Timing the market is usually a futile practice. Ive been trying for two years and got lucky. Emphasis on lucky. Generally, early Spring and Fall will be best unless the metals market needs to replenish stock for some reason (usually heavy building/construction).
As for metal types - first is ferrous (magnetic) and non-ferrous (non-magnetic). Non-ferrous is worth more per weight. Most common non-ferrous metals are; Aluminum, Copper, Lead and Brass. There are several grades of each and price depends on dimensions. Stainless steel can be almost any degree of magnetic; from not at all to very.
Any scrap yard should have published prices. Get a sheet or copy them down. Ask questions; they dont mind. Ask if they publish a guide on how to prepare stuff by size and any special conditions. For any long pieces of iron (includes steel), youll have to decide if its worth the time and expense to cut it down so it'll be worth more overall. If a piece has several types of metal, you might want to just cut off the nonferous sections that are easily separated
Bookmarks