L. L. Hill, "The Hillotype"
from A Treatise on Heliochromy, 1851

• As a Formula
• The Formula explained.
• My method of working the Formula.

• Thoroughly clean a good daguerreotype plate, by means of rotten stone and alcohol. Polish with buckskin and calcined lampblack. Rouge is detrimental. For ordinary experimenting, you may omit the polishing.
  
• Electrotype the plate till its surface assumes a deep blue. The ordinary cyanide of silver solution is far inferior to the following:-Mix solutions of the cyanides of silver, copper, and zinc, in the proportion of 8 parts of silver, 2 of copper, and 1 of zinc. Use two pair of Daniels batteries, and proceed in every other respect as for electrotyping an ordinary daguerreotype plate.
  
• Rinse and dry the plate. If you use artificial heat to dry the plate, let the latter get cold before the next operation.
  
• Place the plate on a level support, and cover it with a well filtered solution of nitrate of mercury-1 grain of the salt to 20 oz. of water. Let this remain on about half a minute. Pour it off and thoroughly rinse the plate, then cover it with a solution of sel d'or (hyposulphite of gold), 20 grains to one quart water, and let this remain on the plate about one minute. Rinse and dry, and again place the plate in the silver solution until it is slightly changed-say from one to five minutes-according to the strength of the solution. Rinse and dry, and buff to a polish, using calcined lampblack instead of rouge.
  
• Now coat the plate over a jar of chloride of iodine-1 oz. chloride to 8 oz. water, until it assumes a bright pink color. Expose the plate a moment to diffuse light, or place it in a camera directed to a white screen as long as you would for a portrait, and then place the plate over mercury, heat to 170°ree; Fah., for about three minutes. Wash with hyposulphite of soda-or, what is better, cyanide of potash, as you would for a Daguerreotype picture; rinse with water, and gild in the usual way with chloride of gold, or sel d'or. Rinse and dry. If you have exposed long enough to light, your plate will now have a bluish light cast, or solarization, similar to overdone linen in a Daguerreotype.
  The whole of the above process will occupy but little more time than is required for producing and finishing an ordinary daguerreotype picture.
  
• Expose the plate, prepared as above, in a jar of chlorine gas, until it takes on a faint yellow the second time. Keep the plate in this state in total darkness, until wanted for use. It greatly improves by keeping. The chlorine for the above coating I conveniently procure as follows, viz.:-I wet several folds of cotton cloth with dilute sulphuric acid, and place them in the bottom of a deep jar. On these I spread one thickness of cotton flannel, dry, and over that I sprinkle about a teaspoonful of dry chloride of lime, and immediately close the jar. In a few minutes a sufficiency of chlorine gas will be evolved to coat the plate. The action will be mild and uniform.
  
• Prepare the following "singular compound:"-In a quart bottle place 4 oz. of common salt, 4 oz. of blue vitriol, each well pulverized, and add 16 oz. of water heated to 122°ree; to 140°ree; Fah. Shake well for five minutes, with the bottle well stopped. Set it aside to cool. When perfectly cold there should be a deposit of sulphate of soda. If there is not, place the mixture in an evaporating dish, and by means of a water bath, slowly evaporate, until, on cooling, a deposit of sulphate of soda takes place. Then decant the clear liquid into a clean bottle with a wide mouth. Fit into the mouth of the bottle the beak of a lead retort. In the retort place 2 oz. of fluoride of calcium, and 4 oz. of sulphuric acid, and apply a very gentle heat. The beak of the lead retort must dip into the liquid in the bottle, and the stem passed tightly through a well fitted cork. In a few minutes the liquid in the bottle will become saturated with the fluo-hydric acid. The fumes of this acid are intensely poisonous, and the operation should be conducted with extreme caution. When the operation is over, let the retort cool, and immediately wash it with abundance of water. To preserve the liquid, it should be transferred to a gutta percha bottle. After the transfer, add to the liquid 4 oz. of pure muriatic acid, and 1/2 oz. black oxyde of copper. Cork tight, and let it stand at least 48 hours, with occasional shaking. Now mix, in an evaporating dish, 1 oz. of peroxyde of iron (common rouge), 5 oz. of pure muriatic acid, and 1/2 oz. of yellow ochre, and apply heat until a deep yellow liquid is formed. Filter into a glass bottle, and add 1/4 oz. of boracic acid, 60 grains of phosphate of soda, 30 grains of per-manganate of potash, 1/>2 oz. of the fuming liquid of Libavius (made by saturating nitro-muriatic acid with tin by the aid of heat), 5 drops of pure bromine, and 3 grains of iodine. Agitate the mixture for about ten minutes, and add it to the other bottle. Shake well and filter. Place the clear liquid in a large earthen glass, or gutta percha platter, and keep it in sunlight until the watery portion is evaporated. The bottom of the dish will be covered with clusters of brilliant, green, needleform crystals. Collect, and keep these in a well stopped bottle. When you wish to form a bath for coating plates, dissolve 2 oz. of these crystals in 4 oz. of water, and add 1 oz. of pure muriatic acid, 1 grain of bi-chromate of potash, 3 grains of sel d'or, and 1/2 oz. of hydrofluoric acid. Shake well, and filter. Place this in an earthen glass, or gutta percha platter-tilt the platter so as to gather the liquid in one end of the dish-lay the plate prepared as in sec. 6, on the bottom of the platter, and lower the latter in such a way as to secure an even flow of the chemical over the plate. Suffer the plate to remain from 3 to 10 minutes, or until it appears nearly black. This should be done by the light of a candle only. Now rinse the plate freely with water, and dry it off with a spirit lamp, as you would finish a daguerreotype. In this state the plate will reproduce the colors, by a prolonged exposure to light, as you will see by pressing upon it a colored engraving by means of a plate of glass, and placing it in sunlight. Many of my experiments were performed in this stage of the process.
• To render a plate, prepared as above, exquisitely sensitive, you have only to immerse it a few seconds in the following preparation, rinse, and dry. In 1 oz. of aqua ammonia dissolve 6 grains of gallic acid, add to this 1 drachm hydrosulphuret of ammonia, 1/2 oz. commom salt, 1 drachm each of strong essence of lavender, cassia, and cloves, 2 drachms of grape sugar, 50 grains of fluoride of potash, 10 grains hyposulphite of copper, and a quart of water. This preparation may be used in a platter like the preceding. When not in use, it should be kept well corked. Its use gave me a great step in advance. It not only quickens the process, but adds greatly to the strength and truthfulness of the coloration.
  
• A further great improvement in the strength and brilliancy of the pictures results from modifying the coating of the plate, as above prepared, by the applications of heat, or by the action of the orange rays of light, or by both combined. If the plate is heated until it assumes a red, it gives the color more brilliantly, and the whites are always good. A plate which would not give yellow and green, will give both after being exposed under a deep orange glass, in the sun, for a few seconds.
  
• In forming a colored image on these plates direct, without a developer, a prolonged exposure is required-from five minutes to half and hour, in sunlight. Your true way will be to use the process thus, by means of superimposing colored engravings, or other transparent objects, until you have thoroughly tested your chemicals, and mastered the process of coating. Then you may proceed to the work of developing the latent colored images, which you will soon be convinced, are formed almost instantaneously. Phosphuretted and sulphuretted hydrogen, and especially a mixture of carburetted hydrogen, phosphuretted ether, and ammoniacal gas, have the property of developing these images. So, also, with hot aqua ammonia, nearly saturated with hyposulphite of silver, and the combined vapors of burning copper, zinc, lead, and antimony. I make an alloy of these metals, and vaporize this alloy in a mercury bath, heat over a small charcoal furnace having a blast. The plate should be raised about 18 inches from the bottom of the bath. My apparatus for heating is an upright furnace, 10 inches in diameter. Attached to one side is a two foot balance wheel over which passes a band which turns a small shaft, and gives a rapid motion to a fan wheel 8 inches in diameter. The wheel is in a hollow disc having an opening in its sides to let in air. This disc discharges the blast through a tube entering the furnace. A common bellows, of good size, will answer.
  
  The ammonio-hyposulphite of silver, named above, I use in a platter, placed over heat. It must be used at the temperature of 150°ree; to 160°ree; Fahr. The plate should be immersed in the liquid for a few minutes only. In this way I have produced some very fine results.
  
  I use the mixed gases, above named, as follows. In a deep jar I place a few sticks of phosphorus, and cover them with sulphuric ether. The fume arising from it is phosphuretted ether. In the same jar I place a small bottle of aqua ammonia, and into the lower part of the jar I inject a small stream of carburetted hydrogen, made by mixing in a retort 4 parts of oil of vitriol with one part of alcohol, and applying a gentle heat. The plate should be held over this mixture for a period ranging from five minutes to half an hour. You may observe the progress by the light of a candle, screened by yellow glass or paper.
  
  Phosphuretted and sulphuretted hydrogen (for the process of making which see chapter on chemicals), I employ in a similar manner, only the exposure need not be so long.
  
• The final fixing and finish is given to these pictures as follows:
  Dissolve in 1 quart water,
  30 grs. hyposulphite of gold,
  15 grs. chloride of zinc,
  20 grs. sulphate of soda,
  1 oz. sulphuric acid-pure.
  Immerse the picture for a few minutes, or until a slight change is apparent. Then rinse the picture, dry, and rub it with a buckskin buff. A little sweet oil will heighten the polish. If you wish a dead surface, you have only to observe extreme cleanliness in the whole process, and omit the final polish.
  

• The first preparation of the plate has for its object the production of a peculiar molecular arrangement in the particles composing the surface. On this one thing-molecular arrangement-the whole phenomenon of coloration depends. Those who work the process, are urged to keep in view the fact that this preliminary method greatly conduces to the final grand result.
  
• The "Singular Compound," described above I will not attempt to explain chemically. I will simply urge the absolute necessity of a strict adherence to the formula, both as regards manipulating, and the character of the chemicals. Lengthy as the process may seem, it is very simple.
  
• The action of heat and of red light, in modifying the surface, is, I think, to disintegrate the coating, and to give it a different molecular structure. The result is an increase of sensitiveness; and this alone would seem to account for the superiority of the results, as to strength and brilliancy, and for the more certain re-production of yellow and green.
  
• The quickening agents I have named may be greatly varied. The object of their use is to aid in reducing the silver compound formed to that condition at which colors are formed. It is difficult to tell what the compound on the plate is, but it is a peculiar compound of silver. Anything that will tend to reduce this salt to the state, or to the point where color forms, will quicken the process. Aldehide is one of the substances which deserve a thorough trial.
  
• The fixing agent acts by way of deoxydizing the chemical surface. It also effects a partial decomposition of the surface, and those combined chemical agents, such as Chlorine, Fluorine, &c., which would otherwise re-act upon the picture, and destroy. All the pictures which I have treated thoroughly have remained fadeless. Those of which have been carelessly done, have faded very much, and some of them have deteriorated in darkness.
  

• In the choice of chemicals I am scrupulously nice. I set it down as a rule not to use a single chemical unless I am sure of its good quality.
  
• I am very particular as to the clearness of bottles, dishes &c., used in making and mixing chemicals. A little neglect here will defeat every other precaution. This is a hint of the utmost importance. It cannot be neglected with impunity.
  
• I am equally particular in cleaning the plates. The polish is not of so much consequence; but an absolutely clean surface is indispensable. What I mean by a clean plate, is one free from such impurities as will injure the chemical effect. I have found no materials better than pure alcohol, good rotten stone, and clean cotton wool. If at any time you find a plate which has become stained in such a way, as that scouring will not clean it, heat it over a spirit lamp, until it whitens, and then scour it.
  
• The chemicals I make in quantity-exactly in accordance with the formula, as I have given for them.
  
• I avoid haphazard experiments. I make up my mind at night what experiment to try next day, and stick to it till I have found the bottom of the idea, or till I am obliged to abandon it. No progress can be made in any other way.
  
• I adopt, as a thing of no small importance, the principle-"creep before you walk." Hence I begin to work in the direct way-viz.-by superimposing a colored print, and exposing until I get a picture without a developer. In this way I learn whether my chemical coating will give colors. This ascertained, I can pass to the developer, the quickening process, &c.
  
• Perseverance, I have found, is a cardinal virtue in an experimenter. The moment we get discouraged, all progress is at an end. I have persevered for about nine years in the face of difficulties sufficient to appal stouter minds than mine, and all my success has depended upon my fixed purpose not to be foiled.
  

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