Physical Analysis

Pigment Analysis

To conduct an analysis of the residue of the paint pigments I collaborated with conservation scientist Prof. David Scott, the Founding Director of the UCLA/Getty Conservation Program. With Prof. Scott’s aid I conducted XRF (X-Ray Florescence) analysis and polarized light microscopy tests to identify the materials used to create the pigments. We conducted tests three different pigments: a green, a red, and a pink pigment.

       

   

The first pigment tested was the green pigment. To do this we took a small sample from the artifact and conducted an XRF analysis of the sample. The results of the test were that the minerals used to create the pigment are celadonite and glauconite. These two minerals are the chief components of a pigment called Green earth. Green earth was discovered by the Greeks in antiquity and was readily available in the Mediterranean world, but was mainly used in late Greek-Egyptian art and in Roman art. One of the most famous mines which produced Green earth is near Verona, Italy and has been discovered in jars in pigment shops at Pompeii.

       

Figure 1

 

Figure 2

   

The second pigment tested was a red pigment residue. For this test we conducted both XRF analysis and polarized light microscopy. The polarized light microscopy test (Figure 1) was conducted to determine if the red pigment was organic or mineral based. To be able to conduct an XRF analysis, the material being tested must be a mineral based material, so it was important to confirm that the sample was a mineral first. By using this test, we confirmed that the material used to create the red pigment was in fact a mineral. The next step was then to conduct an XRF analysis.

The results for the XRF analysis are shown in Figure 2. From this test we discovered that the red pigment is hematite based pigment. Hematite is red iron oxide mineral and is the principal mineral used in the creation of red ochre. Hematite gains its name from the Greeks and is derived from the word hema which means "blood", so Hematite means "bloodstone" because of its deep red color. Hematite is considered amoungst the oldest of all pigments and was used by every major civilization.

       
       

Figure 3

Figure 4

Figure 5

Figure 6

   

The next pigment tested was a pink pigment. Once again we conducted both polarized light microscopy and XRF tests on the pigment. In the polarized light microscopy tests (Figures 3 & 4) the source material seemed to be a mineral based pigment, but the tests were not completely conclusive.

We then ran an XRF analysis of the material. The results for the two samples are found in Figures 5 and 6. Once again these tests were not conclusive. There is a possibility that the pigment is goethite based. Goethite is an iron oxide based mineral and was used in the ancient world in pigments. Goethite is a hydrated iron oxide that changes colors as it is heated. The higher the temperature the redder the mineral becomes. This color change is directly proportional to the level of the dehydration of the mineral.

The original tests conducted were inconclusive so further testing was undertaken. Prof. Scott and I ran the pink pigment tests again and we determined that this pigment is not made from minerals. It is a paint made from organic materials. To determine the materials used, I will be working with Ioanna Kakoulli, a professor at UCLA and scientist at the Getty Villa by conducting Reflectance and luminescence (fluorescence) imaging tests. These tests use photons to determine what oranic material consists of. This should provide some results so we can determine what was used to create the pink paint.

From the tests in Figures 2, 5, and 6 we are able to determine that the binding agent for  these pigments is gypsum. Gypsum, also known as hydrous calcium sulphate, is the most common sulphate mineral and derives its name from the Greek word gypsos meaning "plaster" and was most likely developed for use by the Greeks. Gypsum can also be found in Egyptian pyramids and is reffered to in ancient cuneiform texts written by the Assyrians. Based on these tests we concluded that the pigment materials were mixed with a mixture of gypsum and a medium such as water, natural resins, glues, animal tallow, blood, casein, eggs, urine, oils, wax, or human saliva. This mixture would have then been applied to the terracotta figurine and left to dry.

       
The tests that were run have allowed me to create some digital reconstructions of the objects original appearance. I was able to use the data to identify colors and hypothesize what the object would have looked like.
       
 
    
   
   
   
       

Terracotta Construction

 

Terracotta figurines are clay figurines made from molds on a massive scale. A negative mold, called a matrix, is made from a hand-shaped prototype called the patrix. Only the front part of the mold is shaped in Egyptian terracotta figurines and the back mold is curved and smooth. The front and back of the figurine would have been made separately and then joined before firing and the edges were smeared with clay. A hole was cut in the back of the figurine and the bottom would have been left open. The hole could have been a firing hole, where moisture could escape during the cooking process or it could have been used to hang the objects on a wall or to secure them in one particular place. After firing, a white coating was applied to the front surface and then they were often painted.

By studying the figurine under a microscope, we were able to determine that the mold used contained straw or grass as a binding agent. It would have been used to keep the clay together so the terracotta could have been successfully created. We can see under the microscope indentations on the figurine where grass or straw impressed on the terracotta before firing and is now preserved in the texture of the object.

 
       

Bibliography

Rapp, George. Archaeominerology. 02/2009. Natural Science in Archaeology. University of Minnesota Department of Geological Sciences. Deluth, MN.

http://www.mindat.org/min-1856.html

http://www.naturalpigments.com/detail.asp?PRODUCT_ID=450-31S

http://www.naturalpigments.com/detail.asp?PRODUCT_ID=460-50S

http://www.mindat.org/min-1784.html

http://www.awci.org/cd/pdfs/8503_c.pdf

  • Grant Dixon