The way ABS cement works is to actually melt the ABS material it's contacting, just like welding. "Old" ABS cement has had too much of the acetone (that actually does the melting) evaporate. Up to a point adding a little acetone and mixing it well will renew it. The "point" is where its no longer liquid enough to easily mix. At that point, you can save it, but, to me, it's not worth the effort if I have to dissolve large chunks. The stuffs not that expensive.