Old, old gray formulation?

[rob-cubed]

My mother recently discovered my old LEGO at the back of her closet! My first sets were from the mid-1970s and had the generic "stage extra" minifigs, and the later sets were all Castle-themed—including one of the first sets to use proper grays for the walls, 6073.

Going through my old hoard I noticed that some of them are a different shade of gray. Specifically, the ones in 6073. This specific set of bricks are yellower than the other old gray bricks I have... the difference is subtle but noticeable, this "old old" gray makes the other old grays look downright bley by comparison. I'm aware that this discoloration could be due to sunlight exposure or age, but these bricks are uniformly the same color throughout and all of the grays would have been purchased within a 5 year span in the early 80s and exposed to identical conditions.

Is anyone aware of a change in color formulation in the earliest gray bricks—before the obvious change to the later bley colors?

 

[AmperZand]

@rob-cubed, 

There isn’t an ‘old old’ grey as far as I know. It sounds like the kind of degradation from age that is accelerated by sunlight and/or heat. The reason it is uniform is likely because all the parts came from the same batch or formulation of ABS and additives. 

[rob-cubed]

Thanks. I figured it is likely age-related but it's just so odd that this one set is so different than others that all came from the same era. Unfortunately there were no whites in 6073 and the other included colors like red don't look any different.

[TeriXeri]

I had some old Space baseplates (Crater and Landing Pad from set 928 Galaxy Explorer) that discolored into yellow while not even being in sunlight for the last 10 years, meanwhile other old-grey road plates from around a similar age didn't discolor as much.

Straight Road Plates came on the market in 1978 (Europe), and the Galaxy Explorer in 1979 so the age should be very similar.

Also, the greys from the Galaxy Explorer itself hasn't yellowed as much as the baseplates either.

 

Edited October 31, 2019 by TeriXeri