1. That's wild. I agree your solution is best. The cause is a real who-done-it.

Moisture issue.

I was considering that because the vertical beam is on an outside wall, but the board is perfectly dry and I am also beginning to get the same type of stain on a vertical support on the opposite side of the room (an internal support).

Is there a heating pipe in there for the baseboard heat? Maybe that's helping accelerate the reaction. That looks like the plaster in our 50s house.

Get a moisture reader to check levels there

It looks very much like the type of plaster that was in my 1951 house, so are you sure its even sheetrock? Cud be something from the plaster itself.

The sheetrock plaster is bone dry and there are no baseboard piping runs or risers within that sheetrocked support. In addition, the same staining has been slowly showing up over the past 8 or so years on a similar internal sheetrocked vertical support on the other (internal) support wall. (See photo.) I’m beginning to think the silver coating on the wood may be old adhesive that originally held the sheetrock to the support and has been chemically leaching through it over time — though I have never heard of a metalic-looking glue.

It seems to be sheetrock. This room was once two bedrooms and a wall was taken down to make a large den sometime before we bought the house in 1989. The living room walls are plaster over lattice. This stained wall is plaster sandwiched between two cardboard layers — so I assume old dry sheetrock.

That silvery coating could be aluminum coloured rust-proof paint, it was a popular choice in the 70's and 80's. I don't know why someone would paint a beam that is about to be covered with sheetrock, but perhaps they re-used lumber from another project that was already painted.

Previous owners might have asked advice on Houzz where they were told to coat beams with metallic paint before finishing walls.

That’s interesting regarding the aluminum colored paint. Maybe it was a new type of fire-resistant paint for treating the wood frame when the house was being built? But then again, i dont see any such coating on the floor joists when I an in the basement. I’ll try Googling silver/aluminum paint. Thanks for the suggestion.

Brown often indicates rust which indicates moisture from somewhere.

I do have some occassional water infiltration through the concrete block foundation on that side of the house. Maybe that wood has absorbed some moisture from the humid basement air below it? The stain started at floor level and progressed upward over the years. But the support is not in direct contact with the foundation and does not run all the way down to the basement, though it may be resting upon an iron I-beam that spans the basement and supports the house. Hard to see. But the I-beam is definately not wet and doesnt explain the internal vertical support wall on the other side of the room where the sheetrock is also showing the beginning signs of discoloring. Would be interesting to see if that wood support beam behind that sheetrock also has a metallic coating. But why the staining has traveled up from the floor on the two opposite support walls is a mystery, considering the rest of the walls in that room (even the outside wall) has developed no staining at all. And if it was due to the metallic coating, the entire length of that piece of sheetrock would be stained uniformally. If it is caused by moisture below, it has taken a very long time to develop, but it seems the most logical answer at this point.

Definitely moisture.

Does it smell? I would get a moisture meter and test it.

Capillary action can wick moisture a LONG way. It gets water to the top of the redwoods, after all.

https://www.usgs.gov/water-science-school/science/capillary-action-and-water

Hi, Looks like a moister stain. cool on one side and warm on the other side. Doesn't look damaged. What I have done with water stains, when dry is get a can of spray shellack and apply it to the stain. when dry, about 30 minutes then paint.

Good Luck Woodbutcher

The staining doesn’t smell, even with my nose to it. And I dont think I will be able to find the source of dampness, especially if it is due to capillary action in the wood. The outside wall was insulated and vinyl sided a few years ago and is water tight. I will simply apply a couple of coats of sealer and then paint. Should last another 20 years. Thanks for all of the brainstorming and suggestions you all have peovided me.