T H E THEORY O F DYEING. V BY WILDER D. BASCROFT
I n the preceding papers I have discussed some of the characteristics of acid dyes, basic dyes, and mordants. I n this paper I propose to take up the question of fastness. There is very little satisfaction to be obtained on this point from any book with which I am acquainted. Knecht, Rawson and Loewenthal’ say: “With certain classes of dyed and printed materials, fastness to washing is of prime importance, whereas with others it is not of much moment. Thus printed calicoes which have to be frequently washed (often with soap and soda), and the colours in figured woolen goods which are milled with soap during the process of manufacture, must necessarily be fast to washing. On the other hand, in such goods as carpets, furniture coverings, and fine silks this fastness is not of great importance. “ The behaviour of colours in washing varies enormously. Some, like the ordinary azo-scarlets and oranges fixed on cotton, are stripped by water alone. The direct cotton colours dyed on cotton mostly give up a certain amount of colour to boiling water, but only up to a certain point, and no more can be removed unless fresh water is used. Many colours are little affected by boiling water, but are easily affected by such as contain free alkali. Some colours, lastly, like aniline black, indigo, and other vat dyes, and most of the alizarin colours fixed on chromium mordant are fast enough to withstand the action of boiling soap. Since the production of shades fast to washing generally invol\-es a larger number of operations, the use of more costly dye-stuffs, and, consequently, greater expense t o the dyer, he will, as a rule, employ those dye-stuffs and methods which will bring about the desired result in the least time and a t the smallest cost, and will only produce fast shades where they are a requirement of the trade, or when specially asked for. -.
A Manual of Dyeing,
2,
747 (1910)
146
W i l d e r D . Bancrojt
“The loss of colour in washing is not the only drawback which the dyer has to guard against, especially in dyed yarns, which are used in figured fabrics. Many colours have the disagreeable property, when immersed in water or soap, of colouring other yarns in the piece, especially the whites. Such colours are said to ‘ bleed’ or ‘ run’ and their use in any fabrics which have to stand washing, other than plain ones, must be carefully avoided.” Beech1 is more explicit but unfortunately not accurate. ‘‘ The affinity of the basic dyes for wool increases with increase of temperature. This is a property that has an important bearing on the method of dyeing, and to any person who pays some attention to theory in its practical applications it indicates the most rational method of working, which is t o enter the goods into the bath cold, or, at the most, a t a hand heat, then, after working a short time to get the goods thoroughly impregnated with the dye-stuff , to gradually raise the temperature to the boil and work for from half an hour to an hour longer, even if before this time the dye-bath be exhausted. The reason for giving a fair length of time in the bath is to get the colour properly fixed on the fibre. The combination of the dye-stuff and the fibre is a chemical one, and, as stated above, the dye-stuff has to be decomposed so that the base may combine with the essential constituent of the wool fibre, while it is obvious that this decomposition and then the union of the colour base with the wool must take time, and as it is effected more easily and completely a t the boiling point, it is advisable to work the goods in the bath so as to fully insure that they are given the necessary time for the chemical change to take place.” If the bath is completely exhausted by the fiber, washing cannot extract enough dye to color the water perceptibly and consequently the dyed fabric will be fast to washing with,water. This is often the case with basic dyes and ~ 0 0 1but ; ~ it is more “The Principles and Practice of Wool Dyeing,” 65 (1902). Beech: “The Principles and Practice of Dyeing,” 65 (1902).
difficult to obtain level dyeing if the bath is to be exhausted completely. If the bath is not to be exhausted completely, the goods must be removed before dyeing is completed and when the shade is that which would be in equilibrium with an exhausted bath; or the bath must be so arranged that it has a high stripping power and consequently will dye only to the shade which is in equilibrium with boiling water. Ganswindt’ points out that the color is fast to washing when wool is dyed in an acid bath from which the color is exhausted slowly but very completely, whereas the color is less fast, other things being equal, the less completely the bath is exhausted With the direct colors on cotton, for instance, the bath is rarely completely exhausted.? The same thing is true for many acid colors dyed on ~ o t t o n . “The ~ acid colours, with very few exceptions, cannot be fixed on cotton or linen so as to resist washing. If precipitated in these fibres as metallic lakes with the aid of salts of aluminum, tin, etc. (similarly to the mordant colours), the lakes are decomposed by water and the colour is extracted. The soluble blues or cotton blues resist water slightly better. Of the other acid colours the Crocein scarlets and allied colours only-;. e . , the azo-compounds prepared by combination of amidoazobenzene or amidoazotoluene with betanaphthol sulphonic acid (B .), or the so-called gamma acid, or alphanaphthol disulphonic acid (Sch.)-are of some importance in cotton dyeing, since they resist light better than the benzidine colours and are not sensitive to acids. For this reason they are still employed in cotton dyeing in spite of their inferior fastness to washing. The acid colours have never been used to any large extent in the dyeing of linen, but they find application on jute.” In the dyeing of cotton with acid colors “ t h e dye-bath is used as concentrated as possible to produce full shades; it is never exhausted by the cotton. Both the mordant and dye liquors are used continuously and freshened up regularly. The shades which are Theorie und Praxis der modernen Farberei, 2 , 3 (1913). and Loementhal: A Manual of Dyeing, I b i d , 2, 510, 511 (1910).
* Knecht, Rawson
I,
6 (1910).
W i l d e r D . Bamrojt
148
produced by any one of the methods are not fast even to a light soaping, but they resist light moderately.” There is another factor which is more important in determining the fastness and that is the temperature. The natural thing is to assume as Beech did that the fiber has a greater affinity for the dye at high temperatures. As a matter of fact this is not always so. This follows indirectly from a passage by Ganswindt . “With dyes which are taken up rapidly, it is not unusuaI to get uneven dyeing especially if the wool gets into the hotter portions of the bath or comes in contact with the hot walls of the drum, while other dyes do not show this unpleasant characteristic. This leads one to assume that the affinity of wool for the dyes, which dye unevenly, increases very rapidly with the temperature whereas with the other dyes we must assume that the affinity at high temperatures is not appreciably greater than a t ordinary temperatures. ” Fortunately we do not have to rely on this indirect evidence. Mills and Rennie? have shown that the amount of rosaniline acetate taken up by wool reaches a maximum at 31 O and drops off to a low figure a t 81 ’. Dreaper3 gives some data for the adsorption of certain dyes by wool. The figures in Table I are the percentages of the dye left in solution. TABLEI ~ ~
40’ ______
Dye _____~-__
20’
Acid magenta Tartrazine Indigo carmine Acid green Acid violet 4 B\V
1
60’
80”
IOOO
-
79 46 46 79
‘4 3
4
4 3
I
I
3 18
3 4 4
44
26
20
8
3 5 3 6 20 8
5 6 97
0
6 2 5 2
28 7
This table disposes of the generalization that the adsorption is greater at higher temperatures. On the other hand, it 2 3
Theorie und Praxis der modernen Fhrberei, 2 , 65 (1903). Jour. SOC.Chem. Ind., 3, 215 (1884). “Chemistry and Physics of Dyeing,” I O I (1906).
The Tlzeory o j Dj*ei.tzg
I49
is a general rule to heat the bath after the fabric has been entered. The real object of this is to coagulate or agglomerate the dye, thus making the dye much less soluble. This is a general phenomenon. If soot, intended for lamp-black, is heated too hot or too long, it becomes sandy. After short ignition silica dissolves in a boiling solution of sodium or potassium carbonate; but after long ignition it does n o t 1 When lime has been heated above IIOO’, it takes up carbon dioxide much more slowly than lime which has only been heated to a dull red heat. Freshly precipitated arsenic sulphide gives off a good deal of hydrogen sulphide when boiled with water. If previously heated to 12 j ‘, this is not the case.3 Hydroxides and sulphides on standing or on heating change so that they are less readily dissolved or peptonized. Luppo-Cramer? points out that ‘‘ the state of the silver bromide is of fundamental importance for the emulsification in gelatine by means of bromine ions and the same thing is true for the emulsification of ammonia. The silver bromide gel loses its peptonizable properties completely just by standing. Samples which had stood for one, three, six, ten, and twelve hours in the dark at ordinary temperature showed a gradual decrease in the tendency to form an emulsion. After twelve hours’ standing, no emulsion could be formed at all.” Kutschera and Utz” have done some work on the influence of steaming on the fastness of color lakes after dyeing. ‘‘ Having frequently made the observation that when dyed patterns are dried and steamed, they are faster to soaping than those which are not steamed; an explanation of the cause was sought for. The observation was made with aniline colours dyed on antimony tannate or on catechu grounds, further with alizarin and other colours dyed on aluminum, iron, or chromium mordants. The greatest improvement is always noticed when
* Rammelsberg:
Ber. deutsch. chem. Ges., 5 , 1006 (1872). Raoult: Comptes rendus, 92, 189 (1881). 3 De Clermont and Frommel: Comptes rendus, 87,330 (1878). 4 Phot. Correspondenz, 44, 578 (1907); Jour. Phys. Chem., 14,2 2 (1910). Jour. SOC.Chem. Ind., 5 , 532 (1886).
Wilder D. Bancroft the dye-baths are exhausted at a low temperature. Two suppositions for such a change may be offered. First, that the mordant itself, or the colour-lake already fully formed in the dye-bath, is simply better fixed upon the fibre; or, secondly, that during the dyeing operation the colour-lake is incompletely formed, and only completed during the subsequent steaming. If the first supposition is correct, then the amount, both of mordant and colouring matter present in the fibre, would be larger after steaming than before;’ if the second supposition is correct, then the amount of mordant on the fibre would be the same both before and after steaming, and the deeper colour would be due simply to a larger quantity of colouring matter having been taken up. A trial was made by mordanting cloth with three different amounts of chromium mordant and dyeing with alizarin. The cloth was then divided into two parts, one was dried and steamed, the other not. Finally both were soaped, and the chromium estimated in equal portions of the two patterns. I n two cases the steamed patterns contained an increase of chromium ; but the shades of the patterns were proportionately much deeper than could be ascribed simply to the increase of chromium, and in a third case where the steamed pattern contained less chromium the tone of the dyed colour was nevertheless deeper, hence the authors concluded that the second supposition is the most likely one-viz., that the steaming renders the colour faster to soaping by completing the formation of the colour-lake initiated in the dye-bath.” It is a little unsatisfactory to criticjze an abstract of a paper, especially when that abstract is not written by the authors. Since the original is not accessible, one must do the best one can with the abstract. One cannot be absolutely certain whether the difference in tint is due entirely to less 1 [What the authors mean is that soaping would remove more mordant and dye from the sample which had not been steamed than from the one which had been. As worded they imply that steaming increases the amount of the dye present in the fiber instead of saying that it increases the amount which is not removed by soaping.-W. D. B . ]
The Theory
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sf Dyeing
dye being removed from the fiber or whether there is, as the authors assume, a difference due to the coagulation of the dye. This latter is quite possible because we know, for instance, that very finely ground cadmium yellow is paler than a somewhat coarser pigment. The time to have determined this, however, was before the soaping and not after it. In spite of these difficulties the experiments of Kutschera and Utz show that the high temperature has made the dye faster to washing, Similar results have been obtained by Dreaper and Wi1son.I When Kight Blue is adsorbed by silk above 40' C, " a second and more resistant dyeing effect is produced with part of the dye absorbed; this effect increases with the temperature up to 100' C, a greater proportion of the dye being fixed in this way. When dyed a t 1 5 C, ~ a subsequent soaping a t the boil or treatment with alcohol of ordinary strength will practically remove all the dye from the fibre, but as the temperature of the dye-bath is increased, a certain proportion of the dye is fixed against the action of the solvents. I n all cases the total amount of dye on the fibre was kept constant. This effect has been noticed with other basic dyes, but the dyes vary in their resistance against re-solution in some way yet to be determined, especially with different solvents. " These experiments have been extended t o the so-called acid class of dyes, with interesting results : similar differences have been observed when working with the same fibre, so that this action is not confined to basic dyes, the temperature effect being of a similar order when expressed in terms of the resistance of the dye t o the action of solvents. It has not yet been proved, however, that the dye is fixed in two ways, as in the case of Night Blue, when dyeing at a high temperature, which result may be due to the dissociation of the dye into more basic compounds, which are known to be more insoluble. This point is being investigated. The effect of drying the dyed fibre between the drying and re-solution treatments Jour. SOC.Chem. Ind., 26, 667 (1907).
has no appreciable effect on the proportion of colour removed. At any rate an increased temperature of the dye solution gives an increased resistance against the solvent action of soap solution or alcohol in the case of acid dyes.” m’hile we do not need to believe that the Kight Blue is taken up in two different ways, these and other experiments by Dreaper are convincing evidence that a rise in temperature makes most dyes more resistant to solvent action-a result which is absolutely in line with what we have known for years in regard to hydroxides, sulphides, etc. The effect of liquids and solutions in removing color from the fabric seems to be fairly simple theoretically. If the mordant or the dye is readily soluble in a given liquid or solution or is readily peptonized by it, the mordant or the color will be extracted more completely than by a liquid or solution having less solvent or peptonizing action. Many dyes are more soluble in alcohol, for instance, than in water and more color is taken out in these cases by alcohol than by water. Spring has shown that soap’ will peptonize soot or rouge. If it will peptonize rouge there is no apparent reason why it should not peptonize iron, alumina, and chrome mordants as well as the dye itself. That is why soap strips more than plain water. Further, if wash goods are given a more vigorous soaping after drying than they will ever receive under ordinary conditions, they will be practically fast afterwards in the laundry, always supposing that the heavy soaping left any color. If a mordant is precipitated too rapidly, so that it is chiefly on the outside of the fiber, it and the color will tend to rub off readily and will also be removed easily by soap.? There is another possibility to which I have found no reference in the literature, of the color lake becoming friable and less readily adsorbed. We know that an emulsion will often crack on standing and the emulsifying agent will settle as 1
Zeit. Kolloidchemie, 4, 161 (1909); 6, 11, 109, 164 (1910). Knecht, Rawson and Loewenthal: A Manual of Dyeing, 2, 743 (1910).
a curdy mass. It is conceivable that certain dyes, or still more, certain dyes in certain mordants, may change with time so that either the dyes or the mordants are not adsorbed so strongly by the fibers as before. Cnder these hypothetical conditions the dye would rub off readily. This should shorn itself by the freshly dyed fabric being more resistant to rubbing than a fabric which had been dyed quite a while before. This point may be familiar to practical dyers; but it seems not t o be discussed in the books on dyeing. The whole question of fastness t o light is in a very bad way because so little systematic work has been done. About all that I can hope to do a t present is to point out how much we do not know in the hope that some enthusiastic person will fill in some of the gaps in our knowledge some day. Only light which is absorbed can produce chemical action and all light which is absorbed tends to produce chemical action. JVhether a given substance is changed appreciably by light depends upon the readiness with which the substance reacts or upon the presence of suitable depolarizers. With some silver salts or with Eder's solution of mercuric oxalate, we get visible decomposition by light. With chromium salts we get no measurable change. n'ith some substances the action of light causes fluorescence of phosphorescence, thus indicating the occurrence of chemical changes even though these may be no measurable decomposition. While a copper sulphate solution is apparently not sensitive to light, that is because we usually do not have a sufficiently powerful depolarizer present. What change takes place in any given dye when exposed to light will depend on the chemical characteristics of the dye and on the chemical conditions prevailing when the dye is exposed to light.? Methylene blue, for instance, may fade as a result of reduction3 or of oxidation. The bleaching of methylene blue is usually an oxidation bel
Bancroft: Jour. Phys. Chem., 17, 596 ( 1 9 1 3 ) . Bancroft: Ibid., 16, j 2 9 ( 1 9 1 2 ) . Cf. Wender: Jour. Chem. SOC.,66 11, 1 2 2 ( 1 8 9 4 ) .
Jb-ilder D . Bancroft
I54
cause of the oxygen in the air. In presence of gelatine’ or of stronger reducing agents the bleaching of methylene blue by light is due to a reduction. On standing in the dark the leuco base is oxidized and the color comes back. The presence of a reducing agent should, therefore, make oxidizable dyes more stable, at any rate until the reducing agent itself is oxidized. It is claimed by von Grabowski2 that a preparation of zinc, alkali and sugar will make benzopurpurine, benzo-blue, thiazol yellow, and geranin more stable to light. Editorial comment on this is to the effect that the protection does not last long enough to be worth the extra cost. The alleged beneficial action of sodium hyposulphite with some colors3 is probably due to its being a reducing agent. A more interesting case of protection was discovered by Stobbe? who found that many colors were made more stable to light by the addition of copper sulphate, which was taken up by the fiber to form copper mordant. Stobbe attributed this effect to the fact that the copper mordant cut off some of the rays which did the most damage.j This is in line with what apparently happens with vermilion and madder. Toch6 states that vermilion is remarkably permanent when glazed over with madder after it is thoroughly dry. It is hard to see how the madder can act in any way except as a lightfilter. That the copper salt acts as a screen is made more probable by the experiments of Krais7 who increased the fastness to light for diamine blues, benzo-blues, benzo-green, diamine green, benzo-violet, diamine-violet, benzo-azurine, rosazurine, direct black, and direct orange on cotton by adding copper sulphate or a practically colorless mixture of cobalt and nickel sulphates. No protection was obtained with any Gebhard: Zeit. phys. Chem., 79, 639 (1912). Zeit. Farbenindustrie, 2, 399 (1903). 3 Krais: Ibid., I , 2 2 (1902). 4 Eder’s Handbuch der Photographie, 3rd ed., I, 11, 389 (1906). 5 Cf. Justin-Miiller : Zeit. Farbenindustrie, 3, 300 (1904). 6 “Materials for Permanent Paintings,” 108 (191I ) . 7 Zeit. Farbenindustrie, I , 2 2 (1902). 1
The Theory
OJ
Dyeing
I55
yellow. There are only two weak points about this. One is that Krais found no protecting action in the case of colors on wool. Of course, it may be that copper on cotton cuts out certain rays which are not cut out by copper on wool; but there is no evidence as yet that that is the case. The other weak point is that copper salts often act as oxygen carriers' and it is hard to see why they should not sometimes act in this way with dyes. Of course, a negative catalytic agent would be an ideal thing if one knew where to find one.? It is also possible that a careful study would show the presence of positive catalytic agents and that removal of these would decrease the light-sensitiveness. In most cases the lakes are less sensitive to light than the free color^.^ Purpurine bleaches less readily in presence of lime'salts or of alum. Colors on iron, chrome, and copper mordants are less sensitive than on alumina or tin mordants. Eosine with lead mordant is less sensitive than with tin mordant. The data are not sufficient to let us say that dyes on colored mordants are generally faster to light than those on colorless mordants; but it looks a little like this. Stobbe4 found that the alizarine dyes fade more quickly on zinc mordant than on chrome mordant. A number of dyes bleach more rapidly on starch paper than on gelatine or albumenized paper. I have no idea why this is so. Basic dyes are faster to light when mordanted with tannin and fixed with antimony, the antimony apparently being the important factor. It is even claimed that under these conditions methylene blue on cotton becomes quite fast to light.; The facts in regard to the effect of the fiber itself are very confused. Indigo is faster on wool than on cotton6 and the Lothar l l e y e r : Ber. deutsch. chem. Ges., 20, 30j8 (1887). Bigelow: Zeit. phys. Chem., 26, 493 (1898); Titoff: Ibid., 45, 641 (1903).
Cf. Eder: Handbuch der Photographie, 3rd ed.,
I,
11, 384, 389, 390
(19 io),
Zeit. Elektrochemie, 14, 480 (1908). Knecht, Rawson and Loewenthal: A Manual of Dyeing, Stobbe: Zeit. Elektrochernie, 14,480 (1908).
2,
485 (1910).
W-ilder D . Bamroft
156
same is true for the direct colors’ and most of the basic colors.2 On the other hand, safranine and methylene blue3 are said to be more stable on cotton than on wool. It is probable that this discrepancy is due to an unstated difference in the method of dyeing. The experiments would not be comparable, if, for instance, the methylene blue were fixed on cotton with tannin, and antimony and were not so fixed on wool. There are a good many contradictions to be found in the literature. Thus, it is stated in one place that the lakes made from synthetic alizarine are less sensitive to light than those from the madder roots4 On the other hand, Cajarj says that the madder root gives a faster color than synthetic alizarine, purpurine carboxylic acid being the stable substance. Mumford6 voices a popular but unfounded prejudice in regard to synthetic dyes and mordants. “ Color is the Orient’s secret and its glory. These dark-skinned peoples, lagging so far backward along the pathway of civilization, mastered long ago the chromatic mysteries lurking in the shrubs of their deserts, the vines, leaves and blossoms which make these lands radiant, and they have guarded this subtle knowledge from foreign participation with greater care and jealousy than they seem to have exercised for their bodily welfare, or their place among races. The royal purple of Tyre, which the Phoenicians by some magic won from the molluscs of their seas, is virtually obsolete Science has found, in the refuse of factories, gaudy hues to serve the purpose; but the old dyes of the East still boast a splendor and lastingness which chemistry cannot counterfeit-a permanence emblematic of the countries where alone the marvel of their compounding has been understood. . . . Edcr: Handbuch der Photographie, 3rd ed , I , 11, 384 (1910). Knecht, Rawson and Loewenthal: A Manual of Dyeing, 2 , 463 (1910). Ibid., 2 , 458 (1910). 1 Edcr: Handbuch der Photographic, 3rd ed., I , 11, 391 (1910). Zeit. angcw. Chem., 24, 793 (1911). “Oriental Rugs,” 42, 47 (1901). [Not through lack of knowledge. Cf. Bancroft: Philosophy of Permanent Colours, I , 1 5 7 (1813)l. 2
The Theory o j Dyeing
I57
“ T h e distinctive feature of the old Eastern dyeing system was that nearly every tingent was of vegetable or animal origin, and that similar ingredients were employed for mordants or fixations. The treatment of the yarn with borax, saltpeter, tartar, copperas and the like had not been known. The native dyers held to the merits of the old-fashioned mordants-valonia, pomegranate-rind, sumac, divi-divi, and the barks of different trees, from which they had for so long obtained renowned results.” Mumford evidently believes that natural products are faster to light than are synthetic products. This is rather remarkable in view of the way in which leaves and most flowers change color. It would be more accurate to say that most vegetable colors are not fast at all to light. Jl-hat the old dyers have done is to pick out the most permanent of the natural coloring matters. Even then, the situation is not a t all what Mumford pictures it. The chief dyes used by the Oriental dyers are madder, indigo, cochineal, orchil, turmeric or saffron, and Persian berries. Of these madder and indigo are colors which are made commercially from coal t a r ; the coloring matters in Persian berries are flavonol derivatives and could be made on a large scale if it mere worth while; the coloring matter of cochineal is apparently a naphthaquinone derivative though the structure formula is not yet known definitely, while orchil and saffron are admittedly not fast to light. Of these six dyes, three have been made from coal tar and two are not permanent. The mordants used by the Oriental dyers were chiefly impure tannins. Though Mumford rejoices in the fact that the eastern dyers do not use tartar emetic, yet we know that colors mordanted with tannin and fixed with tartar emetic are faster to light than those which are not so fixed. Where the Oriental dyers may have an advantage is in the use of impure mordants. It is quite possible that a suitable mixture of mordants may work better than any single one.’ It seems probable that work along this line might be very well worth while. Cf. Koechlin: Chem. News, 46, 179 (1882).
W i l d e r D. Bancroft
158
It is quite true that many of the so-called aniline colors are not fast to light but there are also others which are quite permanent. It is merely a question of people insisting on getting permanent colors and of being willing to pay the cost. I know of Japanese grass-cloth papers which have stood twenty years of bright sunshine without any marked fading. The making of modern Oriental rugs to rival the old ones is a question of time, skill, artistic taste, and cost; but it is not a question of secrets of dyeing. It is quite probable that it would help matters if the use of fugitive dyes were discouraged. A list might be made shoM;ing what dyes were permanent in fabrics and paper, and if people would insist on having those, we should hear less about the rapid fading of synthetic dyes. The general results of this paper are as follows : I . If a fabric exhaust the dye completely in a given bath, the dyed fabric will not bleed in an exhausted bath or in any solution having a lesser solvent action for the dye in question. 2 . The object of heating a dye-bath is to agglomerate the dye in the fabric and to make it faster to washing. 3 . Many dyes are faster to light in iron, chromium and copper mordants, than in aluminum or tin mordants. It is probable, though not proved, that this is due to certain wavelengths being absorbed more or less completely by the colored mordants. 4. The contradictory statements in regard to fastness of dyes on different fibers are probably due to unspecified differences in making the experiments. It is not legitimate, for instance, to ascribe to the fiber differences in fastness to light between a basic dye on wool and the same dye mordanted on cotton with tannin and antimony. 5. People can get synthetic dyes which are very fast to light if they will insist upon having them and are willing to pay for them. Cornell Cnmersity
