Gum Process

Gum Bichromate Process

Note: Nine of 17 typewritten pages have been transcribed and the rest will follow later.

My notes indicate my paper was a compilation from a variety of sources, but I believe it is primarily from an unpublished manuscript written by Leyland Whipple that I ran across while researching the history and techniques of Gum Bichromate printmaking in 1979 at Southern Illinois University. This is a retype of a faded and hard to read 1979 photocopy of my typewritten paper from that same year. I offer it here, as is. I will credit all sources that my paper did so at the time written and I apologize if any references are lost and not cited here. Contact me to add any citations you are aware need adding, I do not hold this up as my own original writings. To the best of my knowledge Leyland Whipple’s unpublished manuscript was never copyrighted, and from my 1979 notes and recent Google searches, the original notebook seems to be housed at the George Eastman House (GEH) museum. I have no recollection of how or where I saw the manuscript quoted here.

The Gum Bichromate Printing Process

Light Source:
This is a contact printing process. Large prints are made from enlarged negatives. The light source may be photoflood, diffuse sunlight, or north skylight, or one of several types of fluorescent lights. These tubes should be spaced 2.5 inches (center to center) and for large prints banks of four to eight lamps are needed.

The processing solutions are the coating solutions must be prepared by the printmaker. Yet the materials are inexpensive and the effects are unique and more than offset the troubles entailed if you enjoy work of this sort.

Two brushes for applying the coating to the paper are required. One is for coating and is called the “spreader”, the other used for smoothing and evening out the coating is called the “blender.” Ordinary good quality soft bristle brushes are satisfactory. Both brushes should be flat and the blender should also be thin. Brushes two and a half inches wide will suffice for coating paper to make prints 8×10 inches and smaller. For larger surfaces, correspondingly larger brushes (greater in width) will enable the attainment of smooth coatings.

Printing Frame:
A standard 13-14 inch printing frame with heavy plate glass is necessary if tight contact over the entire area of a large print is desired.

Development is done in plain water and if several trays of water are used and the print transferred from one to the next, changes of water are needed less frequently.

Gum Arabic. Good quality commercial lumps can use used. The more expensive, finely powdered kind dissolves more quickly.

Mercuric Chloride:
An ounce of this chemical (also known as corrosive sublimate) will last for years. A small amount is used in a batch of gum solution as a preservative. [Robert’s notation this is highly poisonous, difficult to obtain, and unnecessary but included for historical purposes.]

Ammonium Dichromate.
This chemical in the USP grade is sufficiently pure for gum printing.

If you find any difficulties arise that you cannot trace to other obvious causes, prepare all solutions with distilled water. If your water is of uncertain chemical analysis use distilled water from the start.

Tube watercolors of the best grade obtainable are recommended for the beginner. Only the purest, finely ground colors will give satisfactory results. When a degree of expertness has been attained, one may change to powdered or dry colors.

The Paper:
Any paper that will stand repeated soaking and drying and will hold its shape is suitable for gum printing. Rough papers will give broader effects; smooth papers will enable the printmaker to exploit the dynamics to be found in uneven brush strokes. And the contradiction of smoother modulations and the granularity of the pigment process. A medium rough surface, such as that of Opal G photographic paper is one of the easiest to coat evenly and handle in the water baths. Strathmore or Bainbridge two-ply artists’s illustration board will give results as good as more expensive water color papers.

Application of a size to the surface of the paper may be required of papers manufactured by American companies. Some of the European watercolor papers, however, are supplied already sized.

Preparation of the Paper:
The paper must be preshrunk by soaking for about 15 minutes in hot water and then hung up to dry. Hang the sheet by its narrow edge. Without question a sprint-clip (such as the wooden ones used for clotheslines) strung on a length of traverse red cord) is the simplest and generally the most convenient drying arrangement. By pre-shrinking, the paper is stabilized so that registration of several printings is possible when multiple-gum work is undertaken.

The sizing operation takes a good deal of time, so a batch of paper should be prepared at the same time. Size as much paper as can be conveniently hung up to dry. Prior to sizing mark the word “top” lightly in pencil on the back of each sheet.

Preparing Size:
The strength of the gelatin sizing bath is not critical. Ordinary kitchen variety Knox or Plymouth Rock packaged gelatin is satisfactory. An envelope of this contains 8 grams of gelatin. Dissolve 24 grams (3 packets) in a quart of hot water and pour the solution into a tray sufficiently large to take the paper you are sizing. This tray should be warm when the size is poured into it.

Immerse the paper, a few sheets at a time, making sure that the solution covers the surface completely, without bubbles or air pockets. Leaf the sheets over several times and take them from the tray one by one, handling them by a corner. Hang each sheet separately by two pins at the top edge (the edge you have marked “top”). As soon as a sheet is hung by the clips squeegee the font and back with a glass rod from top to bottom to remove excess gelatin size. Glass towel rods may be used for this operation. A two-quart solution (twice the size and taking 6 envelopes of gelatin) will permit the sizing of a large quantity of paper. If you plan to use the size for a second batch, store it in the refrigerator and warm it up before using it the second time. Then discard it.

Hardening the Surface:
When the sheets of sized paper are dry, they should be hardened in a weak solution of formaldehyde. Add about 25 cc. Of the household strength formaldehyde solution (available at drugstores) to a quart of water. Make certain that the entire paper surface is covered. Hang the sheets up to dry by the same edge as before. A second coat of size may be given and the sheet hung up by its opposite narrow edge if a uniform coating is desired. Repeat the hardening bath as well.

The Negative:
The negative for gum printing should be soft but not thin. Avoid the excessively thin negative, which will give unsatisfactory results, as will a hard contrasty negative. Negatives of extreme contrast may be used, but only if you wish this effect. Do not try to overcome excessive contrast with printing techniques. Additional contrast may be obtained by multiple printing, however, if that is required in the print. The density range of 0.9 to 1.0 is suitable for gum printing with modulations approximating full scale. Such negatives give good prints on No. 2 paper grades and passable one on No. 3 grades.

To ensure accurate registration in multiple-gum printing, the negative should be prepared before the first print is made. In the center of each side of the negative fasten a ¾ inch strip of black binding tape. Cover about 1/8 inch or more of the negative. Trim off any excess tape that extends beyond the edge of the negative. Then with a clean cutting paper punch make a semi-circular notch in the middle of each of the strips. This will print a dark half moon surrounded by a patch of white in the middle of each side of the edge of the print. This is readily visible through the unprinted coating and gives a visual reference for replacing the negative precisely in subsequent printings (when making multiple-print gum prints). The negative, after it is in register, is taped in place with small pieces of masking tape. The light for this operation may be a 60-watt lamp about four feet from the paper. This level of illumination produces no serious fogging on the slow bichromated coating.

The Gum Solution:
Uniform success in gum printing requires careful standardization of each step. All possible sources of variability must be eliminated if the percentage of failures is to be reduced to a reasonable size. Only a little additional effort to combine the materials in accurate amounts and according to exact formulas will pay big dividends in consistency of performance.

Use Cold Water:
Gum Arabic (or gum acacia) is obtained in the form of an eduding substance of certain species of acacia trees in various tropical regions. The gum may be obtained in “tears”, fine powder, broken chunks, and is available from graphic arts supply houses in liquid form. If the gum is prepared from tear or powdered form (or chunk) it should be dissolved in cold water. It should not under any circumstances be prepared with hot water. Gum Arabic is the only gum recommended for this process.

The gum solution if it is to be stored must contain a preservative. A ¼ of 1 percent (0.25%) solution of mercuric chloride is sufficient. So preserved bum solutions have been kept in usable condition for more than a year at room temperatures. The solution should be stored in dim light and away from excessive heat or sunlight.

The Solution:
Gum solution should be about the consistency of thin syrup. A thicker solution would be almost unworkable. A thinner solution would not permit dilution by the sensitize, without becoming too thin to use. A useful proportion is from the following formula: Gum Arabic 30 to 35 grams. Cool Water 100cc.

One method of making the gum solution is to pour a quart (32 oz) of water into a half gallon wide-mouthed jar. Rain water may be used. But if the water supply has many chemicals in it, use distilled water to eliminate a number of unnecessary variables. Dissolve 2.5 grams of mercuric chloride in this water.

Caution Poison
Mercuric chloride is extremely poisonous. Handle it with care and observe precautions of washing hands carefully if they chance to come in contact with it.

Weight cut 300 grams of gum arabic and place this in a cheesecloth bag or square of cheesecloth tied into a bag. Suspend the bag in the water and let it stand at normal room temperature. In a day or two all soluble gum will have dissolved. Discard whatever remains in the cloth. If powdered gum arabic has been used, filtering will not have much work to do. To ensure uniformity, however, the solution should be filtered through a wad of absorbent cotton in a clean glass funnel. Filtration is slow and may take all of a day. Wetting the cotton before beginning the filtering will balance any moisture loss through evaporation at this point. The filter may be covered with a sheet of thin glass if the air has much dust in it. The resulting gum solution will keep indefinitely if kept cool and away from strong light.

Baume Hydrometer:
A Baume hydrometer and hydrometer jar for use with liquids heavier than water is indispensable for persons doing a lot of gum printing. The following instructions will enable you to test your gum solution for uniformity of strength.

Specific Gravity

Degrees Be Corresponding Specific Gravity Amt. of water to add to each 100 cc. to bring reading to 12.5 degrees Be.
12.5 1.092 0
13.00 1.096 4.35
13.5 1.100 9.00
14.00 1.104 13.00
14.5 1.1085 18.00
15.00 1.113 23.00

Note: 30 grams of gum dissolved in 100cc. of water should give a reading of 12.5 Be. It is better to make the solution a little stronger and dilute it according to the table given above.

Sensitizing Solution:
This is a solution of the dichromate of either sodium, potassium or ammonium. Dichromate is the current USP Spelling of the older name bichromate, and means the same thing. All three of these salts are soluble, and may be used. The potassium salt is the least and the ammonium salt the most soluble. Since the sensitizing effect is due only to the dichromate part, or, as it is known chemically, the dichromate ion, the choice of which salt to use is wholly one of concentration. The greater the sensitivity of the coating.

Potassium Bichromate is the salt used by earlier workers, and is still preferred by some. However, due to the smaller concentration of dichromate ion, it is the slowest printing of the three. When using artificial light for printing it is desirable to have the paper print as fast as possible.

Sodium Dichromate has never been much in favor, probably due to the fact that it is deliquescent and absorbs water from the air so fast that it is difficult to weigh accurately. It is, however, faster printing than the potassium salt.

Ammonium Dichromate in saturated solution gives the greatest concentration of dichromate ions of the three, is clean to handle, and preferred by those who use artificial light.

Light Sensitivity:
The strength of the sensitizer affects the light sensitivity of the coating only, and has little or no effect upon the quality of the print. A convenient strength should be chosen. Always use it at that strength to ensure uniformity in exposure. I use the following dichromate solution:

Ammonium dichromate 29 grams
Hot water (125 F.) 75 cc.
Cool water to make 100 cc.

This is a saturated solution at about 60 F., and when kept and used at about 68 F., is constant in strength. If it should happen to cool and crystals gegin to separate, they will dissolve readily when the solution is warmed.

The Pigments:
Either tube watercolors or dry pigments may be used, which equally good results. The tube colors are easier to handle, as they require no grinding, but they are more expensive. There appears to be some difference in the pigments and the formula used by the various manufacturers, and it may be necessary to try several brands of a particular color to find the one most suitable for gum printing. For the occasional worker, tube colors are recommended. Those doing a great amount of gum printing usually prefer the dry colors for economy and ease of control.

Weigh Pigments:
The practice of “squeezing out an inch of this color and a quarter inch of that” is too inexact to produce comparable results from one print to the next. All pigments, whether tube or dry, should be taken by weight, and a record kept of the amount used in each log of mixture, until you have arrived at the proportion best suited to your manner of working.

Mixing Pigment and Gum:
For weighing out tube colors, a pair of light, shallow plastic dishes about 2-1/2: to 3” in diameter are convenient. Place one on each pan of the balance and adjust any difference in weight by placing small scraps of paper on the appropriate pan. The color is squeezed into one dish and weighed, and the same dish used for mixing the pigment with the gum solution. The required amount of gum may be measured in a cylindrical graduate, a little of it poured onto the pigment and mixed well with a flexible artist’s palette knife.

The remainder of the gum is then added in small lots until the whole batch is thoroughly mixed. If the amount of mixture is too large to handle at one time in the plastic dish, part of it may be poured into a wide mouth jar and the remainder of the gum used to rinse out the dish into the jar. Stir, preferably with a glass rod, to complete the operation. One difficulty with measuring gum solution is the quantity which does not drain out of the measuring cylinder. In small lots of gum, this factor is considerable. For this reason it is better to mix up fairly large lots at a time, say 50 cc. Or more. The drainage error becomes less in this case. Since the gum-pigment mixture keeps perfectly well and only needs stirring to be ready for use, this method also has advantages in the saving of time and giving uniformity in making the coating mixture. I usually make up 100 cc at a time.

Preparing Dry Colors:
Dry colors are handled a little differently. The pigment must first be worked up with a little of the gum solution to a paste-like consistency and manipulated until every particle has become completely incorporated with the medium. A muller and ground glass plate such as was formerly used by artists in making their own oil pains is ideal for this. However, this equipment is rarely available nowadays. The next best thing is a porcelain mortar and pestle .

Thorough grinding is essential. The finely ground particles of pigment are held in suspension in the gum as a sort of colloidal solution. It is necessary that they remain so. Certain pigments have a tendency to clump together and separate out. Agglutinization is the technical term for this. These clumps deposit on the paper in flecks of pigment and are very troublesome. Lampblack is one of the worst offenders in this respect. The presence of mineral salts in the water aggravates this tendency, and unless your tap water is very soft and pure, or you are using distilled water, you may run into difficulties, especially when using dry colors. The tube water colors contain a plasticizer which is designed to offset this, but when using dry colors, for gum printing more care must be taken.

Sugar as Plasticizer:
The method I use for preparing dry colors involves the use of a small amount of sugar which acts as a plasticizer and improves the quality and smoothness of the coating. A little wetting agent also helps in making a homogeneous mixture of gum and pigment.

Sugar Syrup:
Make a thick syrup by dissolving 20 grams of white sugar in 10cc. Of hot water. To this add 80 cc. Of standard gum solution and 8 drops of Photo-flo or similar wetting agent. Label this “Grinding Medium”. It will keep until used up.

Working Gum Pigment Mixture:
To make 100 cc. Of working gum-pigment mixture, weigh out the required amount of dry pigment, place it in a porcelain mortar which will hold at least 100 cc., and add just enough of the grinding medium to form a thin paste when ground. This will be about 3 or 4 cc. For a 100 cc batch. Grind thoroughly until completely smooth and homogenous. Finally add 100 cc. Of standard gum solution in small lots at a time, with continued grinding, until the whole of the gum solution is mixed.

The quantity of pigment to use per 100 cc. Of gum solution is dependent on the requirements of the coating mixture as a whole, and before making specific recommendations for various pigments it will be well to have a good understanding of the functioning of the coating.

The Coating Mixture:
The coating mixture itself, once the normal gum-pigment mixture is decided on, is unvarying . It consists of equal parts of gum-pigment solution and sensitizer, by volume. In order to avoid the “drainage error” in pouring a small quantity of gum out of a measuring cylinder, I use the following procedure, which is typical for a quantity of mixture sufficient to coat three or four 8×10 sheets. [Robert’s inked in note from 1979 says: 10-12 cc gum/pigment/dichromate]

Into a 10 cc cylindrical measuring graduate pour five or six cc of the gum-pigment mix. Read the quantity to the nearest tenth of a cubic centimeter, at the bottom of the meniscus. You will have to learn to estimate this, as the solution is opaque. It is about two-tenths of a cc lower than the edges of the meniscus. Pour this into the saucer that you will use for coating , which should be at least four inches in diameter. Then, using a graduated 10 cc pipette, run into the measuring cylinder exactly the same amount of dichromate solution. You may be surprised to find that the reading is more than a cubic centimeter more, due to the amount of gum which does not pour out. Add this to the gum in the saucer, mix it well with the coating brush, and you are ready to coat. The coating should be done at once, as I will point out in later detail.

Properties of Coating:
An understanding of the physical and chemical properties of the coating mixture is of importance in the choice of correct proportions of pigment as well as in the manipulations of the coating operation.

A given strength of gum solution will hold just so much pigment in suspension and no more. Excess pigment over that which the gum will hold will deposit on the paper either as over-all stain, grainy appearance of pinhead sized flakes. Increasing the strength of the gum solution will enable it to hold more pigment. This is just what the “one printing” workers do. But a stronger gum solution has disadvantages. It is difficult to coat and blend without streaks and the thick, heavily pigmented coating is not well adapted to control in multiple printing.

A weaker gum solution will give inferior results. It will hold only enough pigment for very weak coats, and stays wet on the paper so long when coating that it is apt to soak into the sizing and stain. A gum solution made up as specified is the best concentration for all -around work, and may be taken as standard, thus eliminating one variable. It is so adjusted that when mixed with an equal volume of non-colloidal liquid (the sensitizer) it will be of the proper viscosity for coating.

Factors Affecting Speed of Coating Changes:
The chemical changes that take place when gum is treated with dichromate are somewhat complicated and occur in successive phases. The speed of these changes is affected by heat , light, whether in solution or dry, and, in the dry state, by the humidity of the atmosphere. The most obvious is the effect of light, but the other factors must also be taken into consideration.

It is often stated that the coating mixture is not sensitive to light until it is dry. This not strictly true, although for practical purposes it may be assumed. If the coating mixture is allowed to stand a couple of days in the dark, it will darken and jellify. If the same mixture is exposed to strong light, it will jellify in a few hours. This setting is only the first stage of the chain of chemical reactions, and does not render the gum insoluble.

Setting Time:
When this mixture is thinly coated on a sheet of paper, it rapidly sets under the physical act of brushing. The control of this setting time is vitally important to the success of the operation. If the setting time is too short, it will be impossible to brush it on and blend it properly, while if the setting time is too slow, blending will be prolonged and the coating will run, be uneven and probably stain. The secret of success is to get it just right.

Pigment Concentration:
By keeping the proportions of gum and sensitizer constant and varying the amount of pigment, we find that the more pigment, the shorter the setting time. Conversely, the less pigment, the longer the setting time. One might be tempted to correct this tendency, in the case of strong pigment concentration, by diluting the mixture, such as by using more sensitizing solution. But when we do that we reduce the amount of gum available, and there is a tendency to strike into the sized coating of the paper and cause stain, as well as a likelihood of blobs of the coating flaking off during development.

From this it can be seen that one of the factors to be considered in determining the upper and lower limits of workable pigment concentration is its response to the coating operation.

Coating Thickness:
The coating on the paper must be thin enough so that the action of light affects it through to the paper surface at all points. Under this condition, the insolubility of the gum is proportional to the amount of light received. This is the principle upon which gum printing is founded. If the coating is too thick, the light may not penetrate it in the highlights and middle tones, and these will consequently flake off in the process of development. It is in this respect that the gum process differs from carbon. In carbon printing, the coating is thick , and the hardening action of light takes place not only proportionally, but also progressively in depth, so that it is necessary to transfer the coating to another surface and develop from the back.

Effect on Contrast:
The statement is sometimes made that increasing the amount of pigment in the coating increases contrast. It is a question whether this is not due more to the fact that with a heavily pigmented coating mixture it is difficult, if not impossible, to spread and blend as thin a coating as is desirable for best printing characteristics, and a reduced scale of tones results, which consequent less of highlight detail and blocking up of shadow areas. It will be seen from the above considerations that the efficiency of gum printing is conditioned to a very thin coating. Therefore, at no time should the pigment concentration be great enough to produce a noticeable contraction in the tonal scale. The build-up of shadow tones is done by successive printings. A good safe rule is to make your normal pigment strength such that it will give a good shadow rendition in three printings. [emphasis mine]. The practice of some gum workers of using just as strong a pigment concentration as they can work without staining, in order to avoid extra printings, is not in keeping with the theoretical requirements for best quality gum prints. A pigment concentration well below that which begins to show degradation of whites will give plenty of richness to shadow areas in three printings, or at most, four.

Standard Pigment Concentration:
From the foregoing, we can define the requirements for a standard pigment concentration for best working. It should not be great enough to produce degradation of whites, should be such that it will coat without difficulty, and no greater than to give a sufficient depth of tone in the shadows in three printings. For most pigments the conditions can be met, and a few test printings will give the answer. Once determined and recorded as the “normal” mix, there should be no further trouble.

Some pigments are so low in tinting strength that it is not possible to get sufficient shadow density at concentrations that work well. These are not suitable for use alone, but may often be used in modifying the color of the black. Other pigments, due to their propensity for staining, are of no use in gum printing. A few pigments, particularly those containing chromium, affect the coating mixture chemically, and are unsatisfactory.

In trying out a pigment, an economical way is to make up a batch of gum-pigment stronger than you think you will need, try it out, and then dilute a measured quantity with a measured quantity of gum solution, recording the details in your notebook. A few test runs will tell you the correct proportion and your notes will give you the quantities to use for future mixes. As a guide, I will give some data on a few pigments that I have found suitable for my use. These might have to be modified slightly to meet your personal requirements.

All the pigments listed below are permanent and usable for gum printing.

A nonstaining transparent black of very good tinting strength. It has a very slight greenish cast which is not noticeable in gum prints. For Windsor and Newton Artist Grade tube colors, a strength of 5 grams in 100 cc. Of gum solution gives a satisfactory working concentration. The easiest way to prepare it is to squeeze the entire contents of the small tube into the weighing dish, weigh it, and multiply the number of grams of pigment by twenty to find the number of cubic centimeters of gum solution to dissolve it in. Wide mouth screw-top jars are excellent for keeping the gum-pigment mixture. [Robert’s note: The lids rust, so I found it better to use plastic jars/lids intended for collecting urine samples in medical offices. The sides are calibrated too, which helps when measuring and mixing.]

Manual Typing of Pages 10-17 to come.