The manufacturing process is to have the bottom portion of Lucite holding the chip and then molten Lucite is poured over it. The heat of the molten Lucite is enough to melt the outer surface of clay composition chips and when the whole mess cools, the Lucite and chip are fully fused!

In some examples (not necessarilly the paper weights shown) you can see where the Lucite temeprature was a bit higher and some of the chip material is mixed with the Lucite, giving a fuzzy appearance to the chip. Sometime, the heat is enough to warp the chip!

Jim