Photoacoustic Analysis of Natural Indigo, Palygorskite and Synthetic Maya Blue
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Photoacoustic Analysis of Natural Indigo, Palygorskite and Synthetic Maya Blue Carlos Aldebarán Rosales Córdova1, Antonio de Ita de la Torre1, Rosalba Castañeda Guzman2 1 Área de Ciencia de Materiales, Universidad Autónoma Metropolitana UAM. Av. San Pablo No. 180, Col. Reynosa Tamaulipas, Mexico DF C.P. 02220, Mexico. e-mail: [email protected] 2 Centro de Ciencias Aplicadas y Desarrollo Tecnológico CCADET, Universidad Nacional Autónoma de México UNAM, A.P. 70-186 Mexico D.F C.P. 04510, Mexico. ABSTRACT Maya blue is an organic / inorganic pigment which is composed principally by two elements: a white clay known as palygorskite (in the Mayan dialect Sakalum), and a blue dye called indigo, this dye is extracted from the plant Indigofera suffruticosa. The mixture and the warming of these two elements produce the Maya blue, which exhibits unusual features such as: resistance to the assault of the acids in warm or high temperatures, his persistent color in spite of having been exposed to different climatic conditions as a result of the passage of time and the specific chemical composition that this pigment presents. In the present investigation natural indigo, palygorskite and synthetic maya blue were analyzed with a new implementation technique called photoacoustic analysis, which detects the structural changes that happen in the material under a controlled increase of temperature. Due to the fact that this technology detects with clarity where the structural changes happen but not that type of changes happen, it used as base termogravimetric analysis. One of the important findings, it was that in the spectrum of the synthetic maya blue were detected the structural changes of the clay and the dye, something that with other used technologies had not been achieved to observe. INTRODUCTION Merwin in 1931 discovers a pigment with a blue tone turquoise on having done an expedition to the temple of the "Warriors", which is located in the archaeological zone that today is known as Chichén Itzá, in the state of Yucatan [1]. In 1942 - one year later - the above mentioned pigment was baptized as Mayan Blue by Gettens [2], given the combination of the tone - color - and the representative local civilization where it was discovered. From this year, it began the study of the Maya blue analyzing the paintings that Merwin had found. The first question that arose for the different investigators and scientists was: which are the elements that compose this characteristic pigment? It was then determined that the pigment is formed by a singular association between clays and vegetable inks, being a result of the mixture of two principally components: Paligorskite ―clay― and the dye called indigo contained in the leaves of the plant Indigofera suffruticosa. Palygorskite is a fibrous clay which chemical formula is [Mgi2S4O10(OH) 4·H2O], its average dimensions are between 0.1 and 2 mm, with a width between 100 to 300 Å, it has a structure anisotropic with porous channels and dimensions of 4.1 Å of width x 10.5 Å of high and 16.2 Å of depth.
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