Bronze is best known as an alloy of copper and tin, but copper alloyed with aluminum, arsenic, manganese, phosphorous, silicon, etc., can also referred to as bronzes.
Yes, it is, see this 2010 paper Eating artificially ripened fruits is harmful. Calcium carbide is extremely hazardous to the human body as it contains traces of arsenic and phosphorus. It is banned in many countries of the world, but it is freely used in India, Pakistan, Bangladesh, Nepal and other countries. Thus we are at risk of short-term and long-term health effects simply by eating fruits that are induced to ripen. Calcium carbide CaC2 is used to easily generate low levels of ethylene gas which can hasten the (outside at least) ripening of fruits like bananas, mangoes etc.
Place the plums in a clean paper bag. Any paper bag will do, but it should be empty of all contents except plums. When plums (and other fruit) ripen, they emit ethylene. Placing them in a paper bag with the top folded over keeps the gas close to the plums, speeding up the ripening process. An even quicker method is to put a ripe banana in the bag with the plums. The extra ethylene produced by the banana will cause the plums to ripen fast. Don't place the plums in a plastic bag. Using a nonporous bag will prevent fresh air from entering, and the plums will end up having a funny taste.