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The Origins of Yellow Staining

By Robert G. Wilkin, P.E.

CBI recently completed a project to recoat the stone on a 200-year-old mill building.  The project was designed under the requirements of the Massachusetts Historical Commission, so the recoated stone needed to match its previous appearance.  The stone on this building softens when wet, unlike Portland Cement, the most common type of cement used today.  The stone had a very old thick lime wash applied many years ago before Portland Cement was available, and had be coated with many things over time. 

By the time CBI assessed the building the newer coatings were peeling and flaking off of the soft, friable lime wash that remained.  During the project the remaining materials were taken down to the stone and the joints were cleared out and pointed.  The stone was then coated with an elastomeric latex coating that restored the building

Several months after completion, for no apparent reason several stones were exhibiting yellow stains through the coating.  The building manager applied a small amount of rust stain remover, the type used to remove rust stains on porcelain tubs, and the stains disappeared.  It turns out that these stones contained iron and the iron bled through the coating.  A stain killer was applied and the stones were recoated to stop the stain recurrence. 

An interesting point in this vein has to do with my washing machine.  Rust formed on poorly coated edges around the top-loading door opening.  Because of the rust, yellow stains were appearing on white clothes.  I tried the rust stain remover on the shirts, carefully, since I wasn’t sure what it would do to the fabric.   It worked great, taking out the stains with no damage at all on the fabric.  I coated the edges around the door with silicone sealant and that seems to have stopped the rusting, at least for now. 

It is always interesting to tie what we learn in the field with what we learn in our lives outside of work, and vice versa.