Metal Casting Zone

Hi, I have been using #4 A shape clay graphite crucibles and they seem to hold up pretty well, really well for aluminum.I have begun to notice they dont like the higher temps for copper alloys.After 4 or 5 runs they begin to get small cracks on the outside like spiderwebs.They appear to get bigger and deeper with further use.(looks kinda scary)How do you know when scrap it before something ends up on the floor?

Even with aluminum it will eventually fail. It just takes longer. A quick way to check your crucible is just "ping"it with a fingernail before use, a thunking dull sound indicates a crack, don't use it or at least be prepared for imminent failure. A ringing noise or sharper tone means it's in one piece and okay to use. That surface checking doesn't neccessarily mean it's about to fail, as long as it passes the ping test.
Crucibles really should be considered a consumable item, at higher temps and with more aggressive cover fluxes they are just consumed at a faster rate.
You should also "anneal" your crucibles every now and then, heat it to red hot empty and let it cool slowly in the furnace, this relieves internal stresses and may help them last longer.

Just to add a little to OddDuck's suggestions.. It may not "Ping" well if you have some Al dross stuck inside the crucible. Try to clean it out the best you can. I found this with the clay crucibles I've been using for Al so, I imagine it's the same for others.
I stopped using a graphite crucible when a visible crack on the top rim extended down about a third of the way on the outside and I could see it on the inside to the slag line. I heard it pop after an Iron pour, I didn't get it back in the furnace fast enough.