Phenolic yellowing
A yellow patch exactly the shape of the rug that was lying on it. The carpet made the colour itself, because that patch could not breathe.
In detail
Yellowish discolouration found under area rugs, flat legless furniture and anything else that sits directly on a carpet without a gap. It is named for BHT — butylated hydroxytoluene — an antioxidant present in the latex adhesive of many carpets, and the mechanism is that the covered area gets no air. Deprived of ventilation, a reaction proceeds that produces the yellow. Almost everything about it misleads. The shape is a perfect outline of the object, which reads as a spill. It appears where nothing is being done wrong. And it does not respond to cleaning, because it is not soil — the carpet's own chemistry produced it, like the yellowing of uPVC, where the material changes rather than collects. The first thing to try costs nothing: remove the rug or the furniture and let the area breathe, which can gradually restore the colour on its own. Where it does not, a professional can apply a specific treatment for it. The prevention is a gap — matting or feet that let air move under whatever is placed on the carpet.
Why
- Phenolic yellowing is explained by Damage versus soiling
Sources for this page
Sources
Consumer Floor and Furnishings Care Information: carpet topics
Institute of Inspection, Cleaning and Restoration Certification (IICRC) · Trade or professional body
Relied on for: That yellowing under area rugs laid over carpet is known as BHT or phenolic yellowing, that carpets containing BHT in the latex glue require breathing space, that flat legless furniture or rugs stop the covered area receiving enough air and cause the reaction, and that removing the object and exposing the carpet to air can gradually restore colour. (states this directly)