Is annealing really just a simple matter of heating the metal and letting it cool?
I think this is the first question that comes to mind for someone brand new to working silver and other metals for jewelry design. I know it was for myself.
One important benefit to annealing is that the process makes the silver easier to handle during the bending and shaping process.
Using a higher gas to oxygen ratio and not using the flux will result in very little oxidization with the higher acetylene ratio.
Annealing is the process of making smaller crystal structure in the metal. The larger the crystal in the metal the harder it is to bend and the more likely it is to crack or break. Since metal crystals grow in size based on their cooling conditions, the the size is controlled with the speed which you cool it.
Fast cooling produces small more uniform crystals, slow cooling produces larger more interlaced crystals. THus heating to red and then quickly quenching produces fine crystal structure. Heating metal up and then letting it air cool produces the more brittle material (larger crystals). Holding it at he crystallization point for a long time will grow much larger crystals. (See link below.)
You would normally never anneal a piece after it is complete as that makes it the most soft. You might anneal a part of something that will be forced to flex as that will make it less likely to break, but it’s still only a temporary fix.
Metals also do what is called work hardening. As you hammer, pend or compress the metal it will slowly make it more brittle. Hence you often hammer (as in forging) for a time, then anneal to return it to a softer more more pliable state, hammer again, anneal again, etc. etc.
Just heat the metal to cherry red ?
Metal is a crystalline structure, so when you heat it, the crystals expand, forcing gaps in between the microscopic crystal cells. When you polish, hammer, or even with age, metal crystals compact, making it hard. If you heat it to cherry red, you damage the structure. Yes, it will be soft, but over doing it will cause possible damage that will become evident while working the metal later on. I used to do this, and hammer blows would crack the soft metal or force air pockets into it.
The ink in a Sharpie marker dissipates at the correct temperature for sterling to anneal. I dip my metal into a mixture of alcohol and boric acid, light it, and wait for the flame to finish spreading the boric evenly over the piece. However you can use whatever flux you are familiar with; some just use a charcoal block without any flux. Really, it doesn’t matter since annealing, shouldn’t get close to firescale temps.
Then while the piece is warm and dry, scribble on it with a black Sharpie marker. Heat with a fluffy yellow flame till the marker dissipates and the black marks vanish, then quench in water or pickle. The quench is important to get the softest silver possible. I was told to do use the marker till I get used to watching for the flux to glisten and there to be a slight red glow that can only be seen in the dark. But, since I have great lighting in my studio, I just keep a Sharpie in the pocket of my apron. I guess the Sharpie is my crutch, but I have never over-annealed a piece since.
In reverse, if you have a project, and you want to harden the metal, as in a ring band or chain, you can place it in a kiln at 600 degrees for about an hour, and the crystalline formations will compact, the longer the harder.
From Ganoksin bulletin board
Sterling silver is at its softest when it is annealed at 650C (1202F) then immediately quenched, at this point it’s hardness is 56 Vickers and its tensile strength is 300 Nmm -2. If it is air cooled it will be only slightly harder at about 60 Vickers due to the difference in crystal structure.
Some people confuse air cooling with the age hardening process where sterling silver can be greatly hardened by heat treating. To do this you first heat the sterling to 750C (1382F) and hold it at this temp for 30 minutes then rapidly quench the item is then heated to 300C (572F) for 1 hour. this will result in a sterling piece that has a hardness of 110-120 Vickers and a tensile strength of 350 Nmm -2. The heat treatment can only be done on items that are NOT soldered as 750C will melt all silver solders if the item has been soldered prior to treatment and soldering after treatment will anneal the item and remove all the hardness gained.
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