We are going to cover some rather technical terms like: alloy, temper, hardness, gauge and finish. Please stick with me through this brief article and you will really know what kind of copper you need for your application. Alloy. Copper sheet, foil, bar, rod and tube is available in many different alloys. These different alloys give certain physical properties to copper it doesn’t usually have, but those new properties often bring new limitations as well. What makes the alloy discussion a little easier is that 99% of the copper sold in the United States is represented by the three alloys listed below.Most of the copper sold today in the United States is alloy C11000, also known as Electrolytic-Tough-Pitch. Plain and simply, C11000 is 99.9% pure copper. It is an excellent electrical conductor, makes a fine looking kick plate on your front door, does a great job as a back splash in your kitchen, makes a very classy looking rain gutter and adorns some of the finest looking roofs in the world.Copper used for tube and pipe is usually alloy C12200. You get this alloy by adding a very small amount of phosphorus to pure copper. This makes the copper easier to weld and braze. Unfortunately, it also makes the copper considerably less conductive. Generally speaking, you rarely see this alloy being used for electrical applications, but if you need to weld or braze, this alloy is your best option. If you are going to turn or machine copper, C14500 is your alloy. If copper is alloyed with a small amount of tellurium it greatly improves the ability of copper to “make chips". Pure copper (alloy C11000) tends to be “gummy" (I apologize for using such a technical term) when machining. This gumminess means you have to slow the machining process down quite a bit. Pure copper is also hard on machine tools, which adds to the expense of machining. C14500 is not quite as conductive as pure copper, but it is relatively close.An interesting fact about copper alloys, if you add a little zinc to the metal - you get brass.Temper. Basically, the Temper of a metal refers to its hardness. The two extremes of Temper are denoted as “hard" and “soft". While the Copper Development Association has installed a numbering convention H01(soft) to H04(hard), most people in the industry simply refer to the degree of hardness. “Hey, I’ll take some ‘quarter-hard’ copper sheet," is really all you have to say to your local copper mill* to call out the Temper.So, why does anybody even worry about Temper? To a large degree, the hardness of the metal determines the application. If you were to make a beautiful pot rack out of soft Temper copper bars and chose to hang your grandmother’s favorite cast iron frying pan on your new creation, the weight of the frying pan could well deform the rack. Bottom line, if your copper project involves supporting any kind of weight, stick to harder Tempers.On the other hand, if you are interested in making a copper etching or simply forming the copper by hand, you would be much better off with a softer Temper. Softer Tempers are also easier to cut as well. For decorative and craft applications that do not require supporting much weight, soft Temper should be your choice.One final point about Temper, because of the end use of the product, copper bar is generally available only in the harder Tempers and copper sheet in softer Tempers. This has to do with how the two different forms of the metal are used.Gauge, Oz. and Inches. Calling out the gauge of the metal is another way of saying the thickness. It is important to keep in mind that with gauge, the larger the number the thinner the material. 30 gauge copper sheet is much thinner than 16 gauge. To make matters even more confusing, the roofing industry measure copper thickness in ounces per square foot! 16 oz copper sheet is a very common size of roofing copper. Finish. If you have ever walked in the lower level of a fancy department store and happened to see the gorgeous copper pots hanging in the kitchen department, you have seen an unnatural copper finish. Unless copper is treated with a synthetic lacquer, it will not have that deep mirror like finish. Also, if that lacquer finish should ever crack (even a very small one) and air reaches the copper, it will discolor fairly rapidly.Generally, the finish of copper bar and sheet will provide a dull reflection. Occasionally, the metal will also have a slight waviness to it, although it will be very smooth. Over time the metal will darken with exposure to air. The oxygen in the air is actually corroding the metal. Over a number of years a greenish patina will form on the metal. Most people find the character gained by copper through this corrosion process to be quite attractive. As a matter of fact, you can actually purchase copper that has been treated with chemicals which greatly accelerate the corrosion process. If you want that greenish finish on that copper bay window roof right away, not a problem, chemical engineering has provided the product for you. If you would prefer more of a grayish patina, that’s available as well.Congratulations, you now know the important factors in specifying copper bar and sheet. Good luck with your project!*I am being sarcastic about your local copper mill. There are only two full line copper mills in the US. If you want to go direct to the source, bring your check book. The minimum order is at least 5,000 lbs.