heavier soot along the joists and nailheads, a kind of ghosting effect.  I learned this happens because the soot tends to settle on areas that are cooler.  It all comes down to the physics of heat transfer. (Here's a little pdf write-up by Mr. Fix-It, Tom Feiza, describing the phenomenon.)

Obviously there is much remediation to be done. Here begins the battle with our home insurance provider regarding what needs to be replaced/repaired, and what is covered by our policy.  Several experienced restoration contractors commented that the sheetrock in most of the house needs to be replaced.  Even the city inspector commented that the sheetrock had obviously been baked. The initial proposal from the structural adjustor: clean, seal, and paint.  !!!

Let's examine the role of sheetrock, or gypsum wallboard, in a home. Obviously, it forms the walls and ceilings of the interior of the home. It is the surface that can be painted over, textured, or wallpapered. It is the surface you drill or hammer into when you want to secure a picture, a shelf, decorations, etc. The sheetrock hides the internal "guts" of your home: the electrical, the plumbing, the insulation, the framework that holds the whole thing together. From that perspective, the sheetrock is functioning as a superficial component of the home.  To clean, seal, and paint the wallboard would only address that superficial issue.

But sheetrock plays an even more important role in your home: it serves as a fire barrier.  Gypsum is a hydrated form of calcium sulfate and inherently contains water within its crystal structure. With exposure to elevated temperatures the gypsum undergoes changes, referred to as "calcination", where the bound water evaporates.  The dehydration process begins when exposed to temperatures in excess of 80°C (176°F). Gypsum becomes calcium sulfate hemihydrate when exposed to temperatures of ~100°C (212°F), which then changes to anhydrous calcium sulfate at temperatures of ~180°C (356°F). Exposure to high heat results in loss of mass and decreasing density of the sheetrock, making the wallboard brittle and crumbly.  Discussion of this process is described in this pdf article by Kennedy et al., 2003. The evaporation of water from the gypsum wallboard is what buys you time in a fire and slows the spread of the fire from room to room.

In my quest to "prove" to the insurance company that the sheetrock in my home would not be "pre-loss" condition if simply cleaned, sealed, and painted, I stumbled across a very interesting and insightful article in the Journal of Fire Sciences.  The article by Melinge et al., (2010) describes the discrete mineralogical phase changes that occur over time during exposure to high heat.  Their laboratory analysis involved exposing a very thick piece of gypsum wallboard (40 mm) to high heat over time and documenting the incremental changes to the mineralogy as the calcination process occurs. What is very interesting about their study, is that they were able to document the exposure length of time and temperature and its relationship to the gypsum wallboard across its cross-section.  They could record the phase changes as they occurred on the heat-exposed surface of the wall board and then observed as those changes migrated through the wallboard. Their work showed that alterations of the gypsum board due to exposure to high heat is dependent on the thickness of the gypsum board, the temperature of the fire, and the length of exposure time.

Figure 12 from Melinge et al., 2010



We cut samples of the sheetrock to send off for testing, one from the second story ceiling that was exposed to high heat and another from the lower floor on the far end of the home not exposed to the heat. There were very obvious differences in the material as the samples were cut.  The sample from the ceiling had very irregular edges and was crumblier, although only through about half the thickness of the sample. The piece cut from an unexposed portion of the house had smooth edges and no obvious issues. Moisture testing of sheetrock samples from the heat-exposed ceiling in my home yielded a 8% decrease in water content compared to the sample from an unaffected portion of the home.  It was interesting to find that the same methods for obtaining geotechnical moisture measurements in soils are used to evaluate the moisture content of the sheetrock.  The only difference is that they have to heat the sample more slowly than a typical soil sample so as not to initiate the calcination stage too early. 

It took us over a month to come to a consensus with the insurance company regarding the sheetrock. The insurance company finally agreed to change out 60% of the sheetrock.  We are paying our contractor to remove and replace the other 40% out of our own pocket. In the end, I think they ignored the science and opted to concede only because our alternate living expenses were adding up.  I learned, however, that the sheetrock does more for you than just look nice. If we are unfortunate enough to have to go through this again, I want the peace of mind that any loss was not due to reduced fire retardancy of our walls.

Yay for calcium sulfate dihydrate!!