[Student Projects, Glassmaking]
Redox Reactions of Pigments in Molten Glass
The purpose of our project is to observe the changing conditions of a flame to get different colors from certain pigments. More specifically, we will be adjusting the oxygen level in the flame to see if we can produce oxidation and/or reduction reactions. Whether by reduction or oxidation we will manipulate the torch's flame as well as the pigments being used to extract different colors.
A measured amount of each pigment put into the glass dish
Glass rods are cut into two pieces
Flames are lit, and the heating begins
After rods are heated, the molten glass is dipped into the pigment(s) then put back into the flame.
Observations of the pigment's reaction to the heat are noted and the procedure is repeated a number of times
No particular shapes were intentionally made, but rather the reactions and activity of the pigments were the main points of focus
Using Oxygen/Gas torches allows a glassworker to control the gas to oxygen mixture in the flame being used. This also makes it possible to produce oxidizing or reducing flames depending on the desired effect. This ability to control the oxygen content of the flame allowed for experiments testing redox reactions in pigments within molten glass.
The amount of oxygen present in a flame can be determined simply by color. The more orange a flame is, the less oxygen it has, making it a reducing flame. By that same token, the more blue in a flame, the more oxygen it contains thus making it able to oxidize the pigments. To begin experimentation MnCl2 was used. These experiments served two purposes: First, some practice is necessary to reach a skill level that makes proper experimentation possible. Secondly, since the effects of oxidizing MnCl2 were already known from the previous glass lab we were able to make sure that the flame actually oxidized the pigment. Here is one of our initial experiments with the MnCl2 by itself. Notice the dark purplish pigment in the glass. The MnCl2 appears this way due to the fact that the glass containing the pigment was heated in an oxidizing flame, one which contained a significant amount of oxygen compared to gas.
As our technique improved we began to get better results. By heating copper2+ chloride, which produces a dark green color, under a reducing flame we were able to produce copper+ which gave us a bright red color. What is interesting about this is that copper2+ is the more stable of the two states. When copper+ is heated it tends to oxidize to copper2+ so to get the reaction to go the other direction is quite fascinating.
After our initial success with the copper 2+ we decided to combine the copper with sulfur and put the two under a reducing flame to see if the sulfur would prevent the copper from reducing as it had when it was combined with the manganese in our first experiments. The results were as expected because most of the copper was a dark green showing that it had not reduced, but there were traces of red proving that if the sulfur had not been present the reduction would have taken place . We also combined the copper2+ with the MnCl2 this resulted in a dark purple, the manganese, with traces of red, copper+, and green, copper2+.
Other interesting aspects of our experiments were the different reactions
of each pigment to the molten glass as well as the flames given off by each
pigment when they were being heated.
Manganese was used by Egyptians to color glass as early as 1400 B.C.(Weyl) Today it is not only used as a coolant but also as a decoloriser. Iron which is present in glass sometimes causes a green color; manganese oxidizes the iron as weelas masking the green with its own color.
The deep purple that is associated with the mangense pigment comes from the Mn3+ ion. It is not this ion alone which is responsible for the colorization, however. The number and state of polarization of the surrounding O2- atoms affects the Mn3+ causing the color to change. When mixed with different elements, the manganese can yeld colors ranging from blue to red.
By now, you have heard us mention a great deal about reducing and oxidizing reactions when heating and cooling glass and pigments. We do not expect everyone to know exactly what we are discussing, so with the help of a couple of references, we will try to translate it in a simple fashion.
Oxidation is the loss of electrons. Conversely, reduction is the gain of e-. These two processes are complementary to each other, meaning that without one the other cannot take place. We learned this first hand when started off by heating the glass in an extremely reducing flame then changed it to an oxidizing one, and vice versa. For example, when the copper chloride by itself was heated in a predominantly reducing flame, the result was a dark green. Here are some other results of pigments that were heated in a reducing flame.
We can conclude that the amount of oxygen in the flame affects the oxidation
state of the pigment in the molten glass. This conclusion was reached because
different colors were achieved from each pigment at different levels of
oxygen. We can also conclude from this experiment that the temperature
as well as the length of time a pigment is in a flame affects its color
and texture in the end. In addition, we learned just how stable or unstable
certain pigments are in their oxidation and reduction states.
Dunham, Bandhu Scott. Contemporary Lampworking: A Practical Guide to Shaping Glass in the Flame.
Salusa Glassworks, Arizona. 1997.
Hammersfahr, James E. and Stong, Clair L. Creative Glass Blowing. W.H. Freeman and Company, San
Paul, Amal. Chemistry of Glasses. Chapman and Hall, New York.
Weyl, W.A. Coloured Glasses. Society of Glass Technology. England
JEFF FOSTER AND GRAHAM GREEN, 1998
LINKS TO OTHER SITES:
List of Suppliers of Glassworking Materials
A Glossary of Glass Terms
The National American Glass Club
Some examples of Glassworking