Hadar Jacobson's metal clay comes in powder form. All you need to do is add water to the portion you want to use. No need to worry about the shelf life of the rest. Mix what you want when you want. This clay is very forgiving, pliable and soft. It mixes quickly with no mess.
This product is best fired in the absence of oxygen to prevent oxidation. Cool Tools offers many kits to help achieve this environment.
Did you make too much? Take a look at our clay hydrator; this tool can keep unused portions moist and usable for long periods of time.