Ftir Research Paper

[Barton Ostby]

Thispaper was presented at the Book and Paper Specialty sessionof the Annual Meeting of the American Institute for Conservation ofArt and Historic Artifacts, Cincinnati, June, 1989.1 Specialty Paper Research1.1. Introduction to the Conservation Problems of SpecialtyPapersFor the last several years, research into the composition ofvarious specialty papers has been undertaken at the ConservationAnalytical Laboratory (CAL) of the Smithsonian Institution. Thesepapers include those made with specially treated pulp stock orhaving highly finished surfaces resulting from the application ofcoatings or impregnation with oils and resins. Examples of objectswith these properties include drawings on papers with preparedgrounds, chromolithographic prints, transparent papers, andvarnished papers, such as maps. These types of papers pose specialconservation problems owing to particular properties imparted by thevarious processing treatments.Coated and transparent papers can absorb extraneous material,like resins and oils from tapes and other adhesives, causingembedded stains which are difficult to remove (Figure 1). Thesurfaces of coated papers are easily marred by scratches andabrasions which scar as well as dull or burnished the finish.Several sheets of coated papers, exposed to water, may block orstick together. Options for the conservation treatment of coatedpapers are limited, since use of drycleaning agents or solvents toremove stains could lead to cracking and flaking of the ground(Figures 2 and 3), or changes in the surface appearance, due to achange in color, refractive index, or reflection.Transparent papers often discolor and embrittle upon ageing(Figure 4). Many of them are extremely hygroscopic, which rendersthem particularly susceptible to planar distortions. Conservationtreatments need to compensate for this property. Various solventscan also cause loss of transparency.For the purposes of the current paper, only the work done withthe analysis of the coated papers will be presented.1.2. Compositional ResearchThere are several treatises on the recipes for creatingtraditionally prepared paper . Among the oldest compositions isground bone ash,5 but white lead, calcium carbonate,gypsum, powdered cuttlebone,6 wax, zinc oxide, titaniumdioxide,9 acrylic gesso,10 clay, talc, satinwhite (aluminum hydroxide, calcium hydroxide and calcium sulfate),barium sulfite, and calcium sulfate, sulfite and carbonate," havealso been used. These have been combined with the traditionalbinders such as starch, mucilage and gums (like gum arabic), gelatinand animal glues (such as hide glue), and casein, as well as modernlatexes and synthetic resins (like acrylic emulsions) and peanut andsoybean proteins.There is a great deal of literature on the history and technologyof coated papers. This literature covers not only the evolution ofdifferent coating compositions, but also the developing technologyused to finish or apply various coatings, from burnishers to bladecoaters. There is very little conservation literature on theanalysis of coatings on paper.CAL's investigation involves a review of the literature on thehistory, technology, and conservation problems and treatments ofcoated papers. From this a chronology of materials and techniques isbeing developed and a database of FTIR spectra of known coatings isbeing established to aid in the non-destructive analysis ofunknowns, leading to their identification. Concurrently, papersamples with known coatings are being subjected to various solvents(water, ethanol, toluene, and acetone) applied in different ways(immersion, suction table, and poultice) in order to evaluate theeffects of some conservation treatment procedures on the diversetypes of coatings. Characterization of the coated papers is beingundertaken before and after treatment using FTIR and SEM analysis.The present paper outlines the procedures used for the FTIRanalysis of samples of coated papers, both knowns and unknowns, andthe preliminary findings. A subsequent paper will report on theeffects of the various treatments.2 FTIR Analysis2.1. Introduction to the Use of FTIR with Museum ObjectsThe theory and operation of the FTIR microscope for theexamination and analysis of works of art have been discussed indetail in a previous paper. Briefly, the information obtained issimilar to that obtained by conventional infrared spectroscopy, thedifferences being that the microscope allows the instrument to "see"smaller (as small as 10 microns in diameter) samples and the fouriertransform processing yields clear spectra in very little time. Whilethe equipment is expensive, it has become a standard addition tomost commercial analytical services and can be found in some museumconservation laboratories as well.Identification by infrared spectroscopy is accomplished by eitherassigning chemical groups to the peaks in a spectrum and inferringthe chemical formula of the sample, or by comparing the spectrum tothose of known compounds and making an identification by the bestmatch. In practice, both methods are used on each sample; the listof possibilities is narrowed by first characterizing the generalchemistry of the sample (such as "protein", "oil", "polysaccharide")and then comparing the spectrum with known spectra from that generalclassification. Often, an exact identification of a museum samplecan not be made, due to the similarity of spectra obtained fromdifferent compounds of the same chemical class, but the generalclassification can still aid the conservator. For example, while itis rarely possible to differentiate between the various gums byFTIR, the characterization of the unknown as a gum is oftensufficient to help the conservator with treatment decisions.Figure 1. Ink drawing on coated paper with embedded stains frompressure sensitive tape.Figure 2. SEM photograph showing cracking of aBaS04/CaC03/acrylic coated sample treated withaqueous poultice.Figure 3. SEM photograph showing loss of burnished surface of aZnO/hide glue coated sample treated by acetone immersion.Figure 4: Transparent watermark paper showing discoloration andembrittlement.2.2 Traditional and New MethodsAll FTIR analysis is technically "non-destructive" since theprocedure does not alter the sample, allowing it to be used forsubsequent analysis. However, in a conservation context, the veryremoval of that sample from the object for analysis could beconsidered "destructive". Fortunately, many objects can be analyzedby FTIR in a truly non-destructive method requiring no sampling;this involves using the FTIR in reflectance mode and obtaining thespectrum from bouncing the IR beam off the surface of the object.Unfortunately, microsampling is still necessary if analysis is to bedone beneath the surface or when the reflected signals are not clearenough.Because the FTIR microscope can analyze such small samples, amethod was developed of removing a single fiber from the coatedpaper, pressing it flat between two halves of a diamond sampleholder, and analyzing the results by transmission of the IR beamthrough the sample. The method provided two distinct regions toanalyze: the flattened fiber and the relatively pure coatingmaterial on either side 9f it. Additionally, a microtoming techniquedescribed by Jia-sun Tsang was employed to look at the depth ofpenetration of some of the coatings by transmission FTIR.Presented here are examples of both reflectance and transmissionspectra of the pigments and binders from several coated papersamples, chosen from the many samples of gated paper which have beencharacterized with the FTIR microscope at CAL.2.3 Results2.3.1 Reflectance Infrared beams can penetrate all but the thickest of coatings, sothat a reflectance spectrum of a coated paper is usually a compositespectrum of the substrate (paper) and the coating. Therefore, if thecoating medium or pigment is to be identified, its spectrum must bedifferent from that of the paper. Gum arabic could not be identifiedby this method, because the spectra of gums and cellulose are nearlyidentical. Proteins, however, such as hide glue and casein showed upwell. As shown in Figure 5, the peaks at 1550cm-1 and 1656 cm-1 ,indicative of proteins,are superimposed on the cellulose and pigment peaks, but the 1650cm-1 absorbance can still be distinguished, although itis very close to the cellulose peak at 1635 cm-1. It isimportant to have good reference spectra, taken with the sameinstrument used for the samples, so that such small differences inpeak location can be seen.Upon close inspection, the small peaks between 1300 and 1000cm-1 can be seen to match between the coated paper sampleand the hide glue reference. Since the chemical compositions of thedifferent proteins, such as hide glue, casein and albumin, aredifferent, one should be able to distinguish between their spectra.In practice, it is more difficult than it sounds, but some attemptshave been made . However, the general characterization of "protein"is easily made from the reflectance spectrum.Figure 5. Reflectance spectra of a) CaCO3/hide gluecoated paper; b) CaCO3 reference; c) hide glue reference;d) plain paper reference.Figure 6. Reflectance spectra of a) coated magazine cover and b)kaolin reference; c) transmission spectrum of kaolin.Acrylic emulsions also have spectra which should show up wellagainst cellulose, but an acrylic coated paper could not besuccessfully identified by the reflectance method. It could be seenthat the acrylic emulsion made a much thinner coating than the otherbinders; the thin layer did not produce a strong enough spectrum tobe seen against the very strong paper spectrum.Pigments, such as calcium carbonate, barium sulfate and kaolin,show up clearly with this method. The spectrum of calcium carbonatein Figure 5 contain many strong peaks (2550,1800, 1500-1300, and 890 cm-1) which show up in thespectrum of the coated paper. The strangely shaped peak at 890cm-1 is often found at various wavelengths in reflectionspectra. It is due to a reflectance phenomenon on the surface whichoccurs certain wavelength (called Restrahlen bands) which areparticular to the compound.Kaolin, often found in coated book or magazine stock, showed upclearly in a coated comic book cover (Figure 6),although the spectral anomaly (due to a Restrahlen band) whichshowed up in the reference reflection spectrum of kaolin (around1160 cm-1) makes the comparison difficult; in this caseit was easier to compare the sample to a transmission spectrum ofkaolin, which looks almost like a mirror image of the reflectancespectrum from the sample. The area between 1600 cm-1 and400 cm-1 is enlarged in Figure 7 toshow the similarity between the spectra, which is not obvious atfirst glance. The two reflectance spectra and the transmissionspectrum all have peaks 3, 4 and 5 (which are "up" peaks in thereflectance mode and "down" peaks in the transmission mode). Thecoating spectrum and the transmission reference spectrum also sharepeaks 1 and 2 in the same way. The reflectance reference spectrumdoes not show peaks 1 and 2 due to the large Restrahlen band-causedspectral anomaly that occurs in the same location as peaks 1 and 2.The fact that the anomaly appears in the reference spectrum but notthe sample spectrum suggests a slight difference in chemistrybetween the two, such as a difference in the degree of hydration ofthe two kaolin samples, or a difference in surface texture.Reflection spectra are almost always different from transmissionspectra, due to Restrahlen bands and changes of peak intensitiesbetween the different modes. In Figure 8, thereflectance spectrum of calcium carbonate appears to have more peaksthan the transmission spectrum due to the weakness of those peaks inthe transmission mode. For example, the peak at 2550cm-1, which is so strong in the reflectance spectrum, isbarely noticeable in the transmission spectrum. In the same figure,the reflectance spectrum of barium sulfate is slightly differentfrom the transmission spectrum. It is important, therefore to usereflectance spectra as references for other reflectance spectra andlikewise for transmission spectra. Additionally, in reflectancework, changes in the spectra caused by slight chemical differencesbetween the sample and the reference must be considered andaccounted for.2.3.2 TransmissionThe method of removing a coated fiber from the paper and pressingit flat provided good spectra for the protein and acrylic samples.Figure 9 shows the transmission spectrum of anacrylic coated paper fiber (the acrylic is identified by the peaksat 1725 cm-1 and 1200 cm-1). This was obtainedby collecting the spectrum at the very edge of the flattened fiber,where the acrylic was most concentrated and the interference fromthe paper was minimized. The pigment appears to be calciumcarbonate, as it matched well with the reference spectrum. It wasstill difficult to identify gum arabic, as the spectra of the gumare still too similar to that of the paper.Figure 7. Reflectance spectra of a) coated magazine cover andb) kaolin reference; c) transmission spectrum of kaolin (enlarged toshow detail between 1600 cm-1 and 400 cm-1).Figure 8. Spectra of CaCO3 a) reflectance and b)transmission; spectra of BaSO4 c) reflectance and d)transmission.The depth of penetration of hide glue into paper was successfullymeasured using a microtomed cross-section of the coated paper. Figure 10 shows the spectra of different areas ofthe cross-section, the top spectrum being near the surface and thebottom spectrum being about 50 microns from the surface. The proteinpeaks (1650 and 1550 cm) slowly disappear as the distance from thesurface increases. While this method would not usually be employedon an actual object, it looks promising for our studies into theeffect of solvent treatments on prepared samples of coated papers.3 ConclusionsCertain binders and pigments can be identified without samplingby the reflectance method using the FTIR microscope. Other binderscan only be identified by transmission, which requires sampling, butthe sample can be as small as one fiber, due to the power of theinstrument. The analysis of cross-sections of the coated paperslooks promising for research on the effects of treatments on thecoated papers. Overall, the FTIR microscope proves to be a usefulinstrument for identification of and research on specialty papers.Figure 9. Transmission spectra of a) single fiber fromcommercially prepared coated paper; b) Liquitex acrylic mediumreference; c) plain paper reference.Figure 10. Transmission spectra of cross-section from hide gluecoated paper a) surface layer; b) about 15 microns from surface; c)about 30 microns from surface; d) about 50 microns from surfaceNotes1. In one of the few publications referring to deterioration ofcoated papers, Margaret H. Ellis, in "Metalpoint Drawings: SpecialConservation Problems for Collectors", Drawings, Vol.2, No. 3, Sept.-Oct. 1980, pp. 59-61, notes problems of abrasion,flaking, fingerprint and oil stains, water damage and stains, andfoxing and mold growth on coated papers, and discusses their causesand preventions.2. Very little has been publishedreferring to deterioration of coated papers. Ellis, Op. Cit., p. 60,notes that water can dissolve some grounds, causing dull areas ortide lines.Eric Harding in "Restoration of Drawings executed on PreparedPaper", Conservator, Vol. 7, 1983, pp. 24-28, outlinestreatments using steam on the verso of a secondary support to removeit from a prepared ground paper, and using pastels to obscure adisfiguring dark spot on another ground which had resulted from a19th century repair. He writes that treatment of foxing with aqueoussolutions applied with small sable brushes could disrupt surfacereflective properties.Paula Volent, in "The Conservation Treatment of an OversizeDrawing by Ann McCoy, The WAACNewsletter, Vol. 10, No. 3, September, 1988, pp. 6-8, writes ofremoving masking tapes from the reverse of a photographic backdropprepared with chalky acrylic gesso using heat, while adhesiveresidues were softened by heptane and mechanically removed. Tearsand breaks were repaired with Japanese tissue and wheat starchpaste, reinforced with acrylic gesso.Unpublished preliminary data collected at CAL has indicated thatcracking of coated papers, visible with SEM (Figs.2 and 3), may indeed be induced by water(applied as poultice in diatomaceous earth on a modern commerciallyprepared paper having a barium sulfide/calcium hydroxide/acrylicbinder coating) or by acetone (following immersion of a sample paperprepared with zinc oxide and Liquitex gel acrylic). This work wasdone as part of a project which involves subjecting several types ofcoated and transparent papers to treatment with four solvents(water, ethanol, toluene and acetone) applied in three ways(immersion, suction table, and diatomaceous earth poultice). Thefindings, along with a technical history of coated and transparentpapers, will be published by CAL staff in the near future.3. For instance, additional unpublishedresearch at CAL has indicated that acetone, applied as a poultice orusing a suction disk, can cause loss of transparency of thecommercially prepared test paper (Albanene Prepared Tracing Paper,Keuffel and Esser Co., Persippany, N.J. #105353, which is advertisedas 100% rag, but which FTIR analysis indicated might have someplastic or oil treatment, judging by peaks at 1760 and 1770).Further details of this study will be published, as noted above.4. 0ne of the oldest treatises on preparedpaper, which was used primarily in the 14th-16th centuries, beforethe invention of pencils or drawing materials such as conte crayons,is C. Cennini's 14th century work, The Craftman'sHandbook, Dover, 1933, translated by D.V. Thompson. Othersources of recipes are Watrous, J. The Craft of Old MasterDrawings, University of Wisconsin Press, 1957; Meder, J., andW. Ames, The Mastery of Drawing, Arabis Books, NewYork, 1978; and Wehlte, Kurt, Materials and Techniques ofPainting, Van Nostrand Reinhold, New York, 1982.5. Cennini, Op. Cit., p. 5.6. Meder, Op. Cit., p. 68.7. Pan Jixing, in "Ten Kinds of ModifiedPaper of Ancient China: A Summary", Institute of PaperHistorians Information, Vol. 17, No. 4, 1983, page 152, notesthe use in ancient China of wax mixed with white mineral powders toproduce coated papers.8. Watrous, Op. Cit., p. 19.9. Ellis, Op. Cit., p. 59.10. Volent, Op. Cit., p. 6.11. See, for instance, Clarence J. West's"The Institute of Paper Chemistry Bibliographic Series No. 162:Coating of Printing Papers", Appleton, Wisconsin, 1952, which coversprimarily developments in coating technology to improve printingtechniques.12. Combinations of these materials are usedas well. For instance FTIR analysis of a commercially producedmodern paper found a combination of barium sulfate and calciumcarbonate.13. West, Op. Cit.14. West, Op. Cit.15. For instance, for a synopsis ofdevelopments up to 1940, see Maclead, Martin, "Early History ofCoated Papers-How They Came of Age," Paper TradeJournal, May 27, 1972, pp. 170-175.16. Harding, Op. Cit., p. 27, mentions usingX-ray fluorescence to detect lead in a Botticelli study. Ellis, Op.Cit., p. 6, refers to analysis of metals for metal point drawings,using XRF and neutron activation energy.17. Baker, M., D. von Endt, W. Hopwood andD. Erhardt, "FTIR-microspectrometry: A Powerful ConservationAnalysis Tool," American Institute for Conservation ofHistoric and Artistic Works. Abstracts of Papers Presented at theSixteenth Annual Meeting, pp. 1-13, New Orleans, June 1-5,1988.18. Information about labs using theseinstruments can be obtained by calling the manufacturers(Spectra-Tech: 203-357-7055, Analect: 714-660-9269, and Bio-Rad,Digilab Div.: 617-868-4330); the sales departments keep user listsarranged by location.19. Tsang, J.-S., and R. H. Cunningham,"Some Improvements in the Study of Cross-Sections," TheAmerican Institute for Conservation of Historic and Artistic Works.Abstracts of Papers Presented at the Seventeenth AnnualMeeting, pp. 20-21, Cincinnati OH, May 31-June 4, 1989.20. The examples chosen for this publicationare: 1. A laboratory prepared sample using a mixture of calciumcarbonate (Baker Reagent grade, #1294) and rabbit skin glue(Liquitex Glue Ground #4450) applied to paper; 2. The cover of asoft cover publication (Marvel comics) found to have a coating ofkaolin and an acrylic; 3. A commercially available coated paper(Cornellisan), which was found to have a coating of barium sulfate,calcium carbonate and acrylic. The spectra were collected on aMattson Cygnus 100 FTIR with a Spectra-Tech IR-Plan Microscopeattachment.21. Perron, J., "The Use of FTIR in theStudy of Photographic Materials," Topics in PhotographicPreservation, Vol. 3, 1989, pp. 112-122.22. Restrahlen bands yield "spectralanomalies", which are usually characterized by a sudden rise in thespectrum above the baseline, followed by a sharp dip, usually ofequal magnitude as the rise, giving the "s" or "z" shape to thepeak. Often, the dip is small, giving the appearance of an"upside-down" peak above the baseline. When there are several ofthese bands together, it is difficult to distinguish the spectralanomalies from "normal" peaks. Since the bands are the result of asurface phenomenon, the spectra will change if the surface textureof the sample is changed; this property can be used to distinguishthe bands, as the normal peaks should not change.

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