Pertanyaan

The question is based on the following passage. Identifying ingredients in old paintings can help curators decide how best to maintain, display and restore them. The 17th-century Dutch artist, Rembrandt, was found to use wheat, according to a new advanced analysis of two of his works. It is the first study to identify wheat starch in any of Rembrandt’s work, even though scientists have performed numerous analyses on more than 150 of his. In Rembrandt’s time, artists mixed their own paints, which they then spread onto canvas in layers. Often, individual layers of the same piece of art contained different binding agents, pigments, varnishes and other ingredients. Besides color, each layer was mixed to just the right level of thickness, glossiness, texture, evenness on the surface, drying time and more. However, paint layers are tough to analyze because they are spread so incredibly thin. The thinnest ones rise just a thousandth of a millimeter above the layer below them. Using a variety of chemical and physical analytical methods, along with old written records, scientists have been able to identify pigments and other inorganic materials in many ancient paintings. For the new study, the scientists used some of the most high-tech equipment around to look at the “Portrait of Nicolaes van Bambeeck,” which Rembrandt van Rijn painted in 1641.First they took a cross-section from a miniscule section of the painting. Then they used a variety of methods to probe the layers, including a technique called Time of Fly -- Secondary Ion Mass spectrometry (ToF-SIMS). This technique involves sending a focused, high-energy beam of ions at the layered sample, then observing the ions that bounce back. By analyzing the energy and chemical nature of the ejected ions, scientists can deduce detailed information about the types of elements and chemical bonds held within. For the second greyish layer of paint on the “Portrait of Nicolaes van Bambeeck,” the scan showed Rembrandt mixed oil and a small amount of lead with wheat flour. It’s not clear yet whether Rembrandt used wheat earlier or continued to use the ingredient after painting the “Portrait of Nicolaes van Bambeeck,” who was a rich wool merchant. But the researchers also found wheat in the “Portrait of Agatha Bas,” the merchant’s wife. What does the word they in "… own paints, which they then …"(Paragraph 2, Sentence 1) refer to?