Патент USA US2106368код для вставки
Jan. 25, v1938. w. B. wEscoTT 2,105,358 SHEET ALUMINUM PLANQGRAPHIC PLATE .Filed Nov. 8,- 1955 ` »jg/ffl ' ~ / /0 /0/ - 2,166,368 mentalen. 25, 193s _ UNITED STATES PATENT OFFICE 2,106,368 SHEET ALUMINUMl PLANOGBAPHIC PLATE William B. Wescott, Dover, Mass., assignor, by ' mesne assignments, to Addressograph-Multi graph Corporation, Cleveland, Ohio, a corpora tion of Delaware Application November s, 1935, serial No. 48,935 9 Claims. This invention relates to a method of prepar ing sheet aluminum for planographic printing and to the product of said method; and it com prises treating sheet aluminum in a hot aqueous 5 solution of an alkali aluminate and an alkali hy ’ droxide whereby all traces of grease are removed (Cl. 148-8) , . do not require the adsorption of a lyophilic col 1pm. when the image to be printed has been directly delineated upon the printing surface by means of crayon or the like, it is customary then to “desensitize” the non-printing areas, thereafter to wash out theV image with turpen and both major surfaces of said sheet are pitted. tine, and to replace it with asphaltum or some and sometimes, but not always, coated with a other image-forming material which will take a layer of microporous oxide, and thereafter rins firmer grip on the plate and hence betterwith 10 ing the said so-treated sheet aluminum and stand the attritional effect of the inking rolls 10 sometimes, but not always, washing the same to _during printing; thus assuring the maintenanceA remove any superficial scum which may‘result from said treatment, and then drying the prod uct; whereby are obtained sheet aluminum plan of the. integrity of the image trirougnoutedinons of many thousands of copies. _ ‘ Metal plates are vcustomarily roughened or 15 ographic printing plates, both major surfaces of V grained forl the purpose of minimizing the attri which are “grained” with fine pits characterized _ tional effect of the ink rollers on the image and, by a contained deposit of microporous oxide and perhaps more importantly, .for'the purpose of i which surfaces are sometimes, but not always, further characterized by a surface layer of mi 2o _croporous oxide comprising a trace of alkali, and k which surfaces are basic in reaction and _highly sensitized to direct image-forming materials comprising free fatty acids; all as more fully hereinafter described and claimed. - 25 The art of planographic printing from metal preventing these rollers from too greatly reduc ing the iìlm of moisture which must bel retained .in order -to prevent ink contamination of the non-printing image. 'I'his grain g 'operation is 20 customarily performed by gyrat marbles over ilne sand on the plate; and it is a costly, time `consuming operation requiring much skill. It isan object of -this invention to obviate the plates depends, as does the older lithographie art, l necessity for mechanical graining of plano upon the immiscibility of oil and water and upon ’ graphic plates. It is another object of this in _the preferential retention of a greasy image vention to obviate the heretofore necessary step forming substance by the image areas, and a ofl “counter-etching”, or sensitization, by the 30 similar retention of an aqueous dampening ñuid planographic operator. _It is likewise an object by the non-image areas. In order to> condition of this invention `to obtain a requisite grain and A a metal printing plate for these preferential re sensitization to ink solely by chemical treatment tentions, it has heretofore been customary to of the plate. It is a further object of this in remove all traces of grease and then to “counter vention to prepare aluminum plates for plano 35 etch” the plate to provide a basic surface where- . graphic printing _by treatment with a single with the fatty acid -,component of the greasy chemical reagent» whereby a plate is cleaned, image-forming material might' react. A .so treated metal plate is said to be "sensitized” vto ink. The “sensitization" or “counter-etching” 40 operation has heretofore been performed by the user and usually immediately preceding the de lineation of the image on the plate. After the greasy image has been delineated in known man ner thereupon, the non-image areas are custom- . 45 arily “etched” o_r “desensitized” by treatment with an aqueous solution adapted to at least neu l. e., freed from that contamination by grease which is inevitable -in the production of sheet- aluminum, and is provided simultaneously with an adequate grain and a highly grease-sensitive surface. _ Another object of this invention is to provide aluminum planographic _printing plates which are highly _sensitized to direct image-forming substances comprising fatty acidsand yet are >equally serviceable for the retention of and print tralize the basic condition previously produced, ' ing from photographically produced images. It and preferably to render these areas lyophilic. l.is anotherobject to produce aluminum plano In general _it has been the custom to "desensi graphic printing .plates whereof both major sur 50 tize" by theuse of a weak acid and to .create a faces are conditioned for the reception and re lyophilic condition by means of an adsorbable _ tention of planographlc printing images, and in lyophillc colloîd, of which'gurn arabic is the most particular for such images when they are direct ~ commonly used example. It is also known that ly delineated upon' the said surfaces, as by pen, certain salts of the metal are preferentially wa crayon, or typewriter. A still further .object is to 55 ter-wettable in themselves, and hence if present produce planographic printing plates of sheet 2 2,106,368 aluminum of which both sides are conditioned for use and may be stored indefinitely; and which, after such storage and without further preparation, will firmly retain an image delin common practice, and thus their use affords much saving of time and expense. Moreover, and by reason of the firm retention of the image as orig inally directly delineated on the plate, much less eated with a greasy image-forming material _ skill is required than is requisite when the orig comprising free fatty acid without the necessity inal work has to be washed out and redeveloped, for any “counter-etching” or supplementary. as is the usual practice; and this is of material treatment to sensitize said plates to grease for advantage. Other objects and advantages will ima'ge reception and retention. It is yet another Vbecome apparent as the description proceeds. In the acompanying drawing is illustrated more 10 object of this invention to provide planographic printing plates of sheet aluminum characterized by minute surface pits containing a. vdeposit of microporous oxide of the metal> and further characterized by an alkaline reacting surface. It> 15 is also an object to provide a ñnely pitted‘or grained aluminum planographic printing plate 20 or less diagrammatically a plate formed in ac- , cordance with this invention; ^ Fig. 1 shows a portion of a plate I0 embodying this invention having a character ii upon the 15 surface I2 thereof; Figs. 2 and 3 are enlarged sectional views show with a surface layer of relatively soft micropo rous material comprising an oxide of the metal and an alkali, whereby said plates are particu ing theplate with pits i 5 therein and provided with a coating or layer i6, in Fig. 2 the pits alone being coated and in Fig. 3 both the pits and sur larly sensitive to ordinary carbon paper. face being coated; and It is an object of this invention to provide a process whereby sheet aluminum may be condi tioned to receive and to retain a planographic the plate. printing image, and particularly a directly de 25 lineated image, by a treatment with a single chemical reagent which simultaneously removes all traces of’grease (both superficial grease and >that ground into the metal during the rolling process to which the sheet aluminum is subjected in manufacture) and minutely pits or grains the -surfaces and deposits within the pits an adherent layer of relatively soft microporous material com prising an oxide of the metal and an alkali. It is an object of another embodiment of this inven tion to provide aprocess as above described which, in addition, will coat the said minutely pitted _plates throughout their major surfaces with a layer of the said microporous material. y , Fig. 4 is an enlarged plan view of _ 20 a portion -of - Relatively pure aluminum is particularly suited to the purposes of this invention; and it is pre ferred to use aluminum in the higher grades of 25 purity, although that. alloy »of aluminum and manganese designated by its manufacturers and known to the trade as “38H” may be satisfactorily used, and with particular advantage if a heavy deposit in the pits of the plate or over the pitted surface is desired. A bath suitable for the practice of this inven-tion may be made by dissolving sodium aluminate in ordinary tap water. The sodium aluminate may be replaced either in part or in whole by other alkali aluminates, such» as potassium alu minate; but sodium aluminate alone is preferred. While it has been found convenient to employ the Among the advantages of the process of this ’ best commercial grade of sodium aluminate, so 40 invention may be mentioned the fact that the cost ,called white soluble 90% NaAlOz, the compounds may however be made as required by well-known of preparation of the sheet aluminum for plano methods,` such as by the reaction of sodium hy graphic printing is greatly reduced by the elimi nation of the usual costly and tedious mechanical droxide with metallic aluminum or an aluminum graining operation. Anothervadvantage of the salt. It has been observed that the temperature of 45 said process lies in the fact that the aluminum is thereby both “grained” and sensitized, whereby the bath, the concentration of alkali in the bath, the necessity for "counter-etching” or sensitizing the time of treatment, together with the ratio by the user is avoided. Still another advantage of aluminum surface to bath volume are to some is that both sides of the plates of ,this invention extent compensating variables. Thus,-differences are simultaneously prepared for use and this in concentration can be offset and substantially 50 the same result obtained by inverse adjustment' without resort to building up laminated struc ofthe temperature or time of treatment, or both; tures as has been heretofore suggested. Among the advantagesv of the plates of this and also by varying the ratio of surface tobath invention may be mentioned the fact that they volume. In general, a bath is preferred which contains 55. are useful for the reception and retention of im ages directly delineated by greasy image-forming five grams of the 90% sodium aluminate referred materials comprising fatty acids, and for photo graphicimages as well. Another advantage of the said plates is that the high sensitivity to fatty acids imparted by the process of this invention is not diminished by long storage, and yet their grease-sensitive surfaces may be readily desensi tized by such weakly acidic etches as are buñered at or about the isoelectric point of usual photo graphic image-forming' materials. Still another advantage of the plates of this invention is that they are particularly retentive of poorly coherent greasy image-forming materials‘such as the more or less pulverulent coating of ordinary carbon 70 paper. -A still further advantage of the said plates is that they *arev adapted to print from an image, as directly. delineated in crayon or the. like, without the necessity of washing out the original image forming material and the replace 75 ment thereof with asphaltum or the like as is to above for every 100 cc. of water. Best results are obtained when the bath is maintained at a temperature slightly below the boiling point. A temperature of 195° F. plus or minus 5° has been found satisfactory. When operating continuous ly, a ratio of one square foot of aluminum surface to every five gallons of the preferred solution and an immersion time of 11/2 minutes provide a satis factory relationship.4 A substantially equivalent 05 relationship for processing by the batch method consists of a ratio of one square foot of aluminum surface to every two and one-half gallons of solution, and an immersion time of 11/2 minutes with a pause between batches of 11/2 minutes. If 70 there -were nopause between batches,- the rela tionship would- be the same as that given for the continuous operation, for reasons which will later become apparent. One of the desirable effects of treating alumi 3 :2,106,368l num in a bathof an alkali aluminate is an ero-v mersed in such a freshly made solution; the mol. sion or pitting of the surface of the metal. This , percent of sodium hydroxide drops rapidly as' the erosion is accompanied by loss of Weight and loss aluminum reacts therewith; and then; as hydrol of thickness of the sheet aluminum. 'I'he treat ysis is in some unknown way stimulated as a ment is-considered >to be at itsOptimum when consequence of this reaction, the mol. percent of -maximum pitting occurs with minimum loss of.' sodium hydroxide rises again. By adjustment of u thickness. The optimum treatment of sheet one or more ofthe several mentioned variables, aluminum according to this invention may be the mol. percent of sodium hydroxide may be < obtained over a wide range of concentrations of maintained within the range of that of the bath 10 sodium aluminate by adjusting the above-men as freshly made; and, as stated, a molal concen-l tioned variables, as stated. This adjustment is tration of I15% of sodium hydroxide has been 10 _not critical in so 'far as the production of a found convenient and satisfactory. Thus, when satisfactory pitting or graining is concerned, and there is kept immersed about one square foot of approaches the critical only when it is desired aluminum surface in five gallons of solution con-y 15 so to balance all the variables that the loss of taining flve grams of sodium aluminate for every thickness is held to a consistently low value. The „ 100 cc. of water and maintained at about 195° F 15 above-given example of a practical relationship the bath will remain in substantial equilibrium between concentration, temperature, treatment at about 15 mol. percent sodium hydroxide con time and ratio of aluminum surface to solution centration and the optimum treatment of the Y, volume, is given by way of illustration partly aluminum surface will obtain in about 11/2 min- ` because adherence to'these conditions has yielded » utes. The loss of weight of aluminum is direct _20 25 `satisfactory results in practice, and partly be- » `ly proportional to the time of immersion; and, cause they are particularly convenient. under the stated conditions, _the loss of weight The behavior of hot alkali alumlnate baths, asf per square foot of immersed surface lies in the influenced by reaction with commercial _alurni close vicinity of two grams. When the described num, is not wholly understood: but certain theo retical considerations are-hereinafter given as being useful in clarifying the probable relation Aship of several factors, although not relied upon conditions have been closely maintained, the deviation' from the mean loss of weight has in practice been found to be of t-he _order of plus as factual. y or minus 0.1 gram.l . Inasmuch as some water is lost by evaporation - A freshly prepared solution ‘of sodium alumi , and some solution is entrained with the metal nate in the preferred concentration of five grams when it is removed from the bath, it is periodical of the commercial substance to every 100 cc. of ly necessary to replace both. It has been found ‘ water appears to bel substantially stable over a _convenient to replace lost water by such additions period of lmany hours. When such a hot solution as Will maintain an approximately constant solu is reacted with commercial aluminum- (as for in tion level, and to replace the alkali periodically 35 stance that grade designated by its makers as “3SH”), the bath loses its initial stability and the sodium aluminate hydrolyzes to form aluminum hydroxide or oxide, and sodium hydroxide. The by addition of sodium aluminate in an amount which is indicated by titration. In general, a loss of solution corresponding to 10% of the original total alkali `is permissible._ It has been found in practice that such lossmay occur in from three (0 to six hours of operation depending upon whether former is precipitated, for the most part, as a sediment; the latter is deemed to be the active agent -of the bath, and it reacts with the alumi the operation is continuous or discontinuous. The num to form sodium aluminate. In so `doing it discontinuous, or batch, operation involves use erodes or pits the metal and correspondingly ex of holding means for the individual aluminum hausts itself. For any given initial concentration sheets and hence results in the entrainment of 45 of sodium aluminate, the amount of sodium hy more solution than is occasioned by drawing a droxide in the bath at any time, after hydrolysis continuous ribbon of sheet aluminum through a- ‘ has been initiated by the reactive presence there bath. Continuous operation is preferred. in of metallic aluminum and under otherwise con When operating by the batch method, the indi stant conditions, depends upon the ratio of the vidual sheets of aluminum are preferably main so surface of 'aluminum immersed in the bath to the‘ tained in a substantially vertical position“ and volume of the bath. . Disregarding such small amounts of impurities as there may be, alkali is present in the bath as sodium hydroxide and as sodium aluminate, and each canbe determined as such by titration. It is convenient to express the concentration of sodium hydroxide in terms of mol. percent; that that is to say that the ratio of the mols of sodium hydroxide to the mols of total alkali present may be expressed in percentage. While satis factory> plate surface treatment is obtainable from alkali aluminate baths over a wide range of molal concentrations of sodium hydroxide, it has been found that about 15 mol. percent sodium hydroxide concentration corresponds to substan tial equilibrium at an expedientrate of treatment and at a convenient ratio of bath volume to im 70 mersed aluminum surface. ' spaced apart by at least two inches. In continu ous operation the -desired substantially verticall position of the- aluminum is suñîciently obtained by feeding thegribbon of metal through the bath in one or more loops such that for the most part -the ribbon is vertically pendant therein. Pjor certain purposes, such as forming a direct image by means of a greasy substance compris ing very little free fatty acid or a substance of such friable‘or pulverulent a nature as topoorly contact the plate'surface, it has been found de sirable as part of this invention to substanti-ally coat the entire surface of the plate with' such microporous material as is discernibly retained 65 only in the pits of the plates made as above de scribed. , ‘ / ns a means of coatingthe entire surface of sheet aluminum with the said microporous ma Freshly made solutions of commercial 90% so- ' terial and as a means particularly effective when dium aluminate containing five grams of the ma terial to every 100 cc. of water were found by titration to have between 1l and`16 mol. percentl sodium hydroxide. When aluminum is first im relatively thick coatings are desired, i. e., coat-_ ings as thick as between two ‘and three vone thousandths of a millimeter, the sheet aluminum may be simultaneously pitted y'and coated in a 75 2,106,368 It w111 be obvious to those skilled in the art that the process of this invention is, as stated,> susceptible of wide variation and it is to be un tween 5 and 10 percent, with an immersion time of . derstood that the examples of the preferred rela ten minutes. Under these conditions, the loss of tionship of the several variables are given by way weight is of the order of one "gram per square of illustration only. The effect of the described treatment is three foot of surface and the loss in thickness is negli- . fold: it removes not only the superficial grease gibly small. While baths of higher sodium alumi but also that grease which has been ground into nate content may be adjusted to yield satisfac 10 torily coated plates, less concentrated baths are metal in the process of rolling it into sheet form; _ not in general desirable because they may be, and it so pits thesurface as to provide a ñne “grain”; usually are, ineñective to remove completely from y and it leaves the surface in an alkaline-reacting state.- The complete absence of grease is of . the metal the grease commonly associated there single bath made up of 1.5 of sodium aluminate to every 100 cc. of Water and adjusted to a molal concentration of sodium hydroxide be with. 15 . As stated, aluminum hydroxide is precipitated as a sediment; and it is`convenient to allow this sediment to collect in the bottom- of the tank and to remove it at daily intervals. Best'results are obtained when the ‘sediment is not agitated, 20 although avoidance of agitation is not essential. It is, however, essential for uniform results and therefore desirable that the metalbeing treated should be kept from contact with the concentrated Í sediment’. The treating solution is conveniently heated by jacketing the treating tank in known manner.A , 'I'he treated sheet aluminum should be rinsed in running water immediately after treatment course of vital importance because any residual trace of grease would actas an ink-receptive image and would print a tone in the non-printed areas. Of vital importance also is the alkalinity of the resultant surface, because upon this alka A15 ' linity depends that sensitivity to fatty acids which is necessary to the retention of direct 20 images under printing conditions and for long editions. The “grain” constituted by the pits, while not a prime essential, is nevertheless highly desirable because the dampening solution (with which the plate is wet between successive inking's 25 during printing) is better retained by the plate when the surface continuity of its plane surface is broken up by suchv “grain” as is provided by the pits of this invention; and also,'although less importantly, because this “grain” provides a 30 “tooth” which is of advantage when a direct image is delineated in pencil or crayon. 0n the in order to remove the entrained solution. If 30 rinsing is deferred, an unsightly brown'discolora tion of the metal sometimes occurs; and while this discoloration seems not to adversely affect the behavior of the aluminum as a printing plate, it other hand, the “grain” ofthe plates of this is unsightly, as stated, and for' that reason its .invention is of so ñne a texture that it is not35 formation is preferably prevented. After rins » objectionable when the image is formed photo 35 - ing, the aluminum is swabbedwith a soft rubber graphically from even the ñnest halftones. _While it is feasible to print 4from aluminum -sponge or with cotton wadding. When aluminum alloyed with manganese (that known to .the plates which have been treated by the process of --trade as “3SH”) is treated in the alkali baths of vthis invention merely tothe extent of removing> -this invention, it becomes loosely coated with a the grease and of rendering the surface alkaline 40. dark grayish deposit which is readily removed by reacting but without any substantial pitting, such swabbing. Drying of, or pressure against this limited treatment is not recommended because gray coating should be avoided inasmuch as its not all the advantages of this invention are fully - removal is thereby rendered more diillcult. The 45 purer grades of aluminum do not show this gray deposit but are preferably swabbed to remove any realized when printing from plates wholly devoid of "grain”. These advantages are most fully 45 realized when the-pitted area constitutes between loose precipitate that may survive the 'rinsing 40 and 70% of the total surface. Under the con operation. After the rinsing and swabbing, the ditions of the examples above given, the follow ing relationship obtains. The number of the pits sheet aluminum is dried in any convenient man 50 ner and is then ready for use without any further treatment. . ` - per square millimeter of surface lies between two 50 and ñve thousand. .The individual pits are gen While the plates of this invention may be made erally spaced apart although occasionally they encroach upon each other, and they are of gen under widely variant conditions of bath, the opti erally rounded and approximately circular con mum treatment, for consistent results, will ob tour. The mean diameter of the pits varies over 55 55 tain when the molal concentration of alkali hy droxide in the bath is maintained a constant; that the range lying between something less than 0.001 is, when the rate of the consumption of alkali and 0.015 millimeter; It is observed that in gen hydroxide is equal to the ratev of its liberation through hydrolysis -of the alkali aluminate. Al though high molal concentrations of alkali hy droxide cause rapid loss of weight, equilibrium of thebath at these concentrations requires a low ratio of surface area to~vo1ume; and di' eral the average of the mean diameters of the pits in a given area is roughly inversely proportional to the concentration of pits in that area. Thus when the number of _pits per square millimeter is of the -order of ñve thousand, the average of their . mean diameters is of the order of 0.008 milli minishing returns result. In fact, it has been - meter; and when there are but two thousand pits found that the productivity of a bath (atleast. per square millimeter, the average of their mean in the lower ranges of both the molal concentra ¿diameters is of the order of 0.012 millimeter. The tión' of alkali hydroxide and th‘e concentration ratio of pitted to non-pitted areas appears to vary of total alkali) .is inversely proportional to the much less than does the concentration of pits equilibrium of molal concentration. Hence, it is from one minute area to the next. The depth of 70 in general preferred so to adjust the severalï the pits Vappears to be roughly proportional to variables as to -maintain a relatively low equi- _' their mean diameter. The “grain” constituted by the pits as described and within the above librium' of molal concentration of alkali hydrox ide, since under these conditions the vmaximum limits is very fine when compared to the “grain” yield of treated surface per unit of bath volume - which is obtained by abrasive attrition under gy 75 in unit time is approximated. rating marbles, as is the common practice; and 'l y ‘2,106,868 5 yet it is fully e?'ective to retain, against the , 'I'he microporous deposits on thev plates of this squeegee action of the ink rollers, a sufficiency of invention may be distinguished from both normal dampening solution. It is one of the advantages atmospheric oxide and from anodic aluminum of the plates of this invention that less dampen oxide by two distinct characteristics. Thus, ing solution is required to maintain the non when scratched with a needlepoint, the said de printing portions clear from tone than is re posit appears under the microscope and at- a quired by plates with the finest grain mechani magnification of 100 diameters or so, to consist cally obtainable. This advantageous behavior is of whitish particles suggestive of a relatively thought to be due to the'high moisture retentivity delicate and iinely crystalline ~material and dis 1.o of the open-textured or microporous material de tinctly not comparable to the powdery product posited in the pits and sometimes over the entire of a similarly scratched hard amorphous mass, surface.` The presence of this microporous ma. such as the relatively dense aluminum oxide pro _ terial, in contrast to the extremely thin normal duced by anodic deposition. It is thought that oxide, may be readily observed by microscopic in- . the expression "microporous crystalline oxide” is spection. If the microporous deposit is limited to -useful in distinguishing the oxide deposited by the pitted areas of the plate, its presence though the process of this invention both from normal not its porosity may be observed by inspection of -atmospheric oxide and from oxide produced a microtomed section under vertical incident il anodically. lumination. If the deposit extends beyond the The second distinguishing characteristic is 20 pitted areas, it may be discerned without section vthat the microporous crystalline oxide deposited ing, by microscopic inspection under~ vertical in` by the process of this invention is alkaline re cident illumination. In, either case, whether lim acting. whereas normal atmospheric and anodicited to the .pitted areas or extended over the en oxides are acid reacting. Whether the alkali is tire surface of the plate, the relative thickness merely entrained by, or occluded in, ` the mi of the deposit may be estimated microscopically cropores of the crystalline oxide, or whether it under incident dark field illumination. ' is a component of a compound comprising the The microporosity of the deposit may be dem oxide, is not known; but the` presence of the onstrated in terms of its high adsorptive capacity alkali in the deposited material is of prime ad-for finely -divided pigment, as carbon black for vantage. One of the advantages of the alkaline 30 instance, by the following procedure, which also serves to show the contrasting behavior of the normal oxide coating on the non-pitted areas when the latter are free from the microporous de posit of the process of this invention. Usual ' black lithographie ink is first well rolled onto a restricted area of the plate and then washed out with an appropriate solvent, as carbon tetrachlo ride for instance. The so-treated area, when 5 10 15 20 25 reaction of the deposit is that it provides a pre 30 sensitized surface, i. e.,. a surface with which the fatty acids of direct image-forming materialsv may react; vand thus is avoided the necessity for sensitizing or counter-etching by the user. An other advantage is manifested by the substan .35. tially greater life under printing conditions of an image derived directly from a greasy image forming substance containing a free fatty acid microscopically viewed` at a magnification of reactive with said alkali over the life of any about 100 diameters, will be observed to be of direct image obtainable in the absence of said 40 undiminished metallic brilliancy in the non alkaline component in the deposit. pitted areas and dark gray in the pitted areas, If a restricted portion ofa plate of this inven Repeated washings with solvent are not eñ'ective tion be treated with acid, as dilute phosphoric to appreciably lighten the grayness of the pitted acid for instance; and it be then thoroughly areas; from which observation'it is concluded ,washed and dried, and an image thereafter di 45 that the pigment of the ink'is iìrmly adsorbed rectly delineated upon both the normal surface within micropores of the deposit on the pits. ' and the acid-'treated surface .under like Vcondi When the microporous deposit covers th‘e non tions, it will be found that the printing life of pitted as well as the pitted areas, the above the image on the normal surface greatly exceeds described procedure aifords a test for the con that of the corresponding image on the acid» -5o` tinuity of the deposit as well as for` its micro treated surface. That the relative capacities for porosity. If the deposit is continuous over the 'direct-image retention on the surface of a plate ~ I entire surface, the demarcation of the inked area, which is sensitized by the process of this inven after washing the ink therefrom, will still be ' tion and of a portion of that surface as modi .. macroscopically discernible; although contrast flecl by desensitization with acid indicate not between these and the surrounding normal areas merely a'difference vin degree in sensitivity but a of the plate surface will be very much weaker. diiference in kind 'of surface may be demon When viewed microscopically, the entire inked strated by a simple and expeditious bench test. and washed area will appear markedly darker Thus,_if a plate of this invention be desensitized in color than the surrounding normal plate sur over part of _its area with an acid, as phos 60 face and will comprise at least two distinct phoric for instance; and if, after thorough Wash shades; the darker of these shades will appear as ing and drying, it be rolled up with a usual litho isolated small patches against a continuous back graphic ink and then flooded with a weakly ground of a lighter- and sometimes slightly vary acidic etch, comprising .an acid phosphate such .-ing shade. In Figure 4 of the drawing, the ' as that disclosed in U. S. Patent No. 2,003,268, 65 darker areas- of the coating, corresponding to it will be observed that after standing fora few the pits I5, are indicated by the numeral I'I andl minutes the ink over the desensitized area will the lighter areas by the numeral I8. . ‘I'he ap withdraw from the plate and expose minute areas parent density of the coloring, residual after the of bare metal. It lwill be further observedthat 70 above-described inking and solvent treatment, is these ink-denuded areas gradually increase in believed to be proportional to the thickness of size until they become connected with their ad 70 the coating, whichis greatest in the isolated jacent neighbors; and that the plate becomes areas overlying the deeper pits and is least in> substantially freed from ink without mechanical the expanses of the unpitted or very slightly intervention, unless theink has been rolled up pitted areas lying therebetween. very thickly, in which case the last traces of ink 75 6 2,108,388 may require gentle swabbing for their complete `an alkali aluminate until the surface of said removal. The ink on the sensitized or normal aluminum is freed from grease, is minutely pitted. surface of the plate will be found to be undis turbed by the acidic etch. It is believed that the retention of the ink by the surface of the plates of this invention, against the action of the acidic etch, is due to chemical reaction between the fatty acids of the ink and the alkali component of the oxide ñlm coating the surface of these and is sensitized to fatty acids. plates. poë'ous aluminum oxide comprising a trace of 10 > The presence of alkali, as a component of the oxide deposited on the surface of these plates, may be demonstrated colorimetrically; but, be cause of the extremely small amount of deposited 15 oxide, this determination is tedious and difficult. A characteristic difference between the normal alkaline surface of the plates of this invention and such surface after desensitization by an acid “etch” may otherwise be demonstrated by observ 20 ing the relative wettability of these surfaces by aqueous ethyl alcohol comprising 50 to 55% of water. A drop of this mixture will spread im ~ 3. 'I'he process of preparing vphanographic printing plates from sheet aluminum which com prises immersing said sheet aluminum in a solu tion of an alkali aluminate until the surface of said aluminum is` freed »from grease, and is grained with minute pits coated with a micro- ' al ali. ‘ 4. The process of preparing planographic printing plates from sheet aluminum which com prises immersing said sheet aluminum in a so lution of an alkali aluminate until the surface of 15 said aluminum is freed from grease, is grained with minute pits, and is coated with'a micropo rous aluminum oxide comprising a trace of a1 kali. ~ 5. A planographic printing plate of sheet 20 aluminum characterized by minute pits of the order of from two to ilve thousand per square mediately when applied to the normalA surface of millimeter and of generally .rounded contour in the plate, but will not spread when applied to an otherwise substantially plane surface, the vprinting _surface of said plate being alkaline re 25 the desensitized or acid-treated/ surface. While the planographic printing plates of this acting and highly sensitive to fatty acids. 6. A planographic printing plate of sheet invention which have their entire surfaces cov aluminum characterized by minute pits of the ered with a. continuous layer of microporous de posit (see Fig. 3) are particularlyuseful for the order of from two to five thousand per square 30 reception and retention of images formed from_ millimeter and of generally rounded contour in 30 the less reactive grades of carbon paper and for vvan otherwise substantially plane surface, said free hand work with the drier grades of crayon, pits being partially filled with a deposit of micro porous oxide and said surface being alkaline re lthey are not particularly satisfactory for photo acting and highly sensitive to fatty acids. graphic reproduction. Because of the very open 7. A planographic printing plate of sheet alu-‘_ 35 textured surfacing, which advantageously retains minum characterized by minute pits of the o_rder dampening solution, the unexposed photo-sensi tive material with which they are sensitized for ~ of from two to ilve thousand per square millime photographic reproduction cannot be wholly . ter and of generally rounded' contour inI an other wise substantially plane surface, said pits and washed out of the porous surface layer on devel opment; and hence the obtainment fromvsuch surface'being coated with a deposit of crystalline plates of clear tone-free whites in the non-print lng areas is generally diflicult and more particu larly so with fine halftones. When an image or part of an image is to be formed photographically on plates of this invention, itis preferable to use plates on the surfaces of which the microporous deposit is segregated within the pits and gen erally below the level of the non-pitted areas (see Fig. 2) , It is believed that even though some of the unexposed photosensitive material may remain in the micropores of the deposit within the pits, the dampening solution is so firmly re tained in the pits against the squeegee action ‘ of the ink rolls as to eifectively protect from ink contamination such traces of photosensitive ma terial as may have been there retained. I claim: ' ' . ' 1. In aprocess of preparing planographic print ing plates from sheet aluminum, that step which -microporous oxide thicker in the pits than over the substantially plane surface comprising a trace of alkali and being highly sensitive to fatty acids. ‘ > , 8. A planographic printing plate of sheet alu minum characterized by between two and ñve thousand minute pits of generally rounded con tour to the _square millimeter in an otherwise sub stantially plane surface, said pits having a mean diameter o_f the order of five one-thousandths of va millimeter and said surface being alkaline re acting and highly sensitive to fatty acids. , 9. A planographic‘printing plate of sheet alu minum characterized by between two and ilve thousand minute pits of generally rounded con tour to the square millimeter in anotherwise'sub stantially plane surface,`said pits having a mean _ diameter of the order of ilve one-thousandths of a millimeter and said pits and surface being coated comprises reacting saidaluminum with an alkali ' with a deposit of a crystalline microporous oxide 05 hydroxide in hot solution with an alkali alu minate, whereby the surface of said aluminum is freed from grease and is sensitized to fatty acids. comprising a trace of alkali and being highly sensitive to fatty acids, said deposit h’aving a f thousandths of a millimeter. 2. 'I'he process of preparing planographic print mean thickness of between one and three one ' . ing plates from sheet aluminum which comprises immersing said sheet aluminum in a solution-of WILLIAM B. WESCOTI‘.