Catechol (aka pyrocat) and Pyrogallol are developers which will not only form image silver, but also form a brown stain wherever silver was developed. This gives some degree of image amplification or less grain for same density. Historically Pyrogallol was prefered, but since it is very toxic people moved to Catechol.

There are many resources dedicated to pyrocat developers which list mixing and developing instructions together with properties of their concoctions.