If one had a large tub of old LEGO brick, with some pieces going back to the 1950s, but all mixed together, are there any distinguishing features or appearance to the 1950s cellulose acetate bricks compared to the newer ABS plastic? Do the plastics look different somehow? Or would you just have to look for other features like lack of studs, flat bottoms on 1x1 round pieces, etc?
LEGO switched from cellulose acetate (CA) plastic to ABS plastic in 1963. They continued to distribute CA pieces in sets until their existing supply was exhausted, so during the transition beginning in 1963 both types of pieces were sold in sets. In North America, LEGO pieces were manufactured beginning in 1961 under license by Samsonite, which continued to distribute CA pieces mixed in sets until about 1970.
Distinguishing CA from ABS pieces based on intrinsic criteria is something of an art.
The first thing to look for is warping. CA plastic is much more prone to warp than ABS, which is perhaps the main reason the switch was made. If you find an old piece that is warped, odds are it is CA. You can check for warping by placing a piece on a flat surface and checking for any "wobble." Another good method is to build a simple brick wall with the pieces and look for gaps or poor connections.
Generally CA plastic is much shinier than ABS plastic when it is in good condition. CA pieces will lose the shine with wear, so this is not always possible to see.
There are also some tell-tale clues in the colors.
- Red CA pieces have more orange than the ABS version.
- Yellow CA pieces are not as bright as ABS pieces.
- Blue CA pieces are a brighter shade than the ABS version.
- Black, white, gray and clear CA pieces are harder to differentiate. Transparent CA bricks can be somewhat brownish, while later transparent bricks, made from polycarbonate, can have a bluish cast.
- Green CA bricks are very scarce, but some green plates are CA.
As 62Bricks has said, this can be an art.
Based on my experience (in the UK):
CA bricks tend to have less clutch (stickiness to other bricks) than ABS bricks. I'm not sure if this is because they are more prone to wear, because they tend to warp or just because they're old and worn out. Nevertheless, some of my CA bricks are so non-grippy that they're all but useless.
Due to their age, CA bricks have older logos and are made using older moulds. For this reason, they typically have an older LEGO font on the studs, for example.
CA bricks are more translucent than old ABS bricks. (Although recent ABS bricks can be somewhat translucent, too).
Many parts have only been produced in the post-CA era, so will only exist in ABS. Similarly, some older parts have never been produced in ABS, so will only exist in CA.