Solid-State NMR Studies of Ultramarine Pigments Discloration
- PDF / 1,322,180 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 10 Downloads / 199 Views
0984-MM07-13
Solid-State NMR Studies of Ultramarine Pigments Discloration Eleonora Del Federico1, Jacob Newman2, Lindsey Tyne1,3, Cyndi O'Hern1, Licio Isolani4, and Alexej Jerschow2 1 Department of Mathematics and Science, Pratt Institute, 200 Willoughby Ave, Brooklyn, NY, 11205 2 Chemistry Department, New York University, 100 Washington Square East, New York, NY, 10003 3 Fine Arts Department, New York University, The Conservation Center, The Stephen Chan House, 14 East 78th Street, New York, NY, 10021 4 Fine Arts Department, Pratt Institute, 200 Willoughby Ave, Brooklyn, NY, 11205 ABSTRACT Ultramarines are a family of pigments widely used as colorants in artists’ paints, coatings, plastics, cosmetics, and various industrial materials. They are aluminosilicates characterized by a sodalite cage framework which enclathrate paramagnetic (S3-., S2-.) and diamagnetic (S4 or S3Cl) chromophores responsible for the color of these pigments. Solid-state 27Al MAS NMR studies showed that the fading mechanisms in ultramarine pigments, both in acidic and alkaline environments, is initiated via de-alumination leading to framework destruction which in turn triggers the release of the chromophores. This results in color loss and in the emergence of extraframework aluminum. 29Si MAS studies provide new insight into these processes and suggest that acidic attack leads to the formation of Q1-Q3 silicates and possibly to the emergence of Si(3Al), Si(2Al), Si(1Al) and Si(0Al) fragments indicative of the formation of secondary pores through which the guest chomophores can leave the cage. These findings are important for the design of proper conservation treatments and preservation procedures for artwork containing ultramarine pigments. INTRODUCTION The structure of ultramarine pigments is characterized by SiO44- and AlO45- tetrahedra, which are arranged in a sodalite-type cage forming a cuboctahedral framework, also known as the "βcage" and with the generic formula [Al3Si3O12]3- . The simplified structure is displayed in figures 1a through 1e. Strong paramagnetic (S3-., S2-.) and diamagnetic (S4 or S3Cl) sulfur chromophores are encapsulated within the cages and are responsible for the color of these pigments. Figure 1c shows the enclarthrated S3-. chromophore. The replacement of each Si (in SiO44-) by an Al as AlO45- in a silicate framework introduces one negative charge per each Al atom present. These charges are generally neutralized by Na+ ions which are also encapsulated in the cage (1-2). In sodalite, the paramagnetic chromophors are replaced by Cl- ions.
Figure 1. Sodalite structure (β-cage) showing, a) corner-connected aluminate and silicate tetrahedra , b) simplified version of a), showing only Al and Si in corners, c) S3-. chromophore center and d, e) packed cubic structure.
Historically, ultramarine pigments were extracted from the semiprecious stone lapis lazuli, and were highly regarded by Medieval and Renaissance painters as it provided a vibrant blue color unmatched by any other pigment at the time. Because ultramarines
Data Loading...
