The Science of Work Hardening and Annealing

I had the wonderful opportunity a couple weeks ago to work with Iowa State University Program for Women in Science and Engineering K-12 Outreach Program. They have several opportunities throughout the year for young girls to experience activities that create an interest in science, technology, engineering and math. So they contacted the ISU Memorial Union Workspace where I teach metalsmithing classes and we developed an activity so they could make something and learn something and hopefully even gain some self confidence!

I chose Foldformed Copper Bangles as their project. They turned out great!

These are samples I created for the class

Each of the 54 participants (ages 8-14 in 4 groups) received a 6x1 inch strip of 26 gauge copper. I used 26 gauge because I wanted to be sure that the youngest girls could be successful hammering and unfolding their bangles.  It worked great and if we did it again I think we will use 24 gauge, although 26 is a bit cheaper.

Now the Science

Copper is an element found on the Periodic Table of Elements.  It is a noble metal. It is made up of atoms that line up in a regular pattern that repeats, forming a crystalline structure.  Copper can be soft and it can be hard. Metals can be made stronger by deforming the crystalline structure of the molecules. This is what happens when you hit copper with your hammer. You change the molecular structure of the copper and make it hard. --Work Hardening --  To make the copper soft again, you Anneal the copper.

So as you hammer away on your beautiful copper, you notice it gets stiffer, harder to bend and the sound of your hammering will become higher pitched. The molecular arrangement of the copper has been deformed thus making the copper stronger. If you continue to hammer, the copper will become harder but also brittle. This is why paper clips are easy to break. The metal is hard, but brittle. I also learned from the ISU Engineering Students who were helping the reason why the Titanic broke into two pieces. Apparently the metal that formed the ship was too hard and brittle and that is why the ship broke in two and sunk. wow.

I later watched a documentary on the Titanic and not only was the metal too brittle but the rivets of the Titanic were made of a weaker iron in the front and rear of the ship. When the ship hit the iceberg with the bow, the rivets did not hold and six seams came apart on the ship. The weakness came because there was a larger amount of slag in the iron that made up the rivets. Slag weakens the metal and in tests, metal with a higher amount of slag will not stretch as far and is more brittle than metal with less slag. I also watched a NOVA documentary about the making of viking swords. It was very interesting and helped me to understand more about work hardening and annealing and what happens at the molecular level of the metal. A sword is made of steel and worked by a blacksmith but educating yourself on a variety of art forms only adds to your own knowledge. Also the blacksmith was from nearby Wisconsin and I am now a fan. His knowledge and workmanship was remarkable. Here is a link to the NOVA episode: http://www.pbs.org/wgbh/nova/ancient/secrets-viking-sword.html

Annealing is the process of heating your copper up which helps the copper molecules get back into their nice, tidy, crystalline structure and like magic it is soft again.  I don't know about you but this blows my mind.  I love how you can work with copper over and over and over again and as long as you are careful while annealing you can work with it indefinitely.

When you anneal the copper, turn off your lamp that you use in your torch area. Dim light helps you see the color change of the copper, and that tells you the temperature of the copper. Heat your copper to a dull red. Keep it at a dull red for 10-15 seconds. To do this, keep moving your torch around and over your piece. I have to move the flame off the piece so I can see the color of the metal. You will see that the copper will first start to turn dark and then the dull red. When you move the flame away the dull red will quickly disappear as the metal instantly starts to cool. Immediately put your flame to the metal again, making passes over the metal and watching the color. Then turn off your torch and quench your piece.  Be careful though because dull red can quickly turn to bright red and that is too hot.

If you heat your copper to bright red you risk making your copper very hard -the complete opposite of annealing! So watch the color change very closely. Practice on scrap and have fun!