Recently we got a question from a customer who asked if all our Venetian Beads are annealed. She had heard that annealing makes the beads stronger. What is annealing?
The word comes from the metallurgy and materials science where it has long been recognized it increases the metals ductility and makes it more workable to heat above its critical temperature and then to cool which also relieves the internal stress of the metal.
At left, large muffola in a production furnace. At right vermiculite with beads (you see only the mandrels)
This is where it comes into the glass world. In glass, it is about mainting the glass at a constant temperature, known as soaking at the annealing point where the glass is too cold to continue working, but will continue to relax, so that the entire piece can gradually cool at the same rate. If the glass were left in the open, you can easily see that the outside would cool faster than the inside. Annealing keeps the temperature constant throughout the bead.
For centuries, the Venetians have annealed by:
1) flash annealed, where the bead is put back into the flame one final time to heat the bead to a consistent temperature and
2) slowly cooled the bead in a container of silica which helps hold the temperature. In older days, the Venetians used the sand from the lagoon which is called "fango" in Italian and was also used for the "bead release". Today the world uses vermiculite for this process. Vermiculite is a mineral similar to mica which is fire proof.
The stresses set up in the bead have a lot to do with the amount of time it takes to make the bead, the decorations, etc and frankly the skill level of the bead maker. In our small beads, they are made rather quickly and there is very little value of annealing other than by use of the vermiculite.
For our larger, obviously more expensive, beads we do use an annealing oven - called a muffola in Italian. For our Moretti glass, the annealing temperature is about 940F and it is important to keep them at this level and then bring down gradually. For our blown beads they are all annealed, here you see the small kiln beside the furnace.
Annealing does not make the bead stronger, it insures that there are no stresses set up in the bead during the cooling cycle. Tempering makes the glass stronger - which is the process used for car windows, shatter-proof glass, etc. Below is a type of annealing oven, a kiln produced in the US.