I'm trying to detect lead in my apartment. A wastewater pipe got replaced and the plumbers didn't think to put up any kind of barrier. There is now an impressive layer of metal soot across the entire unit. The pipe is cast iron but I know they are often sealed with lead. So I bought one of the 3M instant lead tests which use tartaric acid. If there is any lead present the swab should turn red/pink. The excessive amount of iron just blackened the swab so I dropped the buffer onto the dusty surface instead.

This picture doesn't do the color justice but the colored specks are pink in real life. In pretty much every case the manual says the entire droplet/surface should turn color. There's nothing about isolated specks. Would this be the result of insoluble lead? Is there any other phenomena that could explain it? If there's a chance it's lead I want to have it sent to a lab. And for scale this droplet is less than 1cm across