I have a question about the word "exert" in this context. I look it up in the dictionary that "exert" means to use power, etc. to affect sth. But I still do not get this sentence in red color. A pigment should not use a harmful chemical reaction to affect what ? I do not get it !
Thanks for your help.
Professor: ...When a pigment is mixed or ground it in a liquid vehicle to form paint, it does not dissolve but remains suspended in the liquid. A paint pigment should be a smooth, finely divided powder. It should withstand the action of sunlight without changing color. A pigment should not exert a harmful chemical reaction upon the medium, or upon other color pigments it is mixed with.
A pigment should not affect the medium, or other color pigments with a harmful chemical reaction.
Take, for instance, physical exercise - you could describe the exertion process as using power/energy/calories - as to exert oneself in such a manner one requires fuel, and therefore the use of something. But here the pigment is performing more of an influencing role, causing something to happen.