Some Notes on the Platinum/Palladium Process
by Phil Davis

Which, of all the historic photographic printing processes, is the most elegant? If you were to pose that question to a panel of experienced photographers, quite a few of them would probably answer "platinum." There is certainly reason for that opinion; a well-made platinum print on fine paper is likely to be a truly beautiful object with a delicacy of gradation that is unique and delightful. Add to this the fact that platinum is the most inert and durable of image metals, and you can see why it was favored by such early notables as P.H.Emerson, Frederick Evans, Alfred Stieglits, Paul Strand and Karl Struss, and why it appeals to such well-known contemporary photographers as George Tice and Irving Penn.

Platinum has another endearing characteristic, too: it's an uncomplicated process that's relatively predictable and controllable. In fact, compared with gum, platinum is downright easy. Once you've calibrated the process, using the sensitometric procedures you're now familiar with, you can reduce waste to a minimum and keep the cost-per-print fairly low despite the fact that the materials are relatively very expensive. You can cut costs still furtherapproximately in halfby using palladium instead of platinum. The two metals are close relatives and, although palladium sometimes produces slightly warmer tones and slightly softer gradation than platinum does, their prints are not always easy to distinguish from each other.

There's an even less expensive alternative, too: If platinum is replaced with silver in a quite similar process the resulting image is called a Kallitype. I don't recommend the kallitype process particularly. Although it's a very flexible process that's capable of exceptionally rich tonal scale, it's much more susceptible to coating streaks and random stains than platinum is and, even when protectively toned, the prints are less stable.

All of these old processes are relatively slow. Platinum and palladium, especially, require long exposure to bright light for proper image formation. Although this can be tiresome, it has one distinct advantage: a darkroom isn't really necessary. You can handle the sensitized materials quite safely, for short periods of time, in dim room light.

As is the case in many of these old processes, paper characteristics will influence the image quality to a considerable degree. Many of the papers that work well for gum printing are also satisfactory for platinum but the reverse is not necessarily true. Cranes kid finish stationery has been a popular paper for platinum printing. Some d'Arches and Strathmore papers are also suitable. One of my personal favorites is Hayle, although not everyone appreciates its rather obvious laid finish and obtrusive watermarks. Some artificial vellums appear to be useful and I've obtained beautiful, translucent prints on Moriki Japanese tissue.

In general I recommend good quality rag papers that are slightly acid or neutral, but avoid the common, relatively inexpensive wood-pulp papers if you want your prints to last. On the other extreme, buffered papersthat is, those that have been laced with some alkaline material such as calcium carbonate to forestall acid deteriorationare not especially suitable either, in my opinion. It appears likely that these alkaline papers may react with the platinum sensitizer (which is quite acid) to reduce its sensitivity. Whether that's the cause or not, some of these "fine" papers require much more exposure than usual, and a few simply will not produce a satisfactory platinum image, no matter how long they're exposed.

Even if this were not the case, buying buffered papers for platinum printing may be a waste of money. By the time the print has been coated with the acid sensitizer and soaked in the acid fixing baths for the recommended time, it's quite likely that the buffering material has been at least partially neutralized, if not totally destroyed.

Although I really prefer not to size paper for any printing process if it can be avoidedmainly because sizing is a messy operation and partly because I can usually get satisfactory results without itI have to admit (reluctantly) that sizing is often beneficial. In many instances it can increase Dmax perceptibly, increase shadow contrast, and make an otherwise poor paper quite usable. Therefore, when I'm serious about extracting as much range and brilliance as possible from these processes, I use one of two relatively easy sizing methods. Although this acrylic size is not very effective on soft papers, and can't be expected to work miracles, it can improve good papers enough, in some cases, to be worth trying.

Liquitex Artists' gloss acrylic medium	100.0 ml
Water	1.0 liter

Soak the paper sheets in this solution for ten minutes or so, then hang them to dry. This treatment makes the paper feel crisp and hardens the surface but doesn't usually affect its appearance. Strengthen the solution for greater effect; dilute it if it begins to glaze the surface visibly.

Although it's more trouble than the acrylic treatment, I prefer a gelatin size for maximum image range and brilliance. It doesn't always affect image color perceptibly but seems to produce slightly cool to neutral tones on some papers. As is true of most home-applied sizes, this one may contribute to "mealiness" or granularity of the image tone. The effect is generally minor, though, and worth the risk when you want maximum black in your prints. Here's the formula I like:

Sprinkle the dry gelatin powder on the surface of the water and allow it to swell thoroughlyabout 20 minutes or so. Then heat the water to about 120F, and maintain the temperature until the gelatin goes into solution. Soak the paper sheets in this warm solution for several minutespreferably one at a timethen drain them briefly, break any air bubbles that may have formed on the surfaces, and hang the sheets by their corners to dry.

You can bottle and save the used size solution for later use (within two or three weeks) if you preserve it with a little Thymol solution (50% in alcohol), mentioned previously as a gum solution preservative. A dropperful will probably be plenty. The gelatin solution will form a soft jelly when it cools and sets, but it can be re-liquified by merely warming the storage bottle in a container of hot water. If the warm liquid appears to contain particles of dirt or lint, strain it through a layer or two of porous plastic foam sheeting before use. An old solution will eventually lose its ability to set. Don't try to keep one indefinitely.

Although most authorities recommend hardening gelatin-sized sheets, I prefer not to; mainly because it's an extra step that (for this relatively thin coating, at least) I haven't found to be necessary. If you believe it's advisable, this formaldehyde bath will do the trick. When the gelatin has set on the paper sheets (it does not have to have dried) soak them for five minutes or so in:

then wash them in several changes of fresh water (to get rid of the alkaline carbonate which will neutralize the sensitizer if left in the paper) and hang again, to dry. For a heavier size coat, increase the proportion of gelatin, or treat the papers twice; for a lighter coat, dilute the size somewhat. CAUTION: formalin is poisonous and its vapors are dangerously irritating. Use it only in a well-ventilated area and avoid breathing the fumes.

Although print image contrast is somewhat controllable, both platinum and palladium are inherently low-contrast processes. Platinum normally works best (on Cranes stationery) with negative DRs(density ranges) of from about 1.1 to 1.4. Other papers may require negatives of greater or less contrast; Hayle, for example, produces lower contrast and requires more contrasty negativesof from about 1.3 to 1.6.

Palladium is even softer; it prefers negative DRs of from about 1.2 to as much as 2.4 or so, and works most comfortably in the 1.5 to 1.6 range.

On any given paper you can modify image contrast to some extent by altering either the sensitizer or developer solutions. Most workers prefer to modify the sensitizer. Here is the traditional formula for platinum:

Solution A
Ferric oxalate solution	25.0 ml.
Solution B
Ferric oxalate solution	25.0 ml.
Potassium chlorate	0.15 gm.
Solution C
Potassium chloroplatinite (platinous potassium chloride)	5.0 gms.
Water, preferably distilled	25.0 ml

Keep these solutions in separate brown dropper bottles, protected from strong light, and in a cool place.

Ferric oxalate solution of the proper strength is available from several sources but I suggest that you consider buying this chemical in powder form from Bostick & Sullivan, P.O. Box 2155, Van Nuys, CA 91404, phone 818/988-6098. Although the powder is difficult to dissolve, the final solution seems to be comparable in all important respects with the prepared liquid form. The powder is much more stable than its solutions are and should remain in good condition for several years if protected from strong light and humidity.

The palladium sensitizer is similar:

Solution C, in both formulas, is approximately saturated and may require a little more wateranother ml or soto dissolve the solid material completely. Don't overdo this. Too much water will weaken the solution noticeably and result in lowered Dmax in the prints.

These C solutions will keep indefinitely, but the ferric oxalate solutions will deteriorate as the ferric salt gradually decomposes to the ferrous state. Depending on the storage conditions this will become apparent within a few days or a few months, during which time the solutions become slightly less sensitive and tend, increasingly, to produce fog.

There's a well-known test that will reveal the presence of ferrous oxalate in the sensitizer: simply add a drop of weak potassium ferricyanide solution to a few drops of the sensitizer solution. No color change, or a very slight one, indicate that the solution is essentially ferrous-free; a pronounced blue color will result if there is ferrous oxalate present.

Although this is an interesting and enlightening chemical test it's almost too sensitive to be practical. I certainly don't want to encourage you to be careless or sloppy but you should be aware that a trace of ferrous oxalate in the sensitizer (certainly more than enough to fail the ferricyanide test) is not going to have any visible effect on your prints. If you discard your solutions as soon as they react visibly with the ferricyanide you're wasting money.

As a rule of thumb, ferric oxalate solutions are probably usable (but not necessarily in first class condition) as log as they remain completely clear and free from any trace of precipitation. But don't interpret this as license to use sensitizers that are truly spoiled. Any suggestion of cloudiness or sediment is cause for real concern.

Ferrous oxalate is certainly a serious contaminant but its gradual build-up in the sensitizer solution worries me less than its much more obvious presence as a dry sage-green crust that's very likely to form around the mouth of the sensitizer storage bottle and on the inside of the dropper cap. Flakes of this material that may fall back into the solution, or onto your freshly-sensitized paper, are genuinely worth worrying about. So are traces of ferrous oxalate that will surely contaminate your coating brush if you don't wash it immediately and thoroughly after each coating operation.

An equally important source of ferrous contamination is the working surface where you coat your paper. The spatters and brush marks that accumulate there decompose rapidly and can easily be picked up by the moist brush, during subsequent coating operations, to cause streaks and spots in your prints. It's easy enough to avoid these problems; simply keep your utensils and working surface clean.

As you work from day to day keep a critical eye on the highlight areas of your prints. At the faintest hint of degradation or fog check the condition of your sensitizer by coating a scrap of paper, drying it thoroughly in the dark, and developing it without exposure. If there is a visible or measurable difference in density between the coated and uncoated areas of the paper your sensitizer is probably reaching the end of its useful life.

NOTE: a sensitizer composed of only solutions A and C, developed in plain developer, will naturally produce some fog density on many papers, even if it's in perfect condition. To make this test valid, include some solution B in the mixture or add a little dichromate to the developer as explained below.

Be sure, if any fog is evident, that it's not being caused by too-bright illumination in the working area. You probably won't have to use an actual colored safelight for platinum printing but keep the room light dim, and protect the coating mixtureand especially the coated paperfrom both daylight and fluorescent light.

When the fog level revealed by this test (or observed in your prints) becomes sufficiently serious to bother you, save the C solution but discard the ferric oxalate solutions A and B and replace them. Be sure to clean out the storage bottles completely before refilling them. The ferrous crust and sediment are virtually insoluble in plain water but can usually be softened and removed easily by soaking the bottles for an hour or so in a 10% (more or less) solution of potassium oxalate.

Your potassium oxalate developer solution can also be used effectively for this purpose but to avoid contaminating it unnecessarily I'd advise using an old, retired solution. Alternatively, dilute one part of ordinary "drug-store" hydrogen peroxide (3% strength) with one or two parts of water. Place this solution in an open container and soak the bottles in it as long as necessary. Don't seal any container of contaminated hydrogen peroxide; the gas evolved can possibly break the container.

Solution B in both formulas above is the contrast control solution. The formulas, as given, are the traditional, often-recommended ones; but I prefer to double the amount of potassium chlorate in bothfrom 0.15 to 0.3 gm. in the platinum formula and from 0.5 to 1.0 gm. in the palladium formula.

A sensitizing mixture blended from equal parts of A and C will produce lowest contrast; equal parts of B and C will yield highest useful contrast. Increasing the amount of potassium chlorate in solution B beyond my recommendations may increase contrast a little further on some papers but it will also reduce sensitivity and image Dmax drastically and greatly increase the likelihood of granular image tones.

Intermediate degrees of contrast can be obtained by varying the proportions of A and B in the final mixture. As a general rule the proportions of A or B, or A and B combined, should equal or be slightly less than the volume of solution C (A + B = C). For example, if 20 drops (or parts) of sensitizer is enough to cover your image area, you can use any of the following mixtures that's appropriate for your purposes:

Obviously these descriptive terms are only relative. You'll have to discover what they mean by personal experience.

Some published formulas for platinum sensitizer include various other chemical ingredients such as lead oxalate or gold chloride. Lead oxalate is a virtually insoluble, very poisonous substance that is both difficult and risky to handle. If it improved the process significantly it might be worth considering but, in my limited experience with it, it seems to increase the sensitivity of platinum emulsions only slightly and actually appears to decrease palladium speedat least on the few papers on which I've tested it.

I've read that lead oxalate added to the sensitizer reduces image granularity and improves gradation but my tests have not confirmed those claim. In fact, I'm not convinced that any chemical additives improve these processes significantly and consistently; and, in general, I don't recommend them. I'm not inclined to argue the point with printers who believe in them, however.

I have similar reservations about double-coating the paper with sensitizera frequent recommendation. Although, if done carefully, this may be worthwhile in some cases, it practically doubles the cost of the process. furthermore, the improvement in Dmax and contrast is by no means certain; careless application of the second coating can drastically decrease both sensitivity and maximum density, and increase the risk of both ferrous contamination and coating streaks. This is a "brute force" approach to achieving decent blacks and richness and I consider it unnecessary, risky, and wasteful, except under rate and special circumstances. If you're using a suitable paper and fresh chemicals, and if you have the process under firm control, I think you'll find that single-coating is generally very satisfactory.

If you elect to control image contrast by varying the proportions of solutions A and B in the sensitizer you can use this traditional developer formula:

Oxalates are poisonous and should be handled with proper care. Avoid prolonged or repeated contact with either the dry chemicals or their solutions. Protective gloves are recommended. After contact with these materials, wash your hands thoroughly before eating or smoking.

This traditional developer formula is nearly saturated and crystals may form in it when it cools. I personally prefer to add another 100 ml. of water to arrive at a 1 to 4 solution. This greater dilution eliminates problems of crystallization, is considerably more economical in use, and has negligible effect on the activity of the solution. Even greater dilutions are possible if you can tolerate some slight loss of developer activity.

This solution does not wear out with use although it will become stained and cloudy. Filter it occasionally and add fresh solution as required to maintain a working volume. The developer can be used more or less indefinitely although, because it gradually accumulates chemical debris from the print coatings and since some of these chemicals (especially additives such as mercuric chloride) may affect image contrast or color, I recommend replacing it occasionally.

You can use this same developer formula for both platinum and palladium prints but for best and most predictable print quality it's probably a good idea to prepare a separate volume of developer for each print type. If you must mix them up, or if you want to use a mixture of platinum and palladium in the sensitizer solution, it's considered less harmful to contaminate the palladium developer with platinum than to get palladium in the platinum developer. Nothing dreadful will happen in either case; blended metal residues in the developer solution are said to inhibit gold toning of the platinum image (if you should want to attempt it) but the effect is not well documented and you're not likely to be aware of any problem at all in ordinary work.

There are quire a few chemicals that can be used as developers but I think you'll find ammonium citrate to be the most satisfactory alternative to potassium oxalate for general use. Other materials such as sodium acetate, sodium or potassium citrate, or various other oxalates, citrate, acetates or tartrates will all develop an image and may be worth investigation if you're inclined to experiment. To make a test developer solution with any of these chemicals it will probably be worthwhile to begin with a nearly-saturated solution.

If you prefer to control image contrast during development, omit solution B from the formulas above and make up the sensitizer solution with equal parts of solutions A and C. This mixture will produce very low contrast and will probably fog the image slightly if developed in the plain oxalate solution, as mentioned above (don't misinterpret this as resulting from stale sensitizer). To increase the contrast and eliminate fogging add appropriate amounts of the following contrast control solution to the developer:

Dichromates are also poisonous and can be absorbed through the skin to cause serious irritation and sores. Don't handle these chemicals without adequate hand protection; also avoid breathing the chemical dust.

Keep this solution in a dropper bottle and add it to the developer, drop by drop, until the desired degree of contrast is produced. Both contrast control methods are satisfactory and they're about equally effective. Each has its advantages: modifying the sensitizer mixture will produce an emulsion of specific contrast (similar to a conventional silver paper contrast grade) that is appropriate for use only with negatives of matching contrast, but it permits the use of the same developer bath for all prints. This method is most useful when you're printing intermittently or making only one or two prints at any one time, using negatives of different density ranges.

Modifying the developer allows you to coat the paper without being concerned about negative DR, but requires separate volumes of developer for each "grade" of contrast desired. It also allows you to make subtle adjustments in image contrast after the paper has been coated. This method is most useful when you're in "mass production." It permits you to mix the sensitizer in fairly large quantities and coat several sheets of paper at one time. This is more efficient and more economical than coating one sheet at a time because you need to wash the coating brush (and waste the precious metal solution it contains) less frequently.

Neither method is truly stable. The potassium chlorate in solution B is said to gradually lose its strength, although in my experience it seems to remain reasonably active as long as the sensitizer solutions themselves last. There's no way that I know of to determine the condition of this solution except by an occasional print test.

Similarly, the dichromate is said to "wear out" with use but that has not seemed to be a serious problem in my experience. It is true that (because of chemical contamination from sensitizer additives?) it sometimes dies spontaneously in just a few days, but that's unusual. I normally expect a volume of dichromated developer to process dozens of prints over a period of several weeks without any appreciable change in contrast characteristic. Fortunately, when the dichromate begins to decompose it often changes color from its original yellow to green or even purple, so you have some general idea of its condition. You can rejuvenate it. Just add more dichromate. The replenished solution may be a strange color but that seems to have no serious effect on its performance.

The developer can be used at any temperature but, because increasing the temperature can (in some cases) increase the effective printing speed of the paper, I suggest that you use the developer at about 90F. Then, if you suspect that you've overexposed the print by a half-stop or so, you may be able to compensate by cooling the developer down to 60 to 70. Similarly, slight underexposure can sometimes be remedied by raising the developer temperature to 110 or more. As suggested above, this is not always an effective control. My tests indicate that palladium is not affected to any useful degree by varying developer temperature from 60 to 120. Platinum responds more readily, but not on all papers. Test this yourself to see how much control your materials and your working conditions affords.

Some contemporary instructions seem to suggest that changing the developer temperature affects platinum image color but, if it does, the effect is minor in my experience. This idea may be based on the fact that in the old days the recommended "cold" developers frequently contained a quantity of some phosphate, which presumably contributed to the "cool" color shift. Palladium images, on some papers, shift toward brown slightly as developer temperature is raised but I have not been able to control platinum image color noticeably and consistently by varying developer temperature, with or without the addition of various phosphates.

Actually, when the sensitizer is prepared normally, image color seems to be most strongly influenced by the characteristics of the paper and the kind of developer used. Coated on the same type of paper and developed in the same sort of developer, platinum images are usually cooler in tone and slightly more contrasty than palladium; but, as mentioned previously, it's not always easy to tell them apart. Near-neutral palladium images are rare but not unheard ofespecially when developed in ammonium citratewhile it's not at all unusual to see distinctly brownish platinum prints. Both metals can be made to produce brown to yellow-brown images if a drop or more of the following solution is added to the sensitizer:

Mercuric chloride is a light, white powder that floats and dissolves very slowly and with great reluctance in plain water. Stir until it's dissolved completely, then store the solution in a dropper in a safe plane. CAUTION: this is a dangerous poison that must be handled with care. Wash thoroughly after handling it, before eating or smoking.

I don't recommend using mercuric chloride routinely for several reasons: it's not a friendly chemical to have around; I don't especially like the image color it produces; it tends to reduce shadow contrast and Dmax; its simple water solution doesn't keep particularly well and may function unpredictably after a few weeks or months; and it's incompatible with the acrylic medium that I often use in the paper coating mixture, causing it to curdle.

Why acrylic medium in the coating mix? Because it helps to prevent "image washoff"one of the most common causes of weak, gray, anemic-looking images. It's easy to see why "washoff" occurs; the paper coating is simply a thin layer of (mostly) water-soluble material, much of which is lying on the surface of the paper. Although development is almost instantaneous, especially in warm developer, it isn't usually completed before some of the coating has dissolved and floated off the paper surface. This dissolved material then develops in solution, producing a black cloud of suspended platinum that should have formed your image shadow details and accents. To compound the disaster, this floating metal is likely to settle back onto the paper, dirtying the highlights and causing unsightly smears.

Some sources say that image washoff results from improper drying of the sensitized paper. I've not been able to prove that, personally, but it may be true. Regardless of the cause (I'll discuss this subject more fully in the next chapter), it's obvious that anchoring the coating to the paper more firmly will alleviate it. One way to do this conveniently is to blend a drop or two of the Liquitex acrylic gloss medium (mentioned previously as a size material) with the sensitizer mixture before coating the paper. You can determine the amount by trial; too little won't have much effect, too much will almost waterproof the paper, inhibit development, and may leave a visible glaze on the surface of the print.

To begin with, try one drop of acrylic for each eight drops of sensitizer mix. This is a procedure that I recommend strongly. Coupled with suitable, properly-sized paper, it can increase image Dmax significantly, improving shadow separation and detail to a degree that rivals or surpasses successful double-coating. Also, by partially waterproofing the image surface, it helps to avoid those troublesome "high water marks" that often mar the image when the exposed paper is not immersed in the developer quickly and evenly. Finally, by extending the coating mixture, it can reduce the per-print cost of your chemicals. Its only apparent drawback is the slightly increased risk of coating streaks and granularity.

Some authorities recommend leaving the print in the developer solution for at least two or three minutes. I suppose this is to allow the developer ( potassium oxalate solution) time to dissolve any ferrous oxalate that may remain in the print. Presumable, if the ferrous oxalate is not removed it may eventually decompose and form brown rusty stains on the image. Since from personal experience I can neither confirm nor deny the necessity for extending development, I generally do it.

Most old published instructions state that, after development, the print should be place directly into the first of three weak hydrochloric acid fixing baths. Although this is an excellent method of fixing platinum prints, hydrochloric acid is capable of bleaching the palladium image noticeably, particularly if the print has been developed in a dichromated developer. To avoid this danger, Dick Sullivan (of Bostick & Sullivan) has suggested fixing palladium prints in EDTA (a sodium salt of ethylenediaminetetraacetic acid) solutiona "chelating" chemical that is widely used for such purposes as separating calcium salts from water.

EDTA has proven to be capable of separating iron from platinum prints without affecting the image at all. Unfortunately, although it also appears to be an effective fixer for palladium prints that have been developed ammonium citrate, it frequently fails to clear palladium prints that have been developed in potassium oxalate. For this reason, and because the traditional HCl fixer is not entirely safe for palladium, I prefer to use a weak (1%) solution of oxalic acid as a fixer for both platinum and palladium. This solutionwhich is also useful for removing iron stains from old or incompletely fixed printsappears to be safe and effective and I recommend it.

For printers who prefer to follow the traditional instructions, the following formula is an effective fixer for platinum prints; doubling the amount of water will make it reasonably safe for palladium prints that have been developed in plain developers, without dichromate.

Hydrochloric acid is a dangerously corrosive chemical with irritating vapors. In case of skin contact with the concentrated acid, flood the affected area with water for several minutes then apply baking soda. To avoid possible spattering when diluting a strong acid such as this, always pour the acid into the water, never the reverse. This diluted solution is safe to handle, although it will corrode iron and some other metals.

Regardless of the fixing chemistry you adopt it's sensible (but probably not necessary) to follow the traditional fixing procedure: fill three trays to a convenient working depth with your chosen solution and treat the print in each bath for five minutes (I suspect this is overkill; two or three minutes in each bath should be adequate) before moving on to the next. The first bath will become stained fairly quickly. When it has acquired a noticeable tan or yellowish tint, throw it out and replace it with the second bath, moving the third bath up to second place. Then prepare a new third bath. Never permit either the second or third bath to become discolored. Transfer the print from the last bath to a tray of gently running water and wash for at least five minutes; then hang the print, or lay it out image side up, on clean blotters or clean cloth, to dry.

Platinum and palladium prints typically exhibit a dull matte surface that is quite susceptible to abrasions marks. Minor spotting can be done effectively using watercolor pigments, blended to match the image color. Don't try to use pencil; you'll almost certainly dent the print surface and the graphite will show as a shiny blemish on the image. The finished prints can be mounted or matted using any of the conventional techniques.

Although platinum is not as sensitive to coating variations as kallitype is, for example, it's not totally forgiving. If the sensitizer is applied unevenly the image may display streaks or blotches, especially in large, light-toned, untextured areas such as skies. If the coating is brushed out too thinly, the image won't reach full density and may turn brownish. A too-heavy coating may not cause serious problems but it's wasteful and may contribute to excessive washoff and bleeding during development.

Every platinum printer has his/her own coating technique and you'll have to find yours, too. For prints no larger than 8"x10" I use the same sort of fan blender that I like for gum printing. Traditionalists will recoil in horror at the thought of using a brush of this sort with a metal ferrule but it has worked well for me for many years. Some of the Pictorialists of the '20s and '30s recommended the "Blanchard" brushsimply a strip of flannel folded over the end of a narrow rectangle of glass and fastened in place with rubber bands. Dick Sullivan has advocated what sounds like a modern version of the Blanchard brush, using velvet in place of the traditional flannel. The familiar foam-plastic paint stripers, inexpensive and available at any paint or hardware store, are popular with some workers. So are the soft-haired wooden handled Japanese "Hake" brushes that are sold in art supply stores; but they soak up a lot of the precious sensitizer, much of which will simply be lost when you wash the brush after each coating operation. There isn't any "best" brush; when you find one that works satisfactorily that's the "best" one for your purposes.

Coating technique is an individual matter, too. I prefer to apply the coating in a quick series of wet strokes, then brush it out until it appears uniform and no longer liquid. Some folks simply pour a pool of sensitizer in the center of the paper sheet and spread it in all directions. I've even heard of air-brushing it but I certainly don't recommend that; these chemicals are toxic, and inhaling them as a mist or spray would be a definite health hazard. In some instances (when sensitizing Japanese tissue, for example) I've diluted the sensitizer with up to 6 parts of water and simply soaked the paper in it, with good results. The actual method of coating doesn't seem to be important. If you can get the sensitizer spread evenly over the paper without damaging the surface you will have solved the problem satisfactorily.

Most of the old references emphasize the importance of drying the coated paper quickly and thoroughly, even to the extent of toasting it before an open fire. Some contemporary writers also stress the necessity of heat drying and suggest "blow-drying" it with a hair dryer. I'm confused by these recommendations. I've never had a problem of any sort that I can attribute to drying conditions and I've dried platinum paper in all sorts of ways. My normal technique is simply to hang the coated paper in a conventional film-drying cabinet. The very mild heat (only a few degrees above room temperature) and slowly circulating air seem to do the job perfectly well in ten minutes or so. My early experiments using high heat for the final drying almost invariably led to uneven image density. No doubt there was originally some reason for the "toast-until-bone-dry" commandment but I have no idea what it was. Local conditions of humidity and temperature may have something to do with it. Then, again, I may be doing something routinely and inadvertently that counteracts the effects of improper drying. Such is the nature of these old processes; you have to find own path to salvation.

The process instructions outlined in the previous chapter are more or less conventional. If you're inclined to experiment a little I think you'll find that you can optimize this process by tailoring it to match your own work habits and your chosen materials. There are several control possibilities that are not generally recognized in print. For example, I think you'll find that adjusting the acid balance of the sensitizer to "match" the paper's characteristics can result in improved blacks and highest printing speed. It's also worthwhile to investigate the sensitizer proportions. Although virtually every published reference advocates using approximately equal parts of the standard ferric oxalate and metal solutions, it appears that the relative percentages of these ingredients can be varied rather widely with potentially useful effects on the image.

To get reliable data from tests of these variables you should have some method of measuring both acidity/alkalinity (pH) and specific gravity (concentration) of the solutions. You can obtain paper "indicator strips" for pH testing from any good chemical supply company. You'll need strips that are sensitive in the range from pH 0.5 to pH 10 or so, with particular sensitivity in the 0.5 to 3.0 acid range.

The normal method for testing specific gravity of solutions is to float a hydrometer in them. Hydrometers are not particularly expensive but they need quite a lot of liquid to float in. That makes them unsuitable for testing ferric oxalate because you'll rarely want to prepare more than 50 ml. or so at any one time. There's an easy alternative, though; just compare the weight of the solution with the weight of a similar volume of water. To be precise about this you should have scales sensitive enough to indicate fractions of a gram, and an accurately calibrated glass cylinder or graduate, marked in milliliters. I recommend a 10 ml. cylinder, obtainable from any chemical supply company.

At approximately normal room temperature a milliliter of distilled water weights one gram. Since specific gravity expresses the weight ratio between water and the test liquid, it's easy to calculate it quite accurately by simply weighing 10 milliliters of the sample and dividing its weight in grams by ten. First, weigh the empty cylinder and record its weight to the smallest possible fraction of a gram (mine weighs exactly 24.6 grams). Then fill it with precisely 10 ml. of sensitizer, measured from the bottom surface of the meniscus, and weigh it again. Subtract its empty weight from the total, shift the decimal point one place to the left, and you have a reasonably accurate measure of the solution's specific gravity.

There are some phenomena that seem to defy analysis. The causes of image "granularity," "washoff," solarization, and image color variations, for example, don't seem to be simple ones. It's not difficult to identify process or material features that influence these things but I'm convinced that several factors are involved. Granularity, for example, is certainly related to paper surface and paper sizing materials, feox (ferric oxalate) concentration, developer type and additives, sensitizer acidity and proportions, and possibly humidity, drying conditions and coating technique. It appears that none of these things, alone, can be held responsible, although it's often true that changing any one of them will affect granularityfor that particular printing situation.

Image washoff is easier to control but, again, there's no single factor at work. Certainly an excess of ferric oxalate in the sensitizing mixture will aggravate it and there's no doubt that some papers are affected more than others. In my experience platinum seems more prone to washoff than palladium is, but since the two metals require different sensitizer proportions and concentration for best results on any given paper, it's not at all certain that the effect is related to the image metal itself. This doesn't have to be a serious problem, however; it can be controlled almost completely by adjusting the sensitizer proportions; and the inclusion of a little acrylic medium in the sensitizer, as mentioned in the last chapter, will provide added insurance.

Image color is also influenced strongly by both sensitizer proportions (of A, B, and C solutions) and by the amount of oxalic acid in the sensitizer. Excess ferric oxalate tends to produce cool image tones (as well as aggravating washoff and granularity) while excess metal tends to increase image warmth and smoothness (at the expense of Dmax and shadow separation). The acid effect is less predictable but an excess often shifts image tones toward brown and reduces granularity. Too little acid tends to increase granularity and washoff. It also tends to reduce Dmax and printing speed and may shift image color toward neutral black.

The developer type is important, too. Potassium oxalate provides warmer tones on both platinum and palladium than ammonium citrate does, for example. The paper type and size material are also influential. Traditionally, British papers were considered to yield neutral tones, while French papers tended toward browns. That may still be true, at least in some instances: RWS, Whatman, and Hayleall British paperstypically give cooler tones than d'Arches and BFKboth French. Domestic papers generally seem to favor warm tones.

Although palladium is generally thought of as producing warm brown prints, and platinum has a reputation for neutral gray tones, their natural tendencies can be overwhelmed quite easily. If all the factors mentioned above are selected and combined for maximum effect, it's possible to produce palladium prints that are almost blue-black in tone and platinum prints that are distinctly brown. By comparison, the effects of developer temperature changes and developer additives (other than mercuric chloride) are not very great.

Bear in mind that these observations are based on my personal experience with my materials in my darkroom. They may or not prove to be valid for you in your environment. Test for yourself; I'm sure you'll soon compile working data that you can rely on.

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