D. ROBERT A. WATSON

Mexican Fugitives: A preliminary investigation of purple in the Torner Collection

A poster presented to the 32nd Annual Meeting of the American Institute for Conservation, June 9-14, 2004 Portland, Oregon.

ABSTRACT

     The Torner Collection of Sessé & Mociño Biological Illustrations at the Hunt Institute for Botanical Documentation comprise almost 2000 watercolor illustrations by the artists Juan Vicente de la Cerda and Atanasio Echeverría y Godoy for the Royal Botanical Expedition to New Spain (Mexico) from 1787 to 1805. On the basis of comparative botanical, and historical analysis of the Datura (Solanaceae) paintings, the light rose red color in these illustrations is believed be a chemically degraded organic purple colorant. Since this may be relevant to early botanical taxonomy in Mexico, a study of materials that could have been used in the watercolors was initiated. Historical references regarding materials and methods for the preparation of watercolors were researched in order to produce standards for analysis. The Mexican dyestuffs examined were obtained from commercial and indigenous sources. Fading rates may indicate standards that stabilize to rose red within the twenty-five year time frame evidenced in the botanical literature. In view of the historical literature cochineal may be the most likely fugitive purple.

INTRODUCTION

      In collaboration with Dr. Robert Bye of the Instituto de Biologia, Universidad Nacional Autónoma de Mexico, I suggested a color change in the 1790 Sessé & Mociño watercolor of Datura inoxia at the Hunt Institute for Botanical Documentation (Watson 2004). The color that is now "rose red" cast doubt on the original taxon Datura meteloides (DC ex Dunal 1852), and my hypothesis suggests this species was named Datura wrightii (Regel 1859) (fig. 1). Both of these taxon are based on illustrations which according to the International Code of Botanical Nomenclature [2000. Art. 44.1] may be used only to validate botanical names published before 1908.

     Dr. Bye asked for proof of the color change on the basis of conservation science. To accomplish this, a set of standards is needed with which to compare to the colorant used in the artwork. Definitive analyses of the watercolors may be conducted using any of a number of non-destructive spectroscopies, or sampling techniques such as chromatography or solution spectroscopy. Therefore, I initiated a search for the indigenous purple colorants which may be compared to the fugitive in the watercolor by Atanasio Echeverría (Torner Collection 6331.0556). The chroma or hue of reference is the violet to purple pigmentation in living flowers of Datura wrightii (fig. 2 and 3).

Figure 1. Datura wrightii Regel t. 260.             Figure 2. Datura wrightii purple. San Emigdio          Figure 3. Datura wrightii violet. U. C. Riverside

METHODS AND MATERIALS

        A reference library of lake pigment watercolor standards is being prepared following historical recipes of Mesoamerican purple colorants. The outcome of this study may be of some value to conservators of pre-Columbian and Spanish colonial art and antiquities. The color library establishes the materials provenance through sources listed at the end of each dyestuff description. The dyestuffs were carefully identified by cross-checking the relevant botanical literature. Equipment made of glass and stainless steel was used exclusively with distilled water or denatured alcohol. Commercial sources of chemicals used are listed in the suppliers section at the end of this paper.

     Protocol for various spectroscopic analyses stated that the colorant extracts should be dried in glass vials. The colorants are also air brushed onto 12.5 cm diameter ashless filter paper (Watman grade) creating four standards 3.0 x 4.7 cm each. Placed in mylar envelopes, two of the standards may be cut apart for spectrometric analyses, a third one is mounted in a color chart used for fade testing (these standards are presented with the dyestuff descriptions), the fourth is the control sample retained in low relative humidity and dark storage for long term preservation.

     A1.1/4 wag represents the label code denoting the initial dye extraction A, or any repeated extractions as B, C etc., followed by a number code which identifies the dyestuff number, separated by a decimal point from the mordant number in Table 1. Forward slashes separate additional dyestuffs or mordants in combinations. Details such as water, alcohol, or gum are listed in lower case initials; other supports include Sanders -SW, Arches -AW, wool -WO, and raw silk -RS. Color standards are labeled with the Genus and species name, number code, technician name, and date.

WATERCOLORS OF COLONIAL MEXICO

     European pigments were certainly available to the Real Academía de San Carlos, Mexico's school of art ordained by Carlos III of Spain, and from where the artists of the Royal Botanical Expedition originated. References to the production of color materials used during the early colonial period in Mexico (1550-1575) are found in the writings of Hernández (1959) and Sáhagun (1950). Using polarized light microscopy, Elizabeth Haude (1998) analyzed watercolors of sixteenth and seventeenth-century New Spain, and she concluded that New World origins were probable. This agrees with the premise that artists of the late eighteenth-century attended to the grinding and gumming of their watercolor paints (Cohn, c1977). Purple colors were at that time absent from the spectra of inorganic pigments. Therefore artists used biological materials for several vivid colors required for painting floral subjects. For this reason I investigated all Mesoamerican dyes known to produce purple hues. However, it must be assumed that the artists of the expedition had some training concerning the colorantes fino used for watercolor painting and those known to be less reliable.

Extraction

     Water soluble dyestuffs are brought to a boil and simmered. The colored solute is evaporated and dried as residual colorant. The extraction requires finite quantities of mordant or additives in order to achieve specific hues. Natural colors are also known to vary with age, by season and geographically. Flowers are softened by boiling, with or without mordant, and the liquid expressed from the pulp. Fruits containing sugars provide biological nutrients and must be fermented into alcohol or vinegar, and then struck on an appropriate base. Fermentation may produce stronger color in some dyestuffs, alcohol being another solvent for many organic colorants. Used by contemporary Spanish artists, “agua ardiente” was a distillation product of high alcohol derived from grapes (Veliz 1986). Once dry the indigoids are insoluble in water, however the solute may be painted onto a substrate or struck on a base, while dried indigo is made soluble in alkali and may not require media for painting purposes. Several plant species produce colored oils and resins that may be used directly for painting.

Mordants

     The mordant forms a base for lake pigments while altering the hue of organic colorants by changing the pH from acid to base and combining metallic ions. Table 1 reconstructs an eighteenth-century mordant list. The basic chemicals used include sulfates, alkalies, and acids. Purple colors result more often when alkalies are used but they also occur with acid mordants, especially tannin. Certain to be among the Aztec pharmacopoeia were lime, potash, soda salts, urea, acetic and tannic acids. According to Hernández (1959) five white powders for setting organic colorants were sold in the colonial markets. In Sáhagun’s chronicle Historia General de los Cosas de Nueva Espania, he wrote that the chemicals alum, ferrous sulfate, and cuperous sulfate were sold in the sixteenth-century markets. Tartaric acid, tin salts, niter, and citric acid from limes may have been introduced with the arrival of the Spanish colonials.

TABLE 1. XVIII CENTURY CHEMICAL MORDANTS

1. ALUM KAl(SO₄) ₂ ^. 12H₂O [Hydrous potassium aluminum slufate] Alunite.

              The most common mordant used for clear colors.

2. FERROUS SULFATE FeSO₄ ^. 7H₂O [Hydrous Iron(ll) sulphate] (green vitrol) Melanterite;

              Hydrotrollite FeSO₄ ^. n H₂O and Pyrites FeS^-. Saddens colors: darkening toward black.

3. CUPEROUS SULFATE CuSO₄ ^. H₂O [Hydrous Copper(ll) sulphate] (blue vitrol) Calcanthite.

              Used for blues and to blue the colors.

4. Lime CaO [Calcium oxide] burnt Calcite, limestone, marble or chalk CaCO₄ .

              When added to water this becomes calcium hydroxide Ca(OH)₂ or slaked lime.

5. Potash KCO₂ [Potassium carbonate] Vascular plant ashes.

6. Soda Na₂CO₃ ^. H₂O [Sodium carbonate] Natron. An arid surface efflorescence used to intensify color.

7. Saltpeter KNO₃ [Potassium nitrate] Niter; including Soda niter NaNO₃ [sodium nitrate].

              Used for bright color, but natron is the more common mineral efflorescence.

8. Tin SnCl [Stannous chloride] Used to intensify color. Historic recipes used hydrochloric acid with oxides of tin.

9. Acedic acid HC₂H₃O₂ (vinegar); including Citric acid C₆H₈O₇ .

10. Tartaric acid HOCC(CHOH)₂COOH (cream of tartar) Used to brighten color.

11. Tannic acid C₇₆H₅₂O₄₆ Bark and leaves; also including Gallic acid C₇H₆O₅ from gall nuts.

12. Ureic acid CO(NH₂)₂ (urea)

Lake Bases

     Along with the mordants, clay may form the base for lake pigments and attapulgite bentonite, illite, kaolinite, montmorillionite, palygorskite, saponite and sepiolite have all been used for this purpose. Clay minerals are generally composed of hydrous magnesium and/or aluminum sheet silicates. The long elusive pigment called Maya blue has come down to us from the ancients who originated the process of dying palygorskite (MgAI)₂ S₄(OH) ^.4H₂O mineral clay with indigo (Jose Yacaman, 1996). In addition to clay serving as a base for lakes, it enhances viscosity while imparting an unctuous quality to the paint. Due to the lower refraction of minerals in media, the color may become dull, and so a very small amount of base will produce the best quality lake. Gypsum CaSO₄ ^.2H₂O [Hydrous Calcium sulfate] was used to lighten colors and as a white mineral it was used as a base.

Media

     Acting as a protective agent, the media can improve flow and render a colorant more sympathetic to brushing. Natural adhesive substances used for painting included gum, resin, latex, wax, oil, albumen, protein, collagen and gelatin. In Europe, gum arabic was added to the dye bath of the organic lakes used in manuscript painting from pre-Renaissance times (Rosetti, 1969). The Naha used the gum of "Huisachuitl" (Acacia farnesiana) and "Achotol" (Bixa Orillinda) for painting media. The Libilus of 1552 mentioned several native plant saps: an adhesive from the Orchidaceae, and the latex sap from some Jatropha species (Emmart 1940; Martínez Cortéz 1970). Of interest is a wax media from the bodies of another coccid insect the "Axe" or the Coccus Aji. Donkin noted production through the late-colonial period along the Sierra Madre Occidental (Donkin 1977).

MESOAMERICAN PURPLE

     This standards library is reconstructed from red, purple and blue organic colorants known to have been used in colonial Mexico. I reviewed the red dyes for the possibility of chemical change resulting from varying pH or the fading of an admixed blue. The scientific nomenclature, chemical dyestuff, and indigenous names encountered in the field are listed for each of the fourteen colorants. Still in use today, the Náhuatl system of plant classification is examined specifically because it was taught in the Aztec school called "Calmec" before the Spanish conquest and it therefore predates the Linnean system (Williams, 1990). Native species from localities that may have provided the historic colorants were photographed at the herbarium of the California Academy of Sciences. Detailed analyses of the biology, history and chemistry may be found in the referenced works.

GENUS [Family] species Author: {synonym}          COLORANT NAME: Dyestuff; “indigenous names”

1. Dactylopius [Coccideae]                      Cochineal, carmine: Female coccid scale insects;

D. coccus L. Costa: {Coccus cacti L.}                         Spanish: Grana; Náhuatl: "Nocheztli" blood of nopal cactus

              Dactylopius coccus is called cochineal in commerce, this being the first and foremost dyestuff of colonial Mexico. The color agent in the coccid insects is a hydroxyanthraquinone or carminic acid (C₂₂H₂₀O₁₃) according to Wallert (1997). Use and trade after the Spanish conquest is recorded through the eighteenth century (Donkin 1977, Wouters and Verhecken 1988). Gettens and Stout (1942) describe purple lake made from alum and lime, or alum, tin, and lime mordants. Purples develop using tartaric acid with sodium carbonate, tannic acid, or ferrous sulfate mordants. Samples of cochineal from Coyotepec (see Wallert 1997) were provided by Bulmaro Perez Mendoza, a traditional Zapotec weaver from Teotitlán del Valle, Oaxaca, Mexico. Kremer Pigmente supplied the genuine carmine naccarat.

2. Caesalpinia [Caesalpiniaea]            Brazilein, Brazilwood; Spanish: Brasil,

C. bonduc (L.) Roxb.:                                                          Pernambuco, Guacolote, Piedra del augila;

{Guilandia bonducella L., C. cristai L.}....................... Náhuatl: "Hoitzquahuitl,” "Huitzquahuitl" Thorn tree

C. melanadenia Rose, C. coriaria (Jacq.) Willd.           Náhuatl: "Cascalote ," "Nacascolote," "Nacascul"

C. cacalaco H&B................................................................ Zapotec: "Xa-gala" Oaxaca: "Chalala"

C. eriostachys Benth., C. mexicana Gray,                      Spanish: Iguanero, Tabachin del Monte; Oaxaca:

C. exostemma DC. ............................................................ "Ebano," "Yagati," "Gueteregi," "Hojasen"

C. yucatanensis Greenm. ................................................. Yucatan Maya: "Xkanpocolcum," "Kanpopol-kun"

C. gaumeri Greenm. .......................................................... Yucatan Maya: "Xcitiuche," "Kitamche," "X-kitin-che"

C. pulcherrima [L.] Sw.                                                      Spanish: Tabachin; Náhuatl: "Chacalxochitl," Shrimp flower;

                                                                                               Maya: "Sik," "K'an-sik'in" Zapotec: "Bandaa-yu" or "-bulaga"

                                                                                               Puebla Náhuatl: "Xiloxochitl" Tarascan: "Sirundanicua"

              Caesalpinia bonduc represents a genus of trees that include many world wide species from which the red dye called Brazilwood extract is derived. Brazilein (C_l6H₁₂O₅) is described as deep red to brown in color. Mineral acids decompose it to a yellow or orange red. It is not a lightfast color. Wallert (1997:61) explains, "The color obtained from this Brazilwood may vary, depending on mordant and pH, from bright orange to a deep bluish purple." The color was boiled with alum then precipitated with lye. Brazilwood was used in the sixteenth century for dying but not used as a painting medium according to Sáhagun (1950-81, 12: 241). Samples of pernambuco wood (Caesalpinia bonduc or C. echinata) and Brazilwood extract were obtained from Kremer Pigmente.

3. Haematoxylum [Leguminosae]        Brazilein, Redwood; Spanish: Brasil, Campeche, Palo de tinta;

H. brasiletto Karst: {boreal S. Wat.}                                   Oaxaca: "Azulillo;" Zapotec: "Yaga-guela-tiguiani"

              Heamatoxylum brasiletto has a greater range than H. campechianum, however the dyewood yields brazilein. Hernández (1959) claimed the species produced a fine purple when long boiled. A sample of “Campeche” obtained through Bulmaro Perez Mendoza from Oaxaca, is the redwood extract of Heamatoxylum brasiletto.

4. Haematoxylum [Leguminosae]       Hematein, Logwood; Spanish: Palo de Campeche, Palo

H. campechianum L.                                                             de Tinta, Tinta; Yucatan: "Ek" or "Er;" Zapotec; "Yaga-

  cohui;" Náhuatl: "Cascalote," "Nacascolote," "Nacascul"

              Heamatoxylum campechianum, large trees indigenous to Mexico, produce the coloring matter hematein (C₁₆H₁₂O₆), and the commercial dye Logwood extract (Robinson 1962). The Mayan name “Ek” [black] indicates the color of the basic dyewood extract from this species (Mc Junkin, 1995). Logwood lake was employed throughout the colonial period (Haude 1998). Gettens and Stout (1966) describe colored lakes prepared with various mordants; "All are insoluble in water and alcohol, are turned bluish violet by alkalis, and are decomposed by mineral acids with the formation of a blood-red solution. The extract was used as a lake pigment in watercolors, but, because it is fugitive in strong light, it has been discarded.". Samples of Logwood chips and Logwood extract were obtained from Kremer Pigmente.

5. Purpura [Murex]                                        Bromoindigotin: Purpura shellfish effluent;

P. pansa Gould, P. patula Gould                                  Náhuatl: "Caracol;" Mixteca: "Tucohoyi," "Tixinda"

              Purpura patula and P. pansa are shellfish that produce a purple dye used through the late eighteenth-century in Mexico. From the secretion of glands comes the purplish red 6,6'- dibromoindigotin (Tyrian Purple) C₁₆H₈₀2N₂Br₂ which is closely related to the blue dye indigo and as such requires no mordent. Although it was early used for painting codical art it is best used for dyeing of yarn. The shellfish are recyclable and their shell functions as a handle while tinting the yarns (Turoc 1988). Difficult to prepare for later use, the shellfish painting must have occurred exclusively along the coast. A sample was obtained from Oaxaca, Mexico, through Miguel Zaffra. A substitute dibromoindigotin was derived using Mullosk shells from the Monterey Bay, California.

6. Jatropha [Euphorbiaceae]                     Naphtaquinone: oil in stems and seeds;

J. curcas L.: {ciliata S&M}                                           Spanish: Sangre de drago, Sangregado, Pinon;

J. dioica Sesse                                                               Veracruz: "Ashte," "Scu-ul'u;" Oaxaca: "Que-ca," "Vico;"

J. glandulifera Roxb.: {triloba S&M}                           Chiapas: "Cuipu," "Nacuala;" Zapotec: "Yaga-be-lape;"

Náhuatl: "Cuauhyohuachtli ," "Quahychtli ," "Esiquitle" Blood

              Jatropha curcas and J. glandulifera were called "Esiquitle" [blood] by the Náhuatl, who utilized the strong purple colorant obtained from the seeds. Wallert says, "The dark purple color that can be found in oil extracts of trunks, branches, and roots of many of the Jatropha species is a naphtaquinone colorant." Standley recorded that it was used for dye and paint. Several other genera contain these shikonin dyestuffs described as Dimethylacrylshikonin (I) C₂₁H₂₂O₆ and acetylshikonin (II) C₁₈H₁₈O₆ (Ballantine 1969). Species of Jatropha containing the red colorant are shrubs locally referred to as the "Sangre de Drago" or "Sangregado." Sesse & Mocifio christened "Jatropha Draco N" [J. dioica] because the blood red sap resembled the old world colorant "Dragon's blood" from the Asian palm Calamus Draco. Seeds of Jatropha curcas were ordered online from Tropilab Inc., Surniam,; however, a sample of this sap has yet to be obtained.

7. Euphorbia [Euphorbiaceae]           Anthocyanin: Flower petals; Spanish: Noche buena;

E. pulcherrima (Willd.) ex Kolt.                                  Catalina; Náhuatl: "Cuitlaxochitl;" Zapotec: "Gule-tiini"

              Euphorbia pulcherrima yields an anthocyanin floral colorant dependent on mordant pH. The poinsettia petals develop purple colorant with alum, ferrous sulfate, lime, potash, soda or vinegar. The pH sensitivity of anthocyanin is well known to chemists, despite the fact that it darkens on exposure and fades readily. Samples were obtained from local greenhouse suppliers.

8. Cosmos [Compositae]                             Anthocyanin: Flower petals; Spanish: Mirasol;

C. bipinnata Cav.                                                          Náhuatl: "Xochipal," "Xochipalli;" Guerrero: "Suchipal"

              Cosmos bipinnata forms a phloroglucinyl (C₆H₃ (OH)₃ ^. xH₂O) colorant, although the red and violet varieties used for purple by the Náhuatl probably combine anthocyanins (Wallart 1997). Early references claim the petals were fixed with nitro, also known as saltpeter (Castillo Yturbide 1988). Seeds were ordered through J. L. Hudson Seedsman, additional flowers were obtained from local gardens.

9. Dahlia [Compositae]                                Anthocyanin: Flower petals; Náhuatl: "Xicamaxochitl ,"

D. variabilis Desf.; D. pinnata Cav.                             "Acocotli"

              Dahlia variabilis is a genus related to Cosmos and probably forms a complex anthocynin. Samples came from local greenhouse suppliers.

10. Morus [Moraceae]                                   Anthocyanin: Mulberry; Spanish: Amacapulin, Moral;

M. celtidifolia H. B. K.: M. tatarica S & M                 Huastec: "Pejon;" Zapotec: "Beyo-zaa ," "Yatga-biyozna"

              Morus celtidifolia forms anthocyanin in the fruit of the mulberry. It retains a purple color when fermented, or extracted by alcohol when dry, using lime or potash mordant. Fruit of this species was collected in Claremont California.

11. Cissus [Vitaceae]                                     Anthocyanin: Wild grape; Spanish: Uvas silvestre;

C. elliptica Cham. & Schltdl., C. oaxacana Lundell, Tripa de Judas; Náhuatl: "Temecatl," "Tlacamazatcazqui"

C. verticillata ( L.) Nicol. & Jarvis: {sicioydes L.}      Yucatec Maya: "Tabkanil," "Ya'ax-tabkanil"

              Cissus eliptica and C. verticillata are among the wild grape species that were called "Tlacamazatcazqui" in Nahuatl, the fruit of which forms a blue anthocyanin (cyanidin 3-diglucoside). Fermentation and treatment with alkali yields purple even with dried raisins (Prillieux 1866:248). A sample of “Uvas Silvestre” or wild grape has yet to be obtained from Oaxaca. Although the genus Cissus is common to the new world tropics, the anthocyanine sample was derived from the old world genus Vitis labrusca and commercial black raisins.

12. Guaiacum [Zygophyllaceae]           Seri blue: Guaiac resin + montmorillionite clay;

G. sanctum L.                                                               Spanish: Palo santo, Guayacan; Maya: "Ken," "Soon"

G. coultari Mill.                                                            Náhuatl: "Huaxaxan," "Matlalquouitl," "Matlali" blue;

G. coultari var. palmari (Nail) I. M. Johnson               Zapotec: "Yaga-gupi," "Yaga-naa;" Chinantec: "Mo-tzi"

              Guaiacum coulteri and G. coulteri var. palmeri forms a blue color from resin that is 70% Guaiaconic acid (C₁₉H₂₀O₅). Mixing the sap with montmorillionite clay provided a source of blue for the Serri tribe of Sonora Mexico (Moser 1964). In the central region of colonial Mexico, a blue dye called "Matlali" in Náhuatl has been traced to the flowers of Matlalquouitl, and according to Martinez (1978) this was Guaiacum coulteri. Sáhagun (1959) recorded the “roots” being sold in markets, although the resin is extracted by chopping and boiling the wood. Maya blue could have been made by adding guaiac resin to attapulgite [palygorskite]. Experiments show that it becomes blue when mixed with many of the adsorbing clays (Pierce, 1964). Stained red with cochineal, Seri blue produces an unsatisfactory grayish purple hue. Attapulgite was obtained from Kremer Pigmente, bentonitic montmorillionite from Yermo California, and samples of guaiac resin were provided by Dr. Jean Langenhiem, University of California at Santa Cruz.

13. Indigofera [Leguminosae]                 Indigotin, Indigo [Maya blue]: leaves;

I. cuernavacana Rose, I. densiflora Mart. & Gal.        Spanish: Anil; Náhuatl: "Pitzahoac," "Xiuquilitl,"

I. microcarpa Desv., I. mucronata Spreng.                  "Huiquilltl" Turquoise herb; Mayan: "Choh;" Oaxaca:

I. suffruticosa Mill.: { Anil tenuifolio L.}                      "Jiquelite," "Xiquelite;" Chinantec: "Mo-i-tza"

              Indigofera suffruticosa forms a blue dyestuff that was used in the painting of manuscripts, and also in the form of Maya blue, it has been found in the same works (Haude 1998). Among the natural plant colorants, indigoten (C₁₆H_I0N₂O₂) is considered one of the most light fast. When used to dye the white clay "Sacalum" [palygorskite], it is called Maya blue. The organic nature of indigo is protected from biodeterioration within lattices in the silicate needle structure of a palygorskite crystal (Jose Yacaman, 1996). Indigo was mixed with reds for purple colors in textiles (Wallert 1997), while Maya blue has been observed mixed with yellow colorants to produce greens for painting (Morris 1931, Haude 1998). Ethanol added to the dye bath yields indirubin purples (Ushida 2003). Samples provided by Bulmaro Perez Mendoza are the traditional “cake” manufacture from Niletepec, Oaxaca (see Ross 1990).

14. Justicia [Acanthaceae]                         Indigotin: leaves; Spanish: Hierba purpirea; Náhuatl:

J. spicigera Schltdl.: {Jacobinia mohintti L.;              "Mohuitli," "Moctli," "Xiuhquilitpatlac;" Mayan: "Yich-caan"

Anil latifolio L.; J. coccinea S&M}                               "Yax an;" Guerrero: "Micle;" Oaxaca: "Moicle"

              Justicia spicigera represents an additional botanical source for indigo that was used by the Huastec, Mayan and Náhuatl for textile dying and painting. The strong reddish purple solution made from fresh bruised leaves boiled in alkali turns to blue on drying. The Spanish called it "Hierba purpirea" considering it inferior to Indigofera. Alcohol added to the dye bath with an alkali mordant creates an indirubin purple hue. Leaf stems were obtained from the Conservatory of Flowers, Strybling Arboretum and Botanical Gardens, Golden Gate Park in San Francisco, California.

CONCLUSION

     The identification of the fugitive colorant that is now rose red in the Torner watercolor 6331.0556 remains uncertain. Nevertheless, we may exclude a few historically obscure purple colorants; Caesalpinia, Guaiacum, Jatropha, the highly fugitive anthocyanins, and the more permanent Maya blue and indigoid dyestuffs. Cochineal and haematein are the principle colorants well documented in commerce during the eighteenth-century and better known by the artists of New Spain.

     Hematein is among the purple candidates for its continuous use in painting, early fading characteristics, and in which the acidic change to red may be relevant. Discoloration of the paper support may indicate the formation of acid radicals derived from the linen fiber and sizing. Condition of the paper has a direct bearing on color stability. Changes in pH can effect pigments and alter the hue of organic colors.

     In view of the historical literature, the purple colorant most used by the artists of New Spain was cochineal. The highly prized commodity produced a brilliant violet in watercolor painting and was the major dyestuff exported to Continental Europe throughout the colonial period. The mordant complexes forming cochineal purples include: reddish violet with lime (CaO), or aulm, lime and tin (SnCl); light violet with tannic acid, or with sodium carbonate and tartaric acid; and deep violet with ferrous sulfate, copper sulfate and tartaric acid, or with lead acetate. Unstable coordinating molecules may chemically degrade to a point where carminic acid reverts to its natural magenta color on the paper substrate. In the case of lime, carbonation of basic calcium hydroxide in air over time produces calcium carbonate and the lower pH values form a reddish color complex.

                                               added to  water                      when dry                       producing in air over time
                        CaO (s) + H₂O (l)   →  Ca+ (aq) + 2OH-(aq)   →   Ca(OH)₂ (s) + CO₂ (g)  →  CaCO₃ (s) + H₂O (g)
                        lime                                 slaked lime                           calcium hydroxide               calcium carbonate     
                                                                                                             pH 11– 12 = VIOLET            pH 7.5 – 9.8 = RED

                                                                                                                                       └ CARMINIC ACID ┘

     Trace metallic mordants remaining in the substrate may be easily detected using noninvasive Raman laser microscopy. While spectroscopic techniques can define the reference spectra of these standards, research on their fading characteristics continues. Fading tests may indicate those standards which stabilize to “rose red” within the twenty-five year time frame evidenced by the botanical literature (Dunal 1852:544). Future work will eventually establish comparative fading rates by means of Blue Wool Standard analysis.

ACKNOWLEDGMENTS

     I would like to thank John Twilley, for instruction in the science of Art Conservation and his encouragement to continue this project. Thanks to Dr. Robert Bye for suggesting the colorant analysis of the Datura illustrations of the Sessé & Mociño lcones Florae Mexicanae. Thanks to Tom Daniel for data adding to the study and to the staff of the California Academy of Sciences. Lugene Bruno assisted in the acquisition of Torner Collection photographic materials. I am grateful for the information generously offered me by Dr. Jean Langenheim, and Dr. George Kremer. Also I must thank Bulmaro Perez Mendoza and Miguel Zaffra for supplying many of the native dyestuffs.

PLATES

1. Dactylopius coccus Costa; Coyotepec, Oaxaca. Insert scale: mm.

2. Caesalpinia bonduc [L.] Roxb.; Puerto Vallarta, Jalisco CAS 761037.

3. Heamatoxylon campechianum L.; E1 Arenal, Tobasco CAS 719383.

4. Heamatoxylon brasiletto Karst; Acapulco, Guerrero CAS 272347.

5. Plicopurpura patula Gould; of the Pacific coast, and Plicopurpura pansa from the Gulf of Mexico.

6. Jatropha cucras L.; 3 mi SW of Tehuantepec, Oaxaca CAS 595517.

7. Euphorbia pulcherrima Willd ex Klot.; 5 km east of Santo Domingo, Oaxaca. CAS 743177.

8. Cosmos bipinnata Cav.; State of Mexico CAS 122022.

9. Dahlia variabilis Desf.; Federal District, Mexico CAS 120368.

10. Morus celtidifolia H.B.K.; Near El Mi1agro, Tamaulipas CAS 221628. Insert: dried fruit.

11. Cissus elliptica Cham. & Schltdl.; Tenosique, Tobasco CAS 643013.

12. Guaiacum coulteri var. palmeri (Nail.) I. M. Johnson; 7 km east of Juchitan, Oaxaca, CAS 799036.

13. Indigofera suffruticosa Mill.; 69 mi NW Tehuantepec, Oaxaca CAS 655643.

14. Justicia spicigera Schltdl.; Tlazontla, Veracruz CAS 560853.

SUPPLIERS SECTION

Kremer Pigmente/ Kremer Pigments Inc., 228 Elizabeth Street, New York, NY 10012 [colorants and mordants]

Sinopia Pigments & Materials, 3385 22nd street, San Francisco, CA 94110 [colorants and mordants]

Earth Guild, 33 Haywood Street, Ashville NC 28801 [general dyer’s mordants]

LabPro, 1290 Anvilwood Ct., Sunnyvale, CA 94089 [potassium nitrate and laboratory equipment]

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D. ROBERT A. WATSON

Artist and Conservator in Private Practice

125-D Post Street, Santa Cruz, CA 95060

drawatson@lycos.com