La Calotipia, o in lingua originale "Calotype", nasce da un sogno visionario di un grande inventore dal nome William Henry Fox Talbot. Inglese nato l'undici febbraio del 1800 a Dorset in Inghilterra che durante un suo viaggio in Italia e precisamente sulle sponde del Lago di Como, con la sua "Camera lucida" o "Camera Ottica" cercava disperatamente di effettuare un disegno del panorama che gli si presentava agli occhi. Tra sé e sé pensò a quanto sarebbe stato bello che fosse stata la stessa luce e non la sua mano a fare quel disegno. Occorreva troppo tempo per fare una bozza di un'immagine senza per'altro poterne curare i dettagli mentre se la stessa luce si fosse impressa in qualche modo sul suo foglio, allora sarebbe stato tutto molto più facile.
Pensò dunque per quale ragione non dovesse essere possibile ottenere un disegno direttamente dalla luce dato che le sue caratteristiche di intensità, di chiaroscuri avrebbero potuto certamente essere impresse in qualche maniera, solo attraverso la stessa intensità della luce ma bisognava capire come...
Talbot, in quel viaggio nell'Italia antica appuntò delle idee che sarebbero potute essere interessanti da provare e sviluppare una volta tornato a Lacock Abbey, sua abitazione inglese. L'argento. Aveva sentito parlare delle spiccate caratteristiche fotosensibili del nitrato leggendo alcuni trattati della seconda metà del settecento in cui il Nitrato d'Argento era secondo alcuni chimici dell'epoca sensibile alla luce. Non rimaneva altro che provare!
Una volta ritornato in Inghilterra dunque si mise immediatamente all'opera. Si procurò del Nitrato d'Argento e lo stese, in soluzione acquosa su di un foglio di carta, sperando che l'esperimento riuscisse.
Il Nitrato si annerì certamente ma il processo era troppo lungo, serviva una luce solare molto intensa e un tempo troppo lungo per essere impiegato con una camera ottica. Provò dunque col Cloruro d'Argento. Niente da fare, i tempi erano decisamente troppo lunghi, iniziò comunque ad impiegare questa primordiale tecnica per imprimere su carta, immagini di foglie e piante che venivano collocate sopra il foglio e sopra di esse veniva posto un vetro per pressare il tutto. Durante la produzione di alcuni fogli, si accorse tuttavia che laddove il sale era in minore quantità rispetto all'argento, la sensibilità aumentava. Proprio questa fu la chiave che gli permise di trovare le giuste combinazioni e proporzioni tra sali ed argento per realizzare le prime immagini con una camera ottica. Le primissime immagini erano ottenute dunque per annerimento diretto all'azione della luce solare e il tempo di posa di una fotografia era di circa 10-15 minuti.
Successivamente Sir William notò l'affinità dello iodio all'argento e iniziò dunque i suoi esperimenti con lo ioduro d'argento, creando una definitiva formula per realizzare delle immagini ma, cosa più importante, scoprì per caso l'effetto rivelatore dell' Acido Gallico, una sostanza in grado di "sviluppare" ovvero rendere visibile un'immagine latente formata con lo strumento ottico a seguito di una brevissima esposizione.
Questa fu la vera chiave del procedimento Calotype.
In seguito e a coronare il tutto, un'altra scoperta interessante suggerita dal chimico Herschel che successivamente collaborò molto con Sir Talbot: il Tiosolfato di Sodio.
Il Sodio Tiosolfato o comunemente chiamato "Iposolfito di Sodio" fu la ciliegina sulla torta di un procedimento quasi perfetto. Infatti fino ad allora i disegni fotogenici venivano fissati con una forte soluzione di Ioduro di Potassio che aveva sì la caratteristica di "frenare" gli alogenuri d'argento non esposti ma anche quella di sbiancare l'argento metallico per le sue proprietà ossidanti.
Alcune "Trappole per topi" di Sir William
Esempio di Calotipo con relativa stampa su carta salata
La comunità della Calotipia non è mai stata molto numerosa, difatti pochi si sono dedicati a tale arte. Ad oggi esistono solo poche decine di veri calotipisti al mondo, forse qualche centinaio ma non di più. Esistono numerosissime formule descritte da quanti hanno contribuito a perfezionare quest'arte davvero completa e complessa nel suo insieme perfetto ma in ogni caso ancora oggi rimane un'arte praticata da pochi. In Italia saremo circa quattro o cinque praticanti tra cui Claudio ed Asia Santambrogio, Giorgio Bordin, Barbara Cattaneo ed il sottoscritto.
Di seguito propongo il brevetto originale depositato da William Henry Fox Talbot nel 1841 nella lingua originale.
TALBOT’S SPECIFICATION AND DISCLAIMER. TO ALL TO WHOM THESE PRESENTS SHALL COME, I, William Henry Fox Talbot, of Lacock Abbey, in the County of Wilts, Esquire, send greeting. WHEREAS Her present most Excellent Majesty Queen Victoria, by Her Letters Patent under the Great Seal of Great Britain, bearing date at Westminster, the Eighth day of February, in the fourth year of Her reign, did, for Herself, Her heirs and successors, give and grant, unto me, the said William Henry Fox Talbot, Her especial licence, full power, sole privilege and authority, that I, the said William Henry Fox Talbot, my e˜xors, ad ˜mors, and assigns, or such others as I, the said William Henry Fox Talbot, my e˜xors, ad ˜mors, or assigns, should at any time agree with, and no others, from time to time and at all times during the term of years therein expressed, should and lawully might make, use, exercise, and vend, within England, Wales, and the Town of Berwick-upon-Tweed, my Invention of ”Improvements in Obtaining Pictures, or Representations of Objects ;” in which said Letters Patent is contained a proviso, that I, the said William Henry Fox Talbot, shall cause a particular description of the nature of my said Invention, and in what manner the same is to be performed, to be inrolled in Her said Majesty’s High Court of Chancery, within six calendar months next and immediately after the date of the said in part recited Letters Patent, as in and by the same, reference being thereunto had, will more fully and at large appear. NOW KNOW YE, that in compliance with the said proviso, I, the said William Henry Fox Talbot, do hereby declare the nature of my said Invention,
Talbot’s Improvements in Obtaining Pictures, or Representations of Objects. and the manner in which the same is to be performed, are fully described and ascertained in and by the following statement thereof (that is to say) :— The first part of my Invention is a method of making paper extremely sensitive to the rays of light. For this purpose I select the best writing paper, having a smooth surface and a close and even texture. First Part of the Preparation of the Paper.—I dissolve one hundred grains of crystallised nitrate of silver in six ounces of distilled water, I wash one side of the paper with this solution with a soft camel hair brusb, and place a mark upon that side by which to know it again. I dry the paper cautiously at a distant fire, or else I leave it to dry spontaneously in a dark place. Next, I dip the paper in a solution of iodide of potassium, containing five hundred grains of that salt dissolved in one pint of water, I leave the paper a minute or two in this solution, I then take it out and dip it in water, I then dry it lightly with blotting paper and finisb drying it at a fire ; or else I leave it to dry spontaneously ; all this process is best done in the evening by candle light. The paper thus far prepared may be called, for the sake of distinction, ”iodized paper.” This iodized paper is scarcely sensitive to light, but, nevertheless, it sbould be kept in a portfolio or some dark place till wanted for use ; it does not spoil by keeping any length of time provided it is kept in a portfolio, and not exposed to the light. Second Part of the Preparation of the Paper.—This, second part is best deferred until the paper is wanted for use ; when that time is arrived, I take a sbeet of the iodized paper, and wash it with a liquid prepared in the following manner :—Dissolve one hundred grains of crystallised nitrate of silver in two ounces of distilled water, to this solution add one-sixth of its volume of strong acetic acid ; let this mixture be called A. Dissolve crystallized gallic acid in distilled water as much as it will dissolve (which is a very small quantity) ; let this solution be called B. When you wish to prepare a sheet of paper for use, mix together the liquids A and B in equal volumes. This mixture I shall call by the name of gallo-nitrate of silver. Let no more be mixed than is intended to be used at one time, because the mixture will not keep good for a long period. Then take a sheet of iodized paper and wash it over with this gallo-nitrate of silver with a soft camel hair brush, taking care to wash it on the the side which has been previously marked. The operation should be performed by candle light. Let the paper rest half a minute, and then dip it into water, then dry it lightly with blotting paper, and lastly, dry it cautiously at a fire, holding it a considerable distance therefrom. When dry the paper is fit for use, but it is advisable to use it within a few hours after its preparation. (Note, that if it is used immediately the last drying may be dispensed with and the paper may be used moist), (Note 2nd, instead of using a solution of gallic acid for the liquid B, the tincture of galls diluted with water may be used, but it is not so advisable. Use of the Paper.—The paper thus prepared, and which I name ”calotype paper,” is placed in a camera obscura, so as to receive the image formed in the focus of the lens, of course the paper must be screened or defended from the light during the time it is being put into the camera. When the camera is properly pointed at the object, this screen is withdrawn, or a pair of internal folding doors are opened, so as to expose the paper for the reception of the image. If the object is very bright, or the time employed is sufficiently long, a sensible image is perceived upon the paper when it is withdrawn from the camera. But when the time is short, or the objects dim, no image whatever is visible upon the paper, which appears entirely blank. Nevertheless, it is impressed with an invisible image ; and I have discovered the means of causing the image to become visible. This is perfomed as follows :—I take some gallo-nitrate of silver, prepared in the manner before directed, and with this liquid I wash the paper all over with a soft camel hair brush, I then hold it before a gentle fire, and in short time (varying from a few seconds to a minute or two), the image begins to appear upon the paper. Those parts of the paper upon which light has acted the most strongly become brown or black, while the parts on which light has not acted remain white. The image continues to strengthen and grow more and more visible during some time. When it appears strong enough, the operation should be terminated and the picture fixed. The Fixing Process.—In order to fix the picture thus obtained, I first dip it into water, I then partly dry it with blotting paper, and then wash it with a solution of bromide of potassium, containing one hundred grains of that salt dissolved in eight or ten ounces of water. The picture is then washed with water, and then finally dried. Instead of bromide of potassium, a strong solution of common salt may be used, but it is less advisable. The picture thus obtained will have its lights and shades reversed with respect to the natural objects ; videlicet, the lights of the objects are represented by shades, and vice versˆa. But it is easy from this picture to obtain another, which shall be conformable to nature ; videlicet, in which the lights shall be represented by lights, and the shades by shades. It is only necessary for this purpose to take a second sbeet of sensitive calotype paper, and place it in close contact with the first, upon which the picture has been formed. A board is put beneath them and a sheet of glass above, and the whole is pressed into close contact by screws. Being then placed in sunshine, or daylight, for a short time, an image or copy is formed upon the second sheet of paper. This image or copy is often invisible at first, but the image may be made to appear in the same way that has already be stated. But, I do not recommend that the copy should be taken on calotype paper, on the contrary, I would advise that it should be taken on common photographic paper. This paper is made by washing good writing paper first with a weak solution of common salt, and next with a solution of nitrate of silver, since it is well known, having been freely communicated to the public by myself, in the year One thousand eight hundred and thirty-nine, and that it forms no part of the present Invention, I need not describe it here more particularly ; although it takes a much longer time to obtain a copy upon this paper than upon calotype paper, yet the tints of the copy are generally more harmonious and agreeable. On whatever paper the copy is taken, it should be fixed in the way already described. After a calotype picture has furnished a good many copies, it sometimes grows faint, and the subsequent copies are inferior. This may be prevented by means of a process which revives the strength of the calotppe pictures. In order to this, it is only necessary to wash them by candle light with gallo-nitrate of silver, and then warm them. This causes all the shades of the picture to darken considerably, while the white parts are unaffected. After this the picture is, of course, to be fixed a second time. The picture will then yield a second series of copies, and a great number of them may frequently be made. (Note.—In the same way in which I have just explained that a faded calotype picture may be revived and restored, it is possible to strengthen and revive photographs which have been made on other descriptions of sensitive photographic paper ; but these are inferior in beauty, and, moreover, the result is less to be depended upon ; I therefore do not recommend them. The next part of my Invention consists in a mode of obtaining positive photographic pictures, that is to say, photographs ,in which the lights of the object are represented by lights, and the shades by shades. I have already described how this may be done by a double process ; but I shall now describe the means of doing it by a single process. I take a sheet of sensitive calotype paper, and expose it to daylight until I perceive a slight but visible discoloration or browning of its surface. This generally occurs in a few seconds. I then dip the paper into a solution of iodide of potassium, of the same strength as before, videlicet, five hundred grains to one pint of water. This immersion apparently removes the visible impression caused by the light (nevertheless, it does not really remove it, for if the paper were to now be washed with gallo-nitrate of silver it would speedily blacken all over). The paper when taken out of the iodide of potassium is dipped in water, and then lightly dried with blotting paper, it is then placed in the focus of a camera obscura which is pointed at an object ; after five or ten minutes the paper is withdrawn and washed with gallo-nitrate of silver, and warmed as before directed ; an image will then appear of a positive kind, namely, representing the lights of the object by lights, and the shades by shades. Engravings may be very well copied in the same way, and positiye copies or them obtained at once (reversed, however, from right to left). For this purpose a sheet of calotype paper is taken and held in daylight to darken it as before mentioned but for the present purpose it should be more darkened than if it were intended to be used in the camera obscura. The rest of the process is the same. The engraving and the sensitive paper should be pressed into close contact with screws, or otherwise, and placed in the sunshine, which generally effects the copy in a minute or two ; this copy, if it is not sufficiently distinct, must be rendered visible, or strengthened with the gallo-nitrate or silver, as before described. I am aware that the use of iodide of potassium for obtaining positive photographs has been recommended by others, and I do not claim it here by itself as a new Invention, but only when used in conjunction with the gallo-nitrate of silver, or when the pictures obtained are rendered visible or strengthened subsequently to their first formation. In order to take portraits from the life, I prefer to use for the object glass of the camera a lens whose focal length is only three or four times greater than the diameter of the aperture. The person whose portrait is to be taken should be so placed that the head may be be as steady as possible, and the camera being then pointed at it, an image is received on the sensitive calotype paper. I prefer to conduct the process in the open air, under a serene sky, but without sunshine ; the image is generally obtained in half a minute or a minute. If sunshine is employed, a sheet of blue glass should be used as a screen to defend the eyes from too much glare, because this glass does not materially weaken the power of the chemical rays to affect the paper. The portrait thus obtained on the calotype paper is a negative one, and from this a positive copy may be obtained in the way already described. I claim, first, the employing gallic acid or tincture of galls, in conjunction with a solution of silver, to render paper which has received a previous preparation more sensitive to the action of light. Secondly, the making visible pbotographic images upon paper, and the strengthening such images when already faintly or imperfectly visible by washing them with liquids which act upon those parts of the paper which have been previously acted upon by light.