Sciendum igitur est

You ought to be aware that many people, when they see that the colour is getting shiny, make a kind of glair which they call „thin glair“ or „weak glair“, which has a half-volume of water mixed with the white of egg and beaten together with it. And since it is leaner, because of the addition of water, a shiny colour which has previously been fattened on the stronger liquid is well tempered with it.