How Stradivari Made his Purfling
Author's Note: The following article was commissioned by "The Strad" magazine in 2015. Shortly after they received it, the editors realized that they had already published a similar piece some years earlier and therefore could not use it! Here are the text and photographs that I submitted:
In 2013 a team of scientists led by Marco Malagodi at the University of Pavia announced that the black strips of Stradivari’s purfling contained iron (II) sulfate along with traces of sulfates of copper, manganese, zinc, and aluminum. The results of this research were presented in an article entitled “A multi-technique chemical characterization of a Stradivari decorated violin top plate” in the journal Applied Physics A: Material Science & Processing. Their analysis was carried out using energy dispersive x-ray fluorescence and Fourier-transform infrared spectroscopy.
Modern-day violin makers might be interested to know that iron sulfate (also known as green vitriol or copperas) was commonly used to make black dyes and inks dating back to the Middle Ages, and probably earlier. Theophilus Presbyter’s De diversis artibus (circa. 1100-1120 CE), a Latin treatise on painting, glassmaking, and metalwork, is one of the earliest writings to describe the making of a black pigment termed atramentum using “thornwood” extract and green vitriol (iron sulfate is green in color). The black color that results is due to a chemical reaction between the tannic or gallic acids in the wood extract and iron in the vitriol that is further enhanced by oxidation. Ebony has always been an expensive wood (it had to be imported from Africa, India, or Sri Lanka), and for musical instrument makers, it was cheaper to stain a wood such as pear for use in purfling, fingerboards, pegs, and the black keys of harpsichords and other early keyboard instruments. Pear is a fairly hard, fine textured wood, and the black stain penetrates sufficiently so that the color remains intense until the wood surface has worn down considerably.
Recipes for ebonizing wood using vitriol abound up through the twentieth century until aniline dyes became ubiquitous and replaced this earlier technique. One such recipe for the use of vitriol can be found in Spons’ Workshop Receipts for Manufacturers and Scientific Amateurs (London and New York, 1909). It states, to ebonize wood, “boil 1 lb. logwood chips 1 hour in 2 quarts water; brush the hot liquor over the work to be stained, lay aside to dry; when dry give another coat, still using it hot. When the second coat is dry, brush the following liquor over the work: 1 oz. green copperas (vitriol) to 1 qt. hot water, to be used when the copperas is all dissolved. It will bring out an intense black when dry. For staining, the work must not be dried by fire, but in the sunshine, if possible; if not, in a warm room, away from the fire.” Logwood (otherwise known as campeachy wood) was imported into Europe from South America as early as the sixteenth century. Decoctions of logwood chips were used primarily as a red dye for textiles, though the tannins in it react with copperas to produce a black stain. Another technique, and one that probably pre-dates the use of logwood and was undoubtedly cheaper, calls for pre-soaking or painting wood with a decoction of oak galls in order to raise the tannin level. Oak galls are plant growths, often formed on leaves, caused by insect infestation. They are not the only source of tannins, which are found in the bark, wood, and leaves of many trees, such as oak and chestnut.
Maple, poplar, beech, and black-stained pear wood are among the woods commonly used to make purfling. The early Brescian makers, Zanetto di Pellegrino, Gasparo da Salo, and Maggini, often used the bark of the fig tree for the white strips. In Cremona, Andrea Amati used maple, though for the blacks, the brothers Amati sometimes used ebony rather than stained pear wood. Nicolo Amati was the first of his family to use poplar for the whites, as did Stradivari. Beech wood was often used by the Ruggieris, as well as many of makers from Bologna, Rome, Genoa, Florence, and Naples, though the Neapolitan Alessandro Gagliano used poplar. For the blacks, other members of the Gagliano family often used dyed paper. The Venetians generally used poplar, though beech was sometimes used. Gragnani of Livorno used whalebone. J. B. Guadagnini sometimes used Acacia wood for the whites. It should be pointed out that American tulip poplar, or yellow poplar, is not a true poplar; its greenish heartwood turns brown and is thus inappropriate for making the white central strip in violin purfling.
Malagodi and his associates report that this technique was used at the Civica Scuola di Liuteria in Milan to make purfling for a copy of a decorated Stradivari violin top, though the unnecessarily complex technique they describe involves a preparatory boiling of the pear wood in sodium hydroxide and mixing ground oak galls with ethanol and acetic acid according to some unspecified “traditional recipe.” To make a typical Stradivari-style purfling, I planed strips of pear wood to a characteristic thickness of 0.3 mm (See figures), and then soaked them in a decoction of oak galls. I let the strips dry and then soaked them in a solution of iron sulfate. When dry, I glued the strips on either side of strips of unstained poplar 0.6 mm thick using hide glue. After the glue dried, purfling strips were cut with a knife and straightedge. The oak galls and iron sulfate were obtained from Kremer Pigmente in New York.