If I were to make a coloured pencil for example, It needs materials like kaolin and wax and so on for it to work as a pencil. It also needs pigment, and the more pigment the stronger the colour is. Problem is if I scimp on the kaolin and wax the pencil will be crumbly and not sharpen well, so I put in the most pigment that I can without the defects happening. This is good quality and the art material has the right balance between extenders and other materials and the pigment. If I were to look at the cost of the pigment and realise that it costs more than wax and kaolin I might decide that 5% more kaolin will give me 5% more profit per pencil. The necessary ingredients have now become adulterants and the artist has a product that is not as good as it could be. It happens, but the point here is that extenders and fillers are necessary often and their presence is often the mark of good workmanship, but that sometimes they are overused for profit reasons.