As many of you know, the STEM field are becoming really popular. So how can other fields keep up? One way is to integrate STEM subjects into your curriculum/studies. Below is a paper that I wrote regarding some of the chemistry concepts that can be found in ceramics. I did quite a bit of research on this, and not all of it was put in this paper, so if you have any questions or want more information, please contact me! Thanks, and hope all of you have a fantastic day!
Cheers,
Kaylee
The Chemistry of Glazes
Although chemistry and art are closely related, many artists do not have a comprehensive understanding of the chemistry behind art. In one field in particular, chemistry is imperative to the art-making process: ceramics. To first understand why art and chemistry are tied so closely, one must examine what makes up ceramics and glazes.
To form clay, rock is weathered down. The types of clay vary in name and composition based on how far they are from their original source. As clays travel farther from the source, they collects more impurities, such as iron and carbon. Clay with more impurities is more plastic, or moldable, than clay with less impurities. Clay can be divided into several groups based on their levels of purity/distance from the source. Clay coming from/found at the original source is classified as primary clay, while clay that travels downstream is classified as secondary. One example of primary clay is porcelain, which can be self-glazing when fired. Primary clay has large particles, a high refractive index (firing temperature) and has a low rate of shrinkage (Hill, P. S., and Marine, S. S., 2009). An example of secondary clay is ball clay, which has smaller particles, a lower refractive index, and a high rate of shrinkage.
Glazes are similar to clays in their composition, but they include a larger amount of elements to help them melt, which are known as fluxes. In addition, they have colorants that some clay doesn’t have. To form the colors of glazes, colorants are added, which are made up of metallic oxides. Metallic oxides consist of a metallic element and oxygen. Typical elements used in colorants are circled on the table below (https://digitalfire.com):
The colorants are affected in the firing processes as well as the mix of fluxes, silicas, aluminas, opacifiers, modifiers, and colorants. Fluxing agents serve the purpose of making the glazes vitrify, or turn to glass because they melt at a point lower than that of the silica. According to Rada (1964), “The body itself is affected while the glaze is melting, and it also partially melted. The point of vitrification of the body must therefore be higher than the melting point of the glaze to prevent softening or possible deformation on the body.”
Silicas and aluminas provide a stabilizing element to the glaze because of their higher melting points. According to Dennis Tobin’s handout titled Glaze Materials, “the alumina makes the glass thick and viscous so that it does not run off the surface.” However, having silicas and aluminas alone would render the glaze useless, because the melting point would be too close to the ceramic ware. As a result, flux is added. According to Rada (1964), “The body itself is affected while the glaze is melting, and it also partially melted. The point of vitrification of the body must therefore be higher than the melting point of the glaze to prevent softening or possible deformation on the body.” The melting points of clay bodies and glazes can be measured in kilns by keeping track of cones, which fall as the kiln becomes hotter. Different cones fire and fall at different temperatures depending on their compositions.
In the field of ceramics, glazes change when they undergo reductive and oxidative firing processes. Colorants are typically made up of metallic oxides, whose main purpose is to provide color to the clay. These colorants can change color depending on whether they undergo oxidation or reduction. For example, copper changes from blue to red depending on whether it undergoes an oxidation or reduction firing. In addition, due to variances in oxygen in the gas kiln, there may be certain places where the ceramic glaze is inconsistent in color due to carbon trapping.
Colorants may be toxic depending on the metals used, meaning that they are not to be used for food/drink-carrying vessels. For example, uranium is poisonous (so it should be used only for decorative wares), but other elements such as copper may be used as colorants for ceramic wares for food. These metallic oxides may change color when placed in a warming environment, such as the kiln. When in the kiln, these elements may also undergo reduction and oxidation reactions, making the number of electrons they carry change. This may be the scientific reason behind the glazes’ color changes as the elements change from cuprous to cupric ions, etc. One of the other things that may affect colorants in the replacement, or “doping” of elements with other elements. This can lead to electron imbalances and change the colors of ceramics (Carter, C., & Norton, M., 2013).
In addition, colorants react differently depending on what ceramic element they are used in. For example, colorants/metallic oxides that are used in glazes are better at different temperatures than those used in underglazes. However, as a whole, colorants are able to express a wider range of colors when fired at lower temperatures (Rada, 1964). Another expression of color to consider in glazes is the opacity of the glaze. Opacifiers make glazes more opaque due to their refractory particles. Refractory particles do not dissolve in a glaze when it melts, but they can also weaken the glaze in regards to weathering over time.
These glazes may also be combined, showing an example of subtractive color mixing. Subtractive color mixing is a process in which colors are mixed so that only certain light waves of colors are allowed to pass through. Because the glazes each subtract the light waves, they demonstrate subtractive color mixing. As a result, layering multiple glazes over one another may lead to a black color depending on how the colorants react with each other (Hill, P. S., and Marine, S. S., 2009).
Through reduction and oxidation firings, glazes undergo a series of chemical changes.The effects of reduction and oxidation effect glazes differently, especially depending on the colorant. For example, when using copper as a colorant, the colors will vary depending on whether the glaze is oxidized or reduced. However, the concentration of coloring pigments may affect the results of the firing. For example, in a study of the concentration of iron oxide, the following results were found:
From left to right: 7%, 6%, 5%, 4%, 3%, 2%, 0% of iron oxide
From left to right: 2%, 0%, 4%, 5%
Concentrations from left to right: 5%, 4%, 7%, 6%
For the highest iron oxide concentration, there was a red slightly showed around the edges/ breaks and there was a red/orange/tan coloring to the glaze. We had predicted this glaze would be red, but it seemed to be primarily a dark brown/black with hints of other colors.
For the 2% concentration of iron oxide, the glaze stayed true to the recipe and was a blue-green. Out of all of the tiles, this was the only one that was blue.
This was our tile to show a baseline for the glaze without colorant. We did this to see if the glaze had any natural color tendencies and to ensure that the batch was clean.
We had predicted that the glaze would start off as a blue-green color because after examining various glazes, many glazes with lower concentrations of iron oxide as the colorant, we saw a trend showing those with lower concentrations as being a blue-green glaze. The concentrations of around seven percent or higher seemed to show red rather than blue. As a result, we predicted that the upper concentrations would show a red glaze and the lower concentrations will be a blue. After analyzing the various glaze recipes available to be viewed, we felt that the lower iron oxide concentration would be blue, then green, then turning to an orange or red around five to seven percent. After about ten to twelve percent, we predicted that the glaze would become black. In addition to using testing the colorant concentration, we also did a glaze of the formula without any colorant.
For these test results, we found that at the glaze with the lower concentration of iron oxide (the colorant) was blue. However, as these increased, the color became black until the last tile, where the colorant was at a percentage of about seven percent or more. On the last tile, the glaze was black and starting to turn orange/yellow. One of the reasons why I think this may have happened was the way that the colorant reacted with the combination of the fluxes, alumina, and silica. In the original recipe for this clay, which consists of 25 g cornwall stone, 25 g EPK, 25 g whiting, 25 g flint, and 2 g of iron oxide. This makes the iron oxide about two percent of the glaze composition.
One of the reasons that the glaze may have been black instead of red is because the colorant was added into the existing glaze each time, not in an entirely new batch. This makes the concentration slightly off because the base glaze was not at a completely correct ratio to the colorant, but it should have been close enough to show a gradient. However, red is known to be one of the least stable glazes, but blue is more stable. The blue test tile turned out as expected, possibly because it was more stable.
One of the scientific reasons why the test tiles in the middle (around 3-6%) were a brown/black color may have to do with the properties of light waves. Glazes use subtractive mixing, meaning that each color subtracts light waves that are being reflected back at the viewer. By having both blue/green and red layered over one another, the light waves for both colors were being absorbed rather than reflected, causing the test tiles to be a brown/black color.
We predicted that the turning point for iron oxide would be around five percent due to observations of existing glazes. In these existing glazes, the red colors seemed to show up when the concentration of iron oxide was five percent and above, while it would cause the glaze to be green or blue in concentrations closer to two percent.
While there are some color changes that can happen in a kiln due to reduction and oxidation, there are a few other phenomena that occur, including bloating. Bloating is the blistering or bubbling of a glaze on a ceramic piece that occurs in the kiln. In the book “Salt-Glaze Ceramics,” Mansfield states that bloating may be avoided in salt glazing by soaking the kiln at 900 degrees Celsius for three hours. As a whole, some aspects of the firing process can be predicted and controlled, but there is also some elements of predictability that can be understood through science.
Through studying things such as colorants, ceramicists are able to better understand the reasons why certain chemicals create certain glazes. However, one aspect that must be considered is that the glazes are not self-contained entities; the glazes may react differently with each firing due to differences in clay bodies and the kiln atmosphere. By understanding the components of each element in the ceramic-making process, we are are able to better understand how they affect each other as well.
Sources:
- Rada, P. (1964). The Book of Ceramics. London: Spring Books.
- (n.d.). Retrieved December 15, 2017, from https://digitalfire.com/4sight/properties/ceramic_property_glaze_color.html
- Parmelee, C. W. (1948). Ceramic Glazes. Chicago, IL: Industrial Publications, Inc.
- Hill, P. S., & Marine, S. S. (2009). The Molecular Basis of Color and Form: Chemistry in Art.
- Mansfield, J. (1992). Salt glaze ceramics: an international perspective. London: A & C Black Limited.
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