Soda ash yellowing silver

Perhaps someone with a chemistry background can help. Our students
usually neutralise their silver in sodium bicarbonate after taking it
from the pickle. Recently they have been neutralising in soda ash
(sodium carbonate) and their silver is going bright yellow.
Occasionally I have seen a faint yellow using sodium bicarbonate but
this is quite bright at times. What is happening? Perhaps it is
contaminated with something?

Would appreciate your technical help.
Regards,
Valerie