ANALYSIS AND REMOVAL OF DEPOSITS FROM THE IRON AGE CERAMICS OF MAPUNGUBWE

Cari Anya Ensio Art Conservation Program, Queen’s University

Many of the earthenware artifacts recovered from the archaeological sites of Mapungubwe in South Africa are covered in disfiguring deposits. In 2008 / 2009, research was undertaken at Queen’s University in order to analyse the deposits and explore chemical methods of removal. The study revealed that the deposits are primarily composed of calcium carbonate and quartz and may be removed using controlled acid treatments with no adverse effects on the ceramic substrate.

INTRODUCTION:

EXPERIMENTAL:

RESULTS AND CONCLUSION:

Mapungubwe: Located in the northernmost region of South Africa, Mapungubwe is the site of South Africa’s earliest known civilization. The kingdom of Mapungubwe flourished from 900 AD – 1300 AD and is considered the most complex early African state in southern Africa. After its fall, the state was largely forgotten until the early 1930s, when archaeological material was discovered on Mapungubwe Hill. Subsequent excavations of the area have yielded thousands of artifacts. These artifacts are housed at the Mapungubwe Museum at the University of Pretoria, Pretoria, South Africa.

Materials:

Samples:30 potsherds from Mapungubwe Hill and K2, on loan from the Mapungubwe Museum

Chemicals: Acids: 1M acetic acid 1M formic acid 1M citric acid 1M 1,2,4-triazole 1M sulfamic acid Poultice: Carboxy methyl cellulose (CMC) Techniques:Polarized Light Microscopy (PLM) X-ray fluorescence (XRF)X-ray diffraction (XRD)Variable pressure scanning electron microscopy - energy dispersive x-ray spectrometry (VP-SEM-EDS)

Methods:

The deposits are primarily composed of calcium carbonate and quartz. They form a discreet layer atop the ceramic substrate and range from soft to compact in structure. With the exception of 1,2,4-triazole, all of the acids effectively removed the softer deposits when applied using cotton swabs and when applied in poultices. None of the acids effectively reduced the compact deposits when applied using cotton swabs, but formic acid, citric acid and sulfamic acid did reduce the deposits to varying degrees when applied in poultices.

Based on the results, a 1M acetic acid solution applied using cotton swabs may be recommended to remove the soft deposits, and a 1M solution of sulfamic acid in a CMC poultice may be recommended to remove the more compact deposits with no apparent adverse effects upon the ceramic substrate.

Left: Location of Mapungubwe in South Africa. Right: Mapungubwe Hill Earthenware vessel from Mapungubwe

Sherd 13 before removal of deposits Sherd 13 after removal of deposits

50 um

Earthenware figurines from Mapungubwe

The Ceramics: The museum’s collection contains hundreds of earthenware figurines and vessels and thousands of potsherds recovered from the archaeological sites of Mapungubwe. Many of the ceramic objects are at least partially covered in disfiguring and potentially damaging deposits. The deposits disguise the finishing, colours, decorative details, and workmanship of the ceramics. A variety of methods, including mechanical cleaning, wet cleaning, and steam cleaning, have been employed in the past in an attempt to remove the deposits; however, these past efforts have met with little success.

Thin cross-section at 100X in transmitted light

Stage I: Analysis of deposits

• Creation of thin cross-sections for analysis by PLM• Creation of particle mount slides of deposits for analysis by PLM• Analysis of deposits, ceramic by XRF• Analysis of deposits by XRD • Examination of 3 sherds by VP-SEM-EDS

Testing grids before and after testing

Stage II: Removal of deposits

• Seven sherds with deposits selected for testing• 10 test areas (each 1 cm x 1 cm) marked on each sherd• Pre-testing documentation of test areas with photomicrographs• Pre-testing analysis of test areas wih VP-SEM-EDS

• Each acid applied to one test area using cotton swab, one test area using CMC poultice• Test areas cleared with distilled water after treatment• Post-testing documentation and analysis of test areas with photomicrographs and VP-SEM-EDS

VP-SEM images before and after testing

The Project:Preliminary testing in the summer of 2008 to explore alternative methods of deposit removal suggested that acids could be used to remove or reduce the deposits. During this testing, a citric acid and CMC poultice was proven to dissolve the deposits, but it was unclear whether the treatment adversely affected the ceramic. Thus, a two-stage research project was undertaken at Queen’s University in 2008 / 2009 in order to analyse the deposits and explore the use and effects of five different acids on Mapungubwe ceramics.

Acknowledgements: Thanks to all who contributed to the project, including Sian Tiley-Nel, Hazel Botha, Marilyn Laver, H. F. Shurvell, Alan Grant, Jane Sirois, and Robert Waller.