US2106368A - Sheet aluminum planographic plate - Google Patents

Description

Jan. 25, v1938. w. B. wEscoTT 2,105,358

SHEET ALUMINUM PLANQGRAPHIC PLATE .Filed Nov. 8, 1955 jg/ffl mentalen. 25, 193s PATENT OFFICE SHEET ALUMINUMl PLANOGBAPHIC PLATE William B. Wescott, Dover, Mass., assignor, by mesne assignments, to Addressograph-Multigraph Corporation, Cleveland, Ohio, a corporation of Delaware Application November s, 1935, serial No. 48,935

9 Claims.

This invention relates to a method of preparing sheet aluminum for planographic printing and to the product of said method; and it comprises treating sheet aluminum in a hot aqueous solution of an alkali aluminate and an alkali hydroxide whereby all traces of grease are removed and both major surfaces of said sheet are pitted.

and sometimes, but not always, coated with a layer of microporous oxide, and thereafter rins- 10 ing the said so-treated sheet aluminum and sometimes, but not always, washing the same to remove any superficial scum which mayresult from said treatment, and then drying the product; whereby are obtained sheet aluminum planographic printing plates, both major surfaces of V which are grained with fine pits characterized by a contained deposit of microporous oxide and 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. The art of planographic printing from metal plates depends, as does the older lithographie art, upon the immiscibility of oil and water and upon the preferential retention of a greasy imageforming substance by the image areas, and a similar retention of an aqueous dampening uid by the non-image areas. In order to condition A a metal printing plate for these preferential retentions, it has heretofore been customary to remove all traces of grease and then to counteretch the plate to provide a basic surface wherewith the fatty acid ,component of the greasy image-forming material might' react. A .sotreated 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 delineation of the image on the plate. After the greasy image has been delineated in known manner thereupon, the non-image areas are customarily etched o r desensitized by treatment with an aqueous solution adapted to at least neutralize the basic condition previously produced,

and preferably to render these areas lyophilic. In general it has been the custom to "desensitize" by theuse of a weak acid and to .create a lyophilic condition by means of an adsorbable lyophillc collod, of which'gurn arabic is the most commonly used example. It is also known that certain salts of the metal are preferentially water-wettable in themselves, and hence if present 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 turpentine, and to replace it with asphaltum or some other image-forming material which will take a firmer grip on the plate and hence betterwithstand the attritional effect of the inking rolls during printing; thus assuring of the. integrity of the image trirougnoutedinons of many thousands of copies.

Metal plates are vcustomarily roughened or grained forl the purpose of minimizing the attritional effect of the ink rollers on the image and,

the maintenanceA perhaps more importantly, .for'the purpose of i preventing these rollers from too greatly reducing the ilm of moisture which must bel retained .in order -to prevent ink contamination of the non-printing image. 'I'his grain g 'operation is 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 l necessity for mechanical graining of planographic plates. It is another object of this invention to obviate the heretofore necessary step ofl counter-etching, or sensitization, by the planographic operator. It is likewise an object of this invention `to obtain a requisite grain and sensitization to ink solely by chemical treatment of the plate. It is a further object of this invention to prepare aluminum plates for planographic printing by treatment with a single chemical reagent whereby a plate is cleaned, 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 printing from photographically produced images. It l.is anotherobject to produce aluminum planographic printing .plates whereof both major surfaces are conditioned for the reception and retention of planographlc printing images, and in particular for such images when they are directly delineated upon' the said surfaces, as by pen, crayon, or typewriter. A still further .object is to produce planographic printing plates of sheet 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 delineated with a greasy image-forming material comprising free fatty acid without the necessity for any counter-etching or supplementary.

treatment to sensitize said plates to grease for ima'ge reception and retention. It is yet another 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 is also an object to provide a nely pittedor grained aluminum planographic printing plate with a surface layer of relatively soft microporous material comprising an oxide of the metal and an alkali, whereby said plates are particularly sensitive to ordinary carbon paper.

It is an object of this invention to provide a process whereby sheet aluminum may be conditioned to receive and to retain a planographic printing image, and particularly a directly delineated image, by a treatment with a single chemical reagent which simultaneously removes all traces ofgrease (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 comprising an oxide of the metal and an alkali. It is an object of another embodiment of this invention 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.

Among the advantages of the process of this invention may be mentioned the fact that the cost of preparation of the sheet aluminum for planographic printing is greatly reduced by the elimination of the usual costly and tedious mechanical graining operation. Anothervadvantage of the said process lies in the fact that the aluminum is thereby both grained and sensitized, whereby the necessity for "counter-etching or sensitizing by the user is avoided. Still another advantage is that both sides of the plates of ,this invention are simultaneously prepared for use and this without resort to building up laminated structures as has been heretofore suggested.

Among the advantagesv of the plates of this invention may be mentioned the fact that they are useful for the reception and retention of images directly delineated by greasy image-forming materials comprising fatty acids, and for photographicimages 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 desensitized by such weakly acidic etches as are buered at or about the isoelectric point of usual photographic image-forming' materials. Still another advantage of the plates of this invention is that they are particularly retentive of poorly coherent greasy image-forming materialssuch as the more or less pulverulent coating of ordinary carbon 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 replacement thereof with asphaltum or the like as is 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 originally directly delineated on the plate, much less skill is required than is requisite when the original work has to be washed out and redeveloped, as is the usual practice; and this is of material advantage. Other objects and advantages will Vbecome apparent as the description proceeds.

In the acompanying drawing is illustrated more or less diagrammatically a plate formed in accordance with this invention;

^ Fig. 1 shows a portion of a plate I0 embodying this invention having a character ii upon the surface I2 thereof;

Figs. 2 and 3 are enlarged sectional views showing 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 surface being coated; and y Fig. 4 is an enlarged plan view of the plate.

Relatively pure aluminum is particularly suited to the purposes of this invention; and it is preferred to use aluminum in the higher grades of 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 portion of 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 aluminate; but sodium aluminate alone is preferred. While it has been found convenient to employ the best commercial grade of sodium aluminate, so- ,called white soluble NaAlOz, the compounds may however be made as required by well-known methods,` such as by the reaction of sodium hydroxide with metallic aluminum or an aluminum salt.

It has been observed that the temperature of the bath, the concentration of alkali in the bath, the time of treatment, together with the ratio of aluminum surface to bath volume are to some extent compensating variables. Thus,-differences in concentration can be offset and substantially the same result obtained by inverse adjustment' ofthe temperature or time of treatment, or both; and also by varying the ratio of surface tobath volume.

In general, a bath is preferred which contains five grams of the 90% sodium aluminate referred to above for every cc. of water. Best results are obtained when the bath is maintained at a temperature slightly below the boiling point. A temperature of F. plus or minus 5 has been found satisfactory. When operating continuously, 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 satisfactory relationship.4 A substantially equivalent 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 there were nopause between batches,- the relationship wouldbe the same as that given for the continuous operation, for reasons which will later become apparent.

One of the desirable effects of treating aluminum in a bathof an alkali aluminate is an ero-v sion or pitting of the surface of the metal. This erosion is accompanied by loss of Weight and loss of thickness of the sheet aluminum. 'I'he treatment is-considered to be at itsOptimum when -maximum pitting occurs with minimum loss of.'

thickness. The optimum treatment of sheet aluminum according to this invention may be because adherence to'these conditions has yielded `satisfactory results in practice, and partly because they are particularly convenient.

The behavior of hot alkali alumlnate baths, asf

influenced by reaction with commercial alurninum, is not wholly understood: but certain theoretical considerations are-hereinafter given as being useful in clarifying the probable relation- Aship of several factors, although not relied upon as factual. y

A freshly prepared solution of sodium aluminate in the preferred concentration of five grams of the commercial substance to every 100 cc. of water appears to bel substantially stable over a period of lmany hours. When such a hot solution is reacted with commercial aluminum (as for instance 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 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 aluminum to form sodium aluminate. In so `doing it erodes or pits the metal and correspondingly exhausts itself. For any given initial concentration of sodium aluminate, the amount of sodium hydroxide in the bath at any time, after hydrolysis has been initiated by the reactive presence therein of metallic aluminum and under otherwise constant conditions, depends upon the ratio of the surface of 'aluminum immersed in the bath to the 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 satisfactory 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 substantial equilibrium at an expedientrate of treatment and at a convenient ratio of bath volume to immersed aluminum surface.

Freshly made solutions of commercial 90% sotitration to have between 1l and`16 mol. percentl sodium hydroxide. When aluminum is first im- 100 cc. of water and maintained at about 195 F mersed in such a freshly made solution; the mol.

, percent of sodium hydroxide drops rapidly as' the aluminum reacts therewith; and then; as hydrolysis is in some unknown way stimulated as a consequence of this reaction, the mol. percent of sodium hydroxide rises again. By adjustment of u one or more ofthe several mentioned variables,

the mol. percent of sodium hydroxide may be maintained within the range of that of the bath as freshly made; and, as stated, a molal concen-l tration of I15% of sodium hydroxide has been found convenient and satisfactory. Thus, when there is kept immersed about one square foot of aluminum surface in five gallons of solution con-y taining flve grams of sodium aluminate for every the bath will remain in substantial equilibrium at about 15 mol. percent sodium hydroxide concentration and the optimum treatment of the Y,

aluminum surface will obtain in about 11/2 minutes. The loss of weight of aluminum is direct- `ly proportional to the time of immersion; and,

, and some solution is entrained with the metal when it is removed from the bath, it is periodically necessary to replace both. It has been found convenient to replace lost water by such additions more solution than is occasioned by drawing a continuous ribbon of sheet aluminum through abath. Continuous operation is preferred.

When operating by the batch method, the individual sheets of aluminum are preferably maintained in a substantially vertical position and spaced apart by at least two inches. In continuous operation the -desired substantially verticall position of the aluminum is suciently 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 comprising very little free fatty acid or a substance of such friableor pulverulent a nature as topoorly contact the plate'surface, it has been found desirable as part of this invention to substanti-ally coat the entire surface of the plate with' such microporous material as is discernibly retained only in the pits of the plates made as above described.

ns a means of coatingthe entire surface of sheet aluminum with the said microporous material and as a means particularly effective when relatively thick coatings are desired, i. e., coatsingle bath made up of 1.5 of sodium aluminate to every 100 cc. of Water and adjusted to a molal concentration of sodium hydroxide between 5 and 10 percent, with an immersion time of ten minutes. Under these conditions, the loss of weight is of the order of one "gram per square foot of surface and the loss in thickness is neglithe metal the grease commonly associated therewith.

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, 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 ing, the aluminum is swabbedwith a soft rubber -sponge or with cotton wadding. When aluminum alloyed with manganese (that known to .the

--trade as 3SH) is treated in the alkali baths of this invention, it becomes loosely coated with a dark grayish deposit which is readily removed by swabbing. Drying of, or pressure against this gray coating should be avoided inasmuch as its removal is thereby rendered more diillcult. The

purer grades of aluminum do not show this gray deposit but are preferably swabbed to remove any loose precipitate that may survive the 'rinsing operation. After the rinsing and swabbing, the

sheet aluminum is dried in any convenient manner and is then ready for use without any further treatment.

While the plates of this invention may be made under widely variant conditions of bath, the optimum treatment, for consistent results, will obtain when the molal concentration of alkali hydroxide in the bath is maintained a constant; that is, when the rate of the consumption of alkali hydroxide is equal to the ratev of its liberation through hydrolysis of the alkali aluminate. Although high molal concentrations of alkali hydroxide cause rapid loss of weight, equilibrium of thebath at these concentrations requires a low ratio of surface area to-vo1ume; and di'- minishing returns result.

in the lower ranges of both the molal concentratin' of alkali hydroxide and the concentration of total alkali) .is inversely proportional to the equilibrium of molal concentration. Hence, it is in general preferred so to adjust the several variables as to maintain a relatively low equiyield of treated surface per unit of bath volume in unit time is approximated.

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 understood that the examples of the preferred relationship of the several variables are given by way of illustration only.

The effect of the described treatment is threefold: it removes not only the superficial grease but also that grease which has been ground into metal in the process of rolling it into sheet form;

it so pits thesurface as to provide a ne grain; y and it leaves the surface in an alkaline-reacting state. The complete absence of grease is of 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 alkalinity depends that sensitivity to fatty acids which is necessary to the retention of direct 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 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 tooth which is of advantage when a direct image is delineated in pencil or crayon. 0n the other hand, the grain ofthe plates of this .invention is of so ne a texture that it is not objectionable when the image is formed photographically from even the nest halftones.

While it is feasible to print 4from aluminum plates which have been treated by the process of vthis invention merely tothe extent of removing the grease and of rendering the surface alkalinereacting but without any substantial pitting, such limited treatment is not recommended because not all the advantages of this invention are fully realized when printing from plates wholly devoid of "grain. These advantages are most fully realized when the-pitted area constitutes between 40 and 70% of the total surface. Under the conditions of the examples above given, the following relationship obtains. The number of the pits per square millimeter of surface lies between two and ve thousand. .The individual pits are generally spaced apart although occasionally they encroach upon each other, and they are of generally rounded and approximately circular contour. The mean diameter of the pits varies over the range lying between something less than 0.001 and 0.015 millimeter; It is observed that in general 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 ve thousand, the average of their mean diameters is of the order of 0.008 milli- In fact, it has been found that the productivity of a bath (atleast.

meter; and when there are but two thousand pits per square millimeter, the average of their mean diameters is of the order of 0.012 millimeter. The

ratio of pitted to non-pitted areas appears to vary much less than does the concentration of pits from one minute area to the next. The depth of the pits Vappears to be roughly proportional to their mean diameter. The grain constituted by the pits as described and within the above limits is very fine when compared to the grain which is obtained by abrasive attrition under gyrating marbles, as is the common practice; and 'l y yet it is fully e'ective to retain, against the squeegee action of the ink rollers, a sufficiency of dampening solution. It is one of the advantages of the plates of this invention that less dampening solution is required to maintain the nonprinting portions clear from tone than is required by plates with the finest grain mechanically obtainable. This advantageous behavior is thought to be due to the'high moisture retentivity of the open-textured or microporous material deposited in the pits and sometimes over the entire surface.` The presence of this microporous ma.-

terial, in contrast to the extremely thin normal oxide, may be readily observed by microscopic inspection. If the microporous deposit is limited to the pitted areas of the plate, its presence though not its porosity may be observed by inspection of a microtomed section under vertical incident illumination. If the deposit extends beyond the pitted areas, it may be discerned without sectioning, by microscopic inspection undervertical in`- cident illumination. In, either case, whether limited to the .pitted areas or extended over the entire surface of the plate, the relative thickness of the deposit may be estimated microscopically under incident dark field illumination.

The microporosity of the deposit may be demonstrated in terms of its high adsorptive capacity for finely divided pigment, as carbon black for 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 deposit 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 ride for instance. The so-treated area, when microscopically viewed` at a magnification of about 100 diameters, will be observed to be of undiminished metallic brilliancy in the nonpitted areas and dark gray in the pitted areas, Repeated washings with solvent are not e'ective to appreciably lighten the grayness of the pitted areas; from which observation'it is concluded that the pigment of the ink'is irmly adsorbed within micropores of the deposit on the pits.

When the microporous deposit covers the nonpitted as well as the pitted areas, the abovedescribed procedure aifords a test for the continuity of the deposit as well as for` its microporosity. If the deposit is continuous over the entire surface, the demarcation of the inked area, after washing the ink therefrom, will still be macroscopically discernible; although contrast .-ing shade. In Figure 4 of the drawing, the

darker areasof the coating, corresponding to the pits I5, are indicated by the numeral I'I andl the lighter areas by the numeral I8. I'he apparent density of the coloring, residual after the above-described inking and solvent treatment, is believed to be proportional to the thickness of the coating, whichis greatest in the isolated areas overlying the deeper pits and is least in the expanses of the unpitted or very slightly pitted areas lying therebetween.

'I'he microporous deposits on thev plates of this invention may be distinguished from both normal atmospheric oxide and from anodic aluminum oxide by two distinct characteristics. Thus, when scratched with a needlepoint, the said deposit appears under the microscope and ata magnification of 100 diameters or so, to consist of whitish particles suggestive of a relatively delicate and iinely crystalline -material and distinctly not comparable to the powdery product of a similarly scratched hard amorphous mass, such as the relatively dense aluminum oxide produced by anodic deposition. It is thought that the expression "microporous crystalline oxide is -useful in distinguishing the oxide deposited by the process of this invention both from normal atmospheric oxide and from oxide produced anodically.

The second distinguishing characteristic is vthat the microporous crystalline oxide deposited fatty acids of direct image-forming materialsv may react; vand thus is avoided the necessity for sensitizing or counter-etching by the user. Another advantage is manifested by the substantially greater life under printing conditions of an image derived directly from a greasy imageforming substance containing a free fatty acid reactive with said alkali over the life of any direct image obtainable in the absence of said alkaline component in the deposit.

If a restricted portion ofa plate of this invention be treated with acid, as dilute phosphoric acid for instance; and it be then thoroughly ,washed and dried, and an image thereafter directly delineated upon both the normal surface and the acid-'treated surface .under like Vconditions, it will be found that the printing life of the image on the normal surface greatly exceeds that of the corresponding image on the acid treated surface. That the relative capacities for which is sensitized by the process of this invention and of a portion of that surface as modiflecl by desensitization with acid indicate not merely a'difference vin degree in sensitivity but a diiference in kind 'of surface may be demonstrated by a simple and expeditious bench test. Thus, if a plate of this invention be desensitized over part of its area with an acid, as phosphoric for instance; and if, after thorough Washing and drying, it be rolled up with a usual lithographic ink and then flooded with a weakly acidic etch, comprising .an acid phosphate such as that disclosed in U. S. Patent No. 2,003,268,

'direct-image retention on the surface of a plate I may require gentle swabbing for their complete removal. The ink on the sensitized or normal surface of the plate will be found to be undisturbed 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 lm coating the surface of these plates.

The presence of alkali, as a component of the oxide deposited on the surface of these plates, may be demonstrated colorimetrically; but, because of the extremely small amount of deposited 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 observing the relative wettability of these surfaces by aqueous ethyl alcohol comprising to 55% of water. A drop of this mixture will spread immediately when applied to the normalA surface of the plate, but will not spread when applied to the desensitized or acid-treated/ surface.

While the planographic printing plates of this invention which have their entire surfaces covered with a. continuous layer of microporous deposit (see Fig. 3) are particularlyuseful for the reception and retention of images formed from the less reactive grades of carbon paper and for free hand work with the drier grades of crayon,

lthey are not particularly satisfactory for photographic reproduction. Because of the very opentextured surfacing, which advantageously retains dampening solution, the unexposed photo-sensitive material with which they are sensitized for photographic reproduction cannot be wholly washed out of the porous surface layer on development; and hence the obtainment fromvsuch plates of clear tone-free whites in the non-printlng areas is generally diflicult and more particularly 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 generally 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 retained in the pits against the squeegee action of the ink rolls as to eifectively protect from ink contamination such traces of photosensitive material as may have been there retained.

I claim:

1. In aprocess of preparing planographic printing plates from sheet aluminum, that step which comprises reacting saidaluminum with an alkali hydroxide in hot solution with an alkali aluminate, whereby the surface of said aluminum is freed from grease and is sensitized to fatty acids. f 2. 'I'he process of preparing planographic printing plates from sheet aluminum which comprises immersing said sheet aluminum in a solution-of `an alkali aluminate until the surface of said aluminum is freed from grease, is minutely pitted. and is sensitized to fatty acids.

3. 'I'he process of preparing vphanographic printing plates from sheet aluminum which comprises immersing said sheet aluminum in a solution of an alkali aluminate until the surface of said aluminum is` freed from grease, and is grained with minute pits coated with a micropo'ous aluminum oxide comprising a trace of al ali.

4. The process of preparing planographic printing plates from sheet aluminum which comprises immersing said sheet aluminum in a solution of an alkali aluminate until the surface of said aluminum is freed from grease, is grained with minute pits, and is coated with'a microporous aluminum oxide comprising a trace of a1- kali.

5. A planographic printing plate of sheet aluminum characterized by minute pits of the order of from two to ilve thousand per square millimeter and of generally .rounded contour in an otherwise substantially plane surface, the vprinting surface of said plate being alkaline reacting and highly sensitive to fatty acids.

6. A planographic printing plate of sheet aluminum characterized by minute pits of the order of from two to five thousand per square millimeter and of generally rounded contour in vvan otherwise substantially plane surface, said pits being partially filled with a deposit of microporous oxide and said surface being alkaline reacting and highly sensitive to fatty acids.

7. A planographic printing plate of sheet aluminum characterized by minute pits of the o rder of from two to ilve thousand per square millimeter and of generally rounded' contour inI an otherwise substantially plane surface, said pits and surface'being coated with a deposit of crystalline -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 aluminum characterized by between two and ve thousand minute pits of generally rounded contour to the square millimeter in an otherwise substantially plane surface, said pits having a mean diameter o f the order of five one-thousandths of va millimeter and said surface being alkaline reacting and highly sensitive to fatty acids.

9. A planographicprinting plate of sheet aluminum characterized by between two and ilve thousand minute pits of generally rounded contour to the square millimeter in anotherwise'substantially 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 with a deposit of a crystalline microporous oxide comprising a trace of alkali and being highly sensitive to fatty acids, said deposit having a mean thickness of between one and three onethousandths of a millimeter.

WILLIAM B. WESCOTI.

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