Copper is a naturally occurring metal whose ore has been mined and smelted since the beginnings of metallurgy. In fact, until about 3000 BCE, copper was the sole metal used in crafting tools and vessels with only occasional pieces of lead, silver, or gold being found. Beginning in the Early Bronze Age (3000 BCE-2600 BCE), which dates closely to the period of Tuval Kayin (Genesis 4:22), many metallurgical concepts were developed, among them the formation of bronzes. Bronze began as an alloy (a blend) of copper and arsenic, with arsenic eventually being replaced by tin. Brass is a rarer alloy of copper and zinc (zinc being less common than tin). By adding varying amounts of these other metals to the copper as it was smelted, the alloy’s properties could be adjusted to optimize workability, hardness, color, and finish. Arsenic and zinc deposits may have occurred together with some copper ores and when such ores were smelted they would form a natural alloy of either bronze or brass.
Thus nechoshes, in a general context, most likely refers to the pure copper. When describing vessels or tools (such as those used in the Tabernacle and Temple, both of which post-date the development of the copper alloys) it would tend to mean bronze or, less commonly, brass, as these were the alloys of choice for their advantages over pure copper.