CA is a moderately hard thermoplastic (a plastic that softens when heated and hardens when cooled). It can have very glossy, vibrant coloration, and good transparency, but it doesn’t hold its color over time as well as other plastics. Most (all?) LEGO CA was Bayer’s Cellidor. The very first LEGO bricks in 1949 were made from CA, and this use continued through the Town Plan and Samsonite eras. However, over time CA has a tendency to warp, often severely. Many of these bricks are now incompatible with modern bricks due to this deformation. LEGO switched to ABS around 1963, and fully phased CA out by 1970, with Samsonite in the USA as the last holdout.
[Original post and comments on Flickr.]