New Light on Robert Boyle and the Discovery of Colour Indicators

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New Light on Robert Boyle and the Discovery of Colour Indicators
  AMB¡X, VOl. 27. yatt 3, \ovemDer rgöo NEW LIGHT ON ROBERT BOYLE AND THE DISCOVERY OF COLOUR INDIC^A.TORS Bv Wrr,rmu EeuoN* Rosnnt Boyle's Exþerirnentø|, History of Col,owrs (1664) is rightly considered a la¡rdmark in the history of analytical chemistry. In that work, Boyle described a number of simple but elegant experiments which ultimately gave chemists a powerful tool for determining the composition of compounds. He found, for example, that the common acids turned theblue syrup of violets red, and that alkalis turned it green. Subsequent experiments with other vegetable juices, including privet berries (Ligustrwm tulgare), buckthorn berries,(Rhamnus cathørticø), cornflowers (Centauris cyønis), and turnsole (Crozoþhora. tinctoria),revealed similar colour changes when treated with acids and alkalis.l Although some of these reactions had been noticed earlier, Boyle was the first to recognize the signiûcance of these substances as chemical reagents, for he alone observed that al,l acids turned such bluevegetable juices red, and lhat øIl alkalis turned them green. Furthermore, he noticed that some substances caused no colour change; these Boyle classified as neutral, neither acid noralkaline, and thus "effectively disposed of the theory that all substances were one or theother".2 Recent scholarship has shown that an extensive literature on the analysis of aqueous solutions existed prior to Boyle's work, notably in the writings of physicians on the medi- cinal virtues of spa waters. Professor Allen Debus, for example, pointed out that Edward Jorden (r569-t6gz), an English physician trained at Padua, observed that bases turnedscarlet cloth blue, while acids turned it red again; the colour change of tincture of violets and of rose leaves in the presence of oil of vitriol was also noticed in the early seventeenth century.8 Curiously, these writings are not mentioned by Boyle, and as Debus suggests, they do not seem to have made any noticeable contribution to Boyle's work on colour indicators.a Another possible inspiration for Boyle's researches, however, has not thus far been seriously considered, and that was the empirical information accumulated by dyers and painters through generations of experimenting with colour changes. That Boyle should have observed these techniques in the workshops of dyers and colour-makers is entirely consistent with his professed adherence to Baconian philosophy. Indeed, if the scientifrc revolution of the seventeenth century was¡ a matter in part of thinking in new ways aboutolder data, Boyle's methodology in this instance is a remarkable example of it. It is not generally known that most of the colour changes that Boyle observed were noticed, and were induced methodically, long before anyone thought of using them as chemical indicators. Medieval dyers and painters knew empirically that plants could be made to yield a wide range of colours, depending on the season of the year in which they were collected and the mordants that were used with them. Unripe buckthorn berries, for example, yielded a fair but fugitive yellow that was used by painters to enrich greens, by leather dressers, and by bookbinders to colour the edges of books.ó If the berries were I Assistant Professor o{ History, New Mexico State University, Las Cruces, New Mexico, U.S,A. -\n earlier version of this paper was presented at the Midrvest Junto of the History of Science Society,Madison, Wisconsin, r r April r98o. Research for this article was supported by a grant from the Arts and Sciences Research Council, New lVlexico State University.  ROBERT BOYLE AND THE DISCOVERY OF COLOUR INDICATORS 2r05 picked Iater in the season, when fully ripe, then mordanted with alum, they produced a much deeper green called sap green , which was widely used by painters as a substitutefor verdigris, and by dyers.o These craftsmen also knew that certain colour changes could be induced chemically, for very few of the vegetable extracts in medieval technology were used in their natural state. With the exception of safiron (the dried stigmas of' Ctocussaliaas), which required no chemical modification, all the plant dyes and pigments weremade by combining the natural juices of plants and flowers with some acid or basic material to develop their colour. With such initial preparation, and by treating the textiles withdifferent mordants, a variety of shades and sometimes even different colours could be obtained from a single dyestuff. Medieval handbooks on dyeing and manuscript illumination give us only partial accounts of the complexity of these arts, but many examples of the colour changes induced on organic materials may be found. The dark blue flowers of iris, for example, appear to be an unlikely source of a green colour. When its juice is first squeezed out, it is purplish in colour, but as soon as it is combined with alum, a clear and beautiful green is formed.  Iris green was the chief rival of sap green in manuscript painting, and wa-s widely used in the fourteenth and frfteenth centuries.? Orchil, a dyestuff derived from several varieties ofthe lichen Rocellø,was used primarily as a purple dye, but its colour ranges from deep red to blue according to its pH. Blue orchil dyes were made by treating the solution with fermented urine or slaked lime to increase its alkalinitf, while acids had a reddening effect.8 This same colour change is characteristic of ma,ny of the other natural red dyes. Brazil, for example, tends to vary from a yellowish colour in strong acids to blue in the presence of alkalis. A recipe in Gioanventura Rosetti's dyeing handbook, thePlictho, is based on this empirical knowledge: a solution of brazil made with vinegar dyes cotton red, whereas the addition of urine to the sediment of the brazil gives the dye a bluish cast.e Perhaps the most versatile colouring agent in the Middle Ages was folium, which wÍu; prepared by soaking pieces of linen cloth in the purplish juice of the turnsole plant (Crozoþhora tinctoria),and hanging the clothlets out to dry.lq When the dried clothlets, or folia, were soaked inwater or glair, a bright red colour would dissolve into the medium. This colour could be heightened by adding vinegar to increase the acidity of the solution. If the linen had flrst been treated with enough lime water to neutralize the natural acidity of the berry juice, however, the clotilets produced a violet colour. The limed clothlets, ûnished as violet, could in turn be made to yietd a somewhat fugitive blue colour by treating them with urine, thereby increasing their alkalinity. Turnsole was thus an extremely versatile colour. It was a late medieval innovation, but once it became established around the early four-teenth century, it became extremely popular because it provided dyers and painters with a convenient and labour-saving source of colour. The dried clothlets, prepared in advance and stored for later use, were always ready to use without elaborate preparation. Almost any vegetable juice could be prepared in this way, and many others were tried, but the turnsole colours proved to be the most satisfactory and enduring.ll It is safe to assume that the late medieval practices described here were still in common use in Boyle's day. Although several important new dyestuffs were introduced into Europe with the discovery of the New World, the materials and techniques of medieval dyeing and colour-making retained their place in the workshops until the eighteenth century.rz In 1662, William Petty presented to the Royat Society his Apparatus to the History of the Common Practices of Dying , a report on current techniques which outlined practices  206 WILLIAM EAMON similar to those recorded in ñfteenth and sixteenth century recipe AlthoughPetty may have consulted some of these works, he had little need to do so: the son of a clothier, Petty knew his subject at frrst hand, and his Apparatus bore the mark of one who was intimately familiar with the workshop.Boyle's interest in the mechanical arts and the crafts is well known. Like many of hiscontemporaries in the Royal Society, he believed that the trades might be improved by scientifrc understanding, and also that science itself would be adva¡nced through thecollaboration of natural philosophers with artisans.l{ One of the essays inhis Useful'ness of Nøtural Philosoþhy was devoted to the proposition that the Goods of Mankind may be much encreased by the Naturalist's Insight into Trades .1ó In typically Baconian fashion,Boyle insisted that oftentimes from those, that have neither frne Language nor ûne cloaths to amuse him with, the Naturalist may obtain informations, that may be very usefull to his design .ro Given Boyle's long experience of collaboration with London craftsmen, it is probable that some of his experiments on colours and colour indicators were suggested by inquiries and observations he made in the workshops of dyers, painters, and colour makers, In fact, Boyle says as much in the Exþerimental, Hislory of Colours. Many experiments , wrote Boyle , may be afforded us to this PurPose by the dyers trade. l? He learned in the dye-houses, for example, that cloth being dyed blue with woad, is afterwards by the yellow decoction of . . . wood-waxli.e., dyer's broom, Genistø tindoria] dyed into a green colour .l8He conversed with a master dyer to learn how the scarlet dye wa-s made,le and he read an Italian book on dyeing to discover how to make løccø.zo In another instance, Boyle noted that oil of tartar turns verdigris blue, and of this experiment he wrote:The hint of this Experiment was given us by the practise of some ltøl'iøn Painters, ñne Azure (as they call it) by Grinding me other Saline Ingredients, and lettingile together in a Dunghill, we suppos'd, Verdþease by this way of Preparation,olatile and Alcalizate Salts, abounding in some of the mingled Concretes, and brought to make a further Dissolution of the Copper abounding in the Verdigrease . . .sl Boyle apparently learned of the specifrc colour changes produced in vegetabie juices by the action of acids and bases through his reading of John Parkinson's popular herbal, Thealrum Bolanicum, a translation of Rembert Dodoens's Kruyileboecå (1554). In that work,Parkinson discussed the uses of buckthorn, privet berries, turnsole, and other plants as dyestuffs. Upon reading Parkinson's discussion of buckthorn, Boyle rushed to a notedcolour-shop to inquire how sap-green was made: I bought them by Questions to confesse to me, that they made their Sap-green much after the ways by our Botanist here men- tioned .a Parkinson's lengthy account of the turnsole colours also impressed Boyle, so much so that he quoted it in full in the Exþerimental Hislory of Col,ours.z¿ Although he does not attest to it in his writings, it is quite conceivable that Boyle even got the idea for making strips of test papers saturated in vegetable dyes from the folium technique devised by medieval As far as we know, no dyer realized that his empirical knowledge of colour changes coulcl be used as a tool for scientifrc research. That was exclusively Boyle's contribution, and it took an entirely new way of thinking about the data to do it. If dyers could produce  ROBERT BOYLE AND THE DISCOVERY OF COLOTIR INDICATORS 207 difierent colours from a single dyestuff by the addition of acids or bases, Boyle reasoned, it should be possible to use these colour changes as a test for the presence of such substances. Certainly Boyle's methodology in this instance is an admirable example of what FrancisBacon had in mind when he invited the virtuosi to compile histories of the trades. Baconinsisted that the aim of the projected histories was not just to improve the arts, but also to furnish data from the workshops relating to the alteration of materials. From such data, the principles and axioms of the new natural philosophy could be drawn. It would be an utter mistake to suppose that my intention would be satisfred by a collection of experiments of arts made only with the view of thereby bringing the several arts to greater perfection ,wrote Bacon, my meaning plainly is that all mechanical experiments should be as streams flowing from all sides into the sea of philosophy .2o If Boyle's experimental investigation of colour indicators represents one of the fruits of medieval craft empiricism, it also exposes the chief limitation of empiricism unaided by theoretical understanding. Techniques in the manual arts changed only through a long process of trial-and-error. Without theory to guide him, the artisan could do little more than try one technique or ingredient, then another, in his effort to reach perfection.z? Boyle was fully aware of this, and he repeatedly expressed his conviction that scientific principles applied to the manual arts would promote industrial improvements. In particular, Boyle noted in the Exþerimentø|, History of Colours that: it may afiord a considerable Hint . . . to know what change of colours mayalready often mention'd, (some or ot reasonable Rates) in the Juices, Dec soluble parts of Vegetables. And,improvement of Knowledge, not of intimate here, That the BIew Liquotwentieth Experiments, are far from being the only Vegetable Substances, uPon which Acid, Úrinous, and Alcalizate Salts liave the like Oþerations to those recited in those two Experiments.æ Although this particular design proved to be impractical, Boyle's vision of applying scientificknowledge to industry was quite prophetic. One of the first fruits of the rapid develop- ments in organic chemistry, two centuries later, was the synthesis of aniline. Almostimmediately, aniline dyes began to replace natural dyes, and by the end of the nineteenth century, practically the only surviving natural dyestuff was litmus (derived fuom Rocel'trø), in the form of our familiar laboratory test papers. RBrrnnucrs r. Robert Boyle, Etþerimcnts anil, Consiilcrøtiotis Touching Colours, London, t664, pp. 245-88. e. Marie Boas, Roberl Boyle ønil Sevenleenlh Cenlury Chemislry, Cambridge, 1958, p. r35. Cf. Boyle, oþ. cit. (¡), pp. zS&g. 3. Allen G. Debus, Solution Analysis Prior to Robert Boyle , Chymio, 8, 4r-6o, 196z. See also idem., Sit Thomas Browne and the Study of C,olour Indicators , Ambir, 10, z9-36, 196z; Ge¡not Rath, Die Anfänge der Mineralquellenanalyse , M¿ilizinìsahe lglonalsschritl, S, S39-.4r, 1949; and  Die Mineralquellenanalyse im 17. Jahrhundert , Sutlholfs Archia, 41, t4, 1957. The history of indicators in the eighteenth and nineteenth centuries has been written by A. Albert Baker, Jr.,  A History of Indicators , Chymia, 9, 147-67, 1964.  2c,8 WILLIAM EAMON 4. Debus, Solution Analysis (ll, SZ. 5. Daniel V, Thompson , The Materiøls anil Techniquas of Medicaal Pointing, 1936; repr, ed., New York, 1956, p. r87. Cf. Mary P. Merriñeld, Original Trealìscs on the Arls of Paínlíng, 1849; repr. ed., New York, 1967,ll, pp. 42r, 683; Cennino d'Andrea Cennini, Tha Craftsman's Høndbooh Il Libro d¿ll'Att¿ , trans. D. V. Thompson, 1933; repr. ed., New York, 196o, p.3z; John Gerarde, 1/¡¿ Hatbalt, or G¿netall Historic of Plønles, London, 1633, p. r33ó; and L[eonard] M[ascall], A þrofitøble bohe,London, 1583, pp, 53-4, Rhømnus was occasionally used rxi a source of yellow dye, but thecolour was so fugitive that it never achieved great popularity, there being many other cheaper and more durable yellows, in particular, weld (Røsado luteolo); cf. Edward Bancroft, Exþerimentøl' Re- seøches Çoncerníng the Philosoþhy of Petmanent Colours, London, r8r3, II, pp. roG-7; and Rita J. .A.drosko, Natural Dyes ønil Home Ðyeing, 1968; repr' ed', New York, î97t, PP' 36-7. 6. Kad Reinking, t)ber die Fårberei der Pfl.anzenfasern im Mittelalte¡ , Melliønds Textilberichle, 19, tgL2oo, ¡938, p.r99; Hans \il'iswe, Mittelalte¡liche Rezepte zur Färberei sowie zur Herstellungvou Farben und Fleckenwasser , Nied,erileulahes Jahrbuch, Slt 49-58, 1958, P. 56; Emil Ploss, Die Färbe¡ei in der germanischen Hauswirtschaft, Zeilsahrífl fur deutsche Philologie, TS, t-zz, t956, pp. r9Þ2o. A recipe for dyeing skins sap green w¿u¡ ¡ecorded in the sixteenth century by Alexis ofPiedmont, Thc Scæetes, trans. William'Warde, London, 1558, fol. 88r-v. 7. Thompson, Maleiøls (S), p. ¡Z¡. Iris green was also used as a dye, according to a fifteenth-century recipe book: Reinking (6), 1938, p. r99, 8. Annette Kok, A Short History of the Orchil Dyes , The Liahenologist, S,248-72, 1966' See also, Albert Berghof, Ðie orgønísahen Føbstoffe úhictischen ønil þflanzlichen Ursþrungøs, Wien, r9oz, pp. 155-ó3; and Ploss (6), 1956, p. 16. A similar technique later produced cudbear, an orchil dyemade from lichens native to the British lsles (Ochrolcahiø sp.), which was developed by George and Cuthbert Gordon in the r75os. g. Gioanventura Rosetti, The Ptíatho, trans. Sidney M. Edelstein and Hector Borghetty, Cambridge, Mass., 1969, p.zz;cf.. WilliamPetty, ApparatustotheHistoryof theCommonPracticesof Dying , in Thomas Sprat, History of the Royal Socíety, London, r667irepr. ed., St' Louis, 1959, PP.297-8,where Petty notes that a drop of Spirit of Vitriol tu¡neth the infusion of Brasil into a purplishyiolet-colour, even although it hath been made yellow before, by the addition of Juyce of Lemmons or Vínegar . ro. The name turnsole has caused some misunderstanding, since it is used in some modern languages (e.g., Fr. to*rnesol, It., tornasole) to identify litmus or orchil extracted from lichens principally of the genus Rocello. ffowever, this has nothing to do with ttre tu¡nsole of the Middle Ages or with theloumcsol en droþeøu of the dyers. There is no doubt that in the Middle Ages and in the earlymodern period turnsole meant Crozoþhora lincloria, since every description of it referred to anherbaceous plant which bears capsules containing three berries, clea.rly tot a lichen. Ifence in sixteentb- and seventeenth-century he¡bals the plant was called Heliotroþdum trìcoccum'. Gerarde, Hctbølt (S), pp. 335-36; John Parkinson, Thcatrum Botanìcum, London, ú4o, p. 439. Andrew Ureattempted to clarify the confusion that later developed over turnsolo when he wrote that archil or litmus is very commonly, but with great impropriety, caf,led linature of tutnsole , Díctionaty of Chemistry, znd ed., London, 1823, s.a. archil , p. t77. See also, Thompson, Malerials (51' pp. r4r-5; and Franco Brunello, The Ail oJ Dyei*g in the History oJ Manhinil, trans. B. Hickey, Vicenza, r.g73, pp. n2-3. Turnsole was used extensively in the frfteenth, sixteenth, and seventeenth centu¡ies not only as a dye aud an a¡tists' pigment, but aJso as a colouring agent in cooking and winemaking; see e,g., Willem Daems, Die Clareit-und Yppocrastezepte in Thomas van der Noots 'Notabel Boecxken van Cokeryen' (um r5¡o) , Fachliúerutur d¿s Mitløløllerc, ed. Gundolf Keil, cú ø1., Stultgart, 1968, pp. zo1-24: and Michael R. Best, The Mystery of Vintners , Agriaulturøl Hislory, 50, 362-76, t976, p. 3Z+.¡ r, A, fifteenth-century painters' handbook gives, for example, recipes for making rag colours (úìlohl,ìn aarwel oÍ cornflowers and bilberries: Viola and Rosamund Borradaile, eds., The Sþøsboutg Mønu- scriþt: A Med,èeual Paínlers' Høndbooh, Now York, 1966, pp. z8-32. Turnsole was ûrst recorded in the twelfth century by the Benedictine monk, Theophilus (probably Roger of Hilmarshausen), in a treatise entitled De iliuwsìs ailibus: Theophilus Fresbyter, On Ðfuets Atls,trans' John G' Hawthorne and Cyril S. Smith, Chicago, 1963, pp. 38-4o, The same recipe subsequently appeared in numeroushandbooks on manuscript illumination, e.g, De coloribus faciendis by Master Peter of St. Audemar,
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