Grin reagent is a catalyst that is extremely sensitive to a grin, and will react to it to create another grin. This allows a p12 oscillator to be made:
A similar reaction can also be achieved with the eater bridge eater, but its fast recovery time disallows a P12 oscillator, and also disallows period-multiplying reactions like the one shown in 65P48 weld.
The name of this object uses "reagent" in a nonstandard way. A stable structure that recovers without permanent damage after an interaction is more commonly called a catalyst. By a similar analogy to chemistry, reagents in Life are usually active reactions that assist in converting another object from one form to another, but are used up in the process.