advice for cotton polyester baseball cap
In addition to not having iron, copper does not have cobalt or zinc (also magnetic metals). In fact, because copper is an element, those details are true by definition. Anyway, putting a bar of metal near a magnet will have no affect (answer A.). However, sliding magnets and copper past each other, specifically if you have a copper tube and slide a magnet through it, they will drag, or go slow. For more details/info on that, see the website under sources.
it really depends on fabric of your baseball cap, as we know ,most of baseball caps were made of cotton twill, this kind of baseball cap if washing many times , the color will fade quickly. The correct way of cleaning the cap, is to use the transparent tape to stick it again and again, this will remove the dirt. If you really need to wash it, just keep the cap strait on your hand and use wash it surface, do not put it all on water, because this will make your cap out of shape soon.
If it's really dirty or sweat stained, let it soak in a bowl of cold water with laundry soap (which is formulated to remove sweat and dirt from fabric, rather than dish soap, though that would work, too). The next day, change the water, add a little more soap, and scrub gently with a soft brush (an old toothbrush or a washrag works great) on the problem areas, like on the brim and sweat band. Rinse thoroughly with cold water and dry by putting it on a bowl or ball that is about the same size as your head so it doesn't get warped and odd looking (unless your head is warped and odd looking, then just let it hang). Set it in the sun so it dries completely. It's a hat, so it was meant to get wet from time to time, but the brim can be warped if mistreated by putting in a washer or not drying correctly. Personally, I like to keep my ball caps clean because I think the style of wearing an old, grimy, ratty looking hat is kind of gross unless you're actually a farmer or some sort of tradesman.
Depending on the fabric of the cap, (i.e. part nylon vs. full cloth cap) washing is the least of your problems. Avoid the dryer . . . they are kryptonite to caps. A gentle liquid laundry soap is just fine for a baseball cap. Unless, of course, the cap is seriously soiled. The biggest problem is drying it without shrinking or mal-forming the cap. They make cap holders that you put the cap in and put it into the dryer. I wouldn't do this at all, IMHO. I'd allow the cap to air-dry over anything else. It will slowly, but gently, dry the cap minimizing the chance that you'll deform the cap or shrink it. If it's a cap that is dear to your soul, I'd hand-wash it with Woolite or some comparable product used for delicates. I like the poster's advice of putting it on a bowl to help retain its shape. Most guys almost never launder a bb cap. Yea, we're pigs and slobs, but some of us like laundered items to wear. ;-)
Copper has no iron in it. It will be unaffected.
