Background
The Chemistry of Art and Artifacts 65.470A
Edited 31 July, 2014
This is a course that was started in the 1908s, and was given as recently as about 2011. The course was designed for Art History students, but in fact it was mostly taken by Gallery and Museum Technicians. No textbook was available, so “The Chemistry of Art and Artifacts” was written for this course. It is no longer in print.
The level of chemistry in the course is fairly low, but deals with unusual topics, such as the nature of colour.
Some Chemical Background
Here are some initial topics with which most students will find familiar. Atoms, Ions and Molecules The world of chemistry is made up of atoms. These, in turn, are made into molecules, crystals, and the more common things of daily life. Elements are the pure building blocks of chemicals, and contain only one kind of chemical atom.- there are now about 110, of which about 40 are important Compounds are made of several different kinds of atoms, bonded into molecules. Atoms are very small – about 2 – 10 x 10-10 metres in diameter. 6.022 x 1023 atoms in what is called a “mole” 1 atom of carbon weighs (about 2 x 10-23 grams) so that a mole of carbon atoms weighs about 12 grams. The “atomic mass” of an element – 12 grams, in the case of carbon – is the mass of one mole of atoms The “molecular mass” of a compound is the sum of the atomic masses of all the atoms in the molecule. Atoms are made up of a nucleus – protons and neutrons – which has a positive electric charge and electrons, which have a negative electric charge. The nucleus is very small (100,000 times smaller than the atom, and lies at the centre of the atom. The nucleus carries most of the mass of the atom. The electrons fill the space surrounding the nucleus, and are held in place by the attraction of the negative to the positive charges. Some typical atoms are, with their normal chemical symbols Hydrogen H Carbon C Oxygen O Nitrogen N Iron Fe Molecules are composed of groups of atoms – sometimes only two, other times many thousands. These molecules are held together by what are called “chemical bonds”. These are really wayward electrons attracted to the positive charge on the nucleus of a neighbouring atom. The reality is, of course, much more complicated, but that catches the essence. Some typical molecules, with their chemical formulas, are: Carbon dioxide CO2 Water H2O Ammonia NH3 While molecules are electrically neutral, many of the interesting molecules are electrically unbalanced, or “polarized”. That is, the electrons are drawn toward one atom of the molecule, so that one end of the molecule may be a little bit negative and the other end a litte bit positive. This is important, because these molecules attract each other more strongly than unpolarized molecules. Important polarized molecules include most molecules with OH groups, such as water, alcohols and sugars. Ions are atoms or groups of atoms that have lost or gained one or more electrons. The sodium atom, for example, has 11 electrons. If one of these gets lost, and the atom now has only 10 electrons, there will be a net electrical charge of +1. The atom is now called an ion – actually a cation because it is positive. Similarly, the chlorine atom normally has 17 electrons. If it gains an electron, and has 18, then the atom has a net charge of – 1. This is now called an anion, because it is negative. Two oppositely charged ions will attract each other and form an ionic compound. In this case it is NaCl – sodium chloride, or common salt. Some typical ionic compounds, with their formulas, are: Calcium carbonate CaCO3 Sodium nitrate NaNO3 Calcium phosphate Ca3(PO4)2 Acids and Bases The most common acids are those that, when dissolved in water, dissociate into hydrogen ions and a corresponding anion. Examples will be Hydrochloric acid Hcl which gives H+ and Cl- Nitric acid HNO3 which gives H+ and NO3- Sulphuric acid H2SO4 which gives H+ and HSO4-and SO4-2 To counteract acids, there are bases which dissociate in water to give Hydroxyl (OH-) ions and a corresponding cation: Sodium hydroxide NaOH which gives Na+ and OH- Calcium hydroxide Ca(OH)2 which gives Ca+2 and OH-Ammonium hydrxide NH4OH which gives NH4+ and OH-. Solubility and solvents. Solid substances, when placed in liquids, will often dissolve. Familiar examples will include sugar and salt dissolving in water. These will dissolve up to a limited concentration, usually expressed in moles per litre or grams per litre. Two quite different principles are applied to molecular solids and ionic solids. Molecular solids dissolve best in liquids of similar polarity. Sugar will dissolve in water, while grease will dissolve in gasoline. For dissolving paints and varnishes, solvents can be ranked as to strength. This is described further in “The Chemistry of Art and Artifacts”. This ranking is of considerable importance in the cleaning of paintings. Ionic solids dissolve best in water (sometimes with the help of an acid). Here the solid dissociates into its ions to the point where the product of the concentrations of the ions (the “solubiity product”) reaches a maximum depending on the individual compound. Understanding of the solubility product is important to understanding the action of acid rain on some building stone. This is described further in “The Chemistry of Art and Artifacts”.