It can happen, and if the congo red is negative, I know that sirius
red procedure is sometimes used. When doing modified Puchler-Sweat congo red
method, and using saturated solutions, it is important that the
solutions ARE saturated, and follow instructions exactly.  The congo red
solution shelf life for the method is 1 year.

Do you look at the congo red with polarized light, and are the sections
thicker than 5 micrometers?

Out of curiosity, what is the method you use?

Gayle Callis