[COLOR="Green"]Wasn't it Tony Williams who once, upon being asked if his (whatever) cymbal was a crash or a ride, replied :
"a cymbal...is a cymbal"
???
I think so.
Not being a wiseass, here. You want an 18" Avedis which works as well as a crash as it does a ride. The 80's ones certainly still have their labels. But most pre-mid 70's ones were stamped neither; or if they were, those logos are long gone.
So...OK, you have the cymbal, the stamp to ID its era.
You have to ask about the weight, next.
A thin 18" will be around...1200g. This would be a thin crash. You couldn't ride it, it'd be hella washy and lose the stick tic fast.
A heavy 18" would be 1800g. This is a heavy cymbal....it'd make a lousy ride, probably, except for a headbanger....it'd be clangy and pingy. It likely wouldn't make much of a good crash either, but maybe as a rock crash it'd be OK. Only Turkish handmades can pull off a heavy 18" and make it sound good, IMHO.
So...if I were to take an educated gamble on a 70's or earlier Avedis 'crash-ride' cymbal of 18"...I'd look for something about 1450-1650g in weight. Then ask the seller if it works better as a crash or a ride.
You want a ride which crashes, basically. This is a generalization, BUT....as a rule, in order for a ride to ride, it needs to have some weight to it....but in order for a ride to crash, it can' be too heavy or have too little taper from shoulder to edge.
I have had heavy rides which don't sound heavy and which have decent crashes because most of their weight is in the bell and bow, and very little toward the edge. But these are the exception rather than the rule. You sorta luck out when you land something like that.[/COLOR]
[COLOR="Gray"]This reminds me of a story...I once landed an 18" Ajaha (UFIP stencil) cymbal which made me really happy...because I had a pair of 'hats, and needed (um, wanted) an 18.
I figured I could use it as a ride. I tried every stick on it imaginable, and it just sounded so damn dead. Nuthin'. Not responsive. Just muddy. I decided it was a dog and put it in my closet.
I would take it out now and again, wanting to like it....to no avail. Then one day.... I just whacked it ...hard.
It was an awesome crash. Fantastic...big and full with a nice quick decay. So it came back out and I used it solely as a large crash ![/COLOR]