Adamson thinks he  knows some things others do not...

"You ask "can it be kept secret " I can only say that Adamson says the manipulation is very liable to go wrong in the hands of most people – that though several have now and then produced a good specimen – they find it difficult to succeed often – and the arrangement of the picture is as much an effort of the artist as if he was in reality going to paint it ...... I  (illegible) therefore that in the common acceptability of the word it cannot be called Secret – tho Adamson thinks he knows some things others do not ." (1)

The Words of David Octavius Hill in a letter to the London landscape painter David Roberts during  March 1845 and of course this is the  partnership of Robert Adamson and David Octavius Hill during the infancy of  photography in the mid 1840s.
But what did Adamson know ?   He Knew many things some of which we are still uncertain and some of which have been hidden in open view since the late 1840s .  He was the finest Calotypist of his day and in his partnership with Hill together they produced some almost miraculous work . Adamson was a fine photographer in modern terms but Hill transformed it into an art form. It must be remembered that Adamson already had a successful photographic studio before he was introduced to Hill and it was his work that convinced Hill of the merits of the Calotype.  Individually they were good but as a partnership they were ground breaking visionaries.

The Calotype was discovered in September 1840 by William Henry Fox Talbot the Wiltshire Polymath . He had been working on his Leucotype direct positive process and trying out Gallic acid as a sensitizer when he stumbled upon the properties of gallic acid (from Oak galls) as a developer , creating of course a latent image.  The story told by Talbot is that he briefly exposed a piece of paper prepared with Silver Iodide and gallic acid . Upon returning to his dark room he was disappointed to find no image (up to this point images were printed out) so he put the paper to one side and left . On his return he was astonished to find an image and it dawned on him that a brief exposure to light was enough if the paper was consequently developed with Gallic acid. This explains why Talbot used Gallic acid as both a sensitizer and as a developer.

The word Calotype means beautiful impression and interestingly Talbot had thought of the word in January 1840 !  It is my assumption that Hill is referring to the negative image but it was the habit of the St Andrews photographers to refer to their prints as Calotypes as well or indeed photography in general . Witness their many beautiful albums of amazing prints entitled Calotypes on the cover and Thomas Rodgers paper to the RSSA called On Collodion Calotypes dated 1856. I fear that we may never know how Robert Adamson made his prints.

Note that Talbot was using a paper coated with silver iodide , this was a sudden and unexplained  development unmentioned in his notebooks . Up until this point he had been working with silver chloride and silver bromide . He would dip the paper in salt or Potassium Bromide , dry the paper then coat the paper with the light sensitive Silver Nitrate , some times giving the paper multiple coats to increase the sensitivity  .  This multiple coating  was  retained in his Calotype process. The most likely explanation for the use of Silver Iodide is that he had by this time seen the chemistry for the rival Daguerreotype which of course used Silver Iodide.

On the 8th February 1841 Talbot patented his process . He did this under advice from his good friend  Sir  David Brewster who had experienced much frustration when his invention of the Kaleidoscope was in modern terms pirated . The patent applied across England and Wales and was exorbitant.  Sir David also persuaded Talbot to exempt Scotland from its restrictions. As a result the Calotype struggled in England but thrived in Scotland – in Sir Davids home town of St Andrews in particular. The driving forces behind the development of the Calotype process in St Andrews were Sir David Brewster , Sir Hugh Lyon Playfair Provost of the university and Dr John Adamson , later they were joined by John Adamsons younger brother Robert who became his pupil in the new art . These were highly intelligent and determined men and they all struggled to master the process at first.

The Calotype is made using fine writing paper as a substrate and gives a negative image that  could subsequently be used to make multiple positive prints. This was its chief advantage over its competitor the French Daguerreotype which produced a one off image that was both negative and positive depending on how it was viewed . In comparison to the Daguerreotype the equipment required was less and the procedure was simpler.

There are several stages to making a Calotype.
These are the

Paper selection.
And fixing in other words making the resultant image permanent.
Each step has its own set of potential problems .

The ingredients are few , they are Potassium Iodide,  Silver Nitrate, Acetic acid , Gallic acid  and a fixer which could be Potassium Bromide,  common salt or sodium thiosulphate  . There is of course the paper and most important – sunshine , without strong light it rarely excells.,

Talbots method is important not only because of its historical significance but  because the early practitioners followed it slavishly or at least didn't question the method or chemistry involved in the various stages. Broadly speaking his process as out lined in the patent is as follows.
Select a fine writing paper with a smooth surface and a close and even texture.
In the evening under candle light , wash one side of the paper using a soft camel hair brush with a solution of 100 grains of crystallized silver nitrate in 6 ounces of distilled water . Allow the paper to dry in the dark or at some distance from the fire.
Dip this paper in a solution of potassium iodide – 500 grains dissolved in one pint of water. Leave the paper for a minute or two in this solution. Dip in water , dry with blotting paper and then at the fire or leave it to dry spontaneously.  This Iodised paper is barely sensitive to light and should be kept in a portfolio or some dark place.
When the paper was required for use , the paper is washed with a solution of 100 grains of crystallized silver nitrate  dissolved in two ounces of distilled water to this add one sixth of its volume of strong acetic acid. This is solution A. Dissolve crystallized gallic acid in distilled water as much as it will dissolve (which is a very small amount) This is solution B.
Mix equal volumes of A and B to make gallo nitrate of silver . Mix no more than is immediately required because the solution will not keep long . Brush on the iodized paper with a camel hair brush . Taking care to wash it on the previously prepared side. Let the paper rest for half a minute then dip it into water . Dry  with blotting-paper  and then by the fire . This will last some hours . If being used immediately it can be used moist. All this is performed by candle light.
This is called Calotype paper.
The paper was then exposed in the camera . On a bright sunny day you could expect a faint outline on the paper when it was withdrawn , on a dull day it would be blank. Nevertheless it is impressed with an invisible image ; and I have discovered the means of causing the image to become visible.
To develop the picture , the paper was again washed with the Gallo Nitrate of Silver solution again with a camel hair brush . Talbot then held it before a gentle fire , the image then starts to show and develop on the paper . When this was complete he then moved on to the next stage , which was to fix the image on the paper.
To fix the paper he first dipped the paper into water and partially blotted it dry , he then washed it with a solution of Potassium Bromide containing 100 grains to eight to ten ounces of water .  The picture is then washed and finally dried. He also mentions that a strong solution of common salt could be used to the same purpose but advised against it .
This then was Talbot's Calotype process and if you followed these instructions you would undoubtedly fail . Talbot has given an outline of the process but has left out the finer details . I suspect that this was intentional . If you wanted to know how to make the process work then you had to pay .

A brief note on weights and measures.
The grain was part of the apothecaries scale of weights and measures. There was 60 grains to the Drachm and 8 drachms to the ounce. There are 15.438 grains to one gram and 31 grams to the ounce  This refers to dry chemical weights . Fluid measures are in the imperial scale and slightly different ! A fluid ounce is still 8 drachms but a drachm is 3.55 mill and a fluid ounce is 28.4 mill .

Chemically there are the following key words . The Iodisation process involves a Double Wash the paper is first washed with Silver Nitrate and then it is dipped in Potassium Iodide. The Sensitiser is made of two parts,  Solution A being composed of silver nitrate and acetic acid and called Aceto Nitrate . Solution B is Gallic acid . The mixture of solutions A and B is called Gallo Nitrate of silver.

The following is a Table outlining the method of the various British Calotype processes .


Date Process Iodizer Sensitizer Developer Fixer
1841 Talbot dw gns gns kbr
1842 Playfair dw gns gns kbr
1843 Thornthwaite dw gns gns kbr
1844 Brewster dw gns gns kbr
1844 Cundell dw gns gns st
1847 Bingham dw gns gns kbr st
1848 Jordan sw      
1849 Anonymous sw acn gns st
1853 Brodie dw acn ga st
1853 Playfair sw      
1853 Buckle sw gns gns st
1854 Long sw gns gns st
1854 Long 2nd process sw acn ga st
1855 Sutton sw gns gns st
1856 Diamond sw gns gns st
1856 Llewellyn dw gns gns st
1856 Furlong sw gns gns st
1858 Otte sw gns gns st

Dw  - double wash
Sw -  single wash
Acn  Aceto Nitrate.
Ga -  gallic acid
gns – gallo nitrate of silver.
Kbr – potassium bromide
St – sodium thiosulphate

The process to note here is the anonymous one in 1849 . It was printed in Chambers Information for the people volume 2 1849. It was reprinted in the same publication in 1857. It will come as no surprise that Chambers lived in St Andrews and less of a surprise that the author of this piece was John Adamson . (It is mentioned in his obituary ) The date is one year after his Brothers untimely death in 1848, they appear to have been close and Dr John was deeply effected . It is unlike every article before or after in that it's directions are not set in stone it gives enormous latitude for different circumstances. It also likely that the brothers shared their discoveries and I believe that here we have Robert Adamsons methods and their combined expertise and experience  for making the Calotype. Amazingly this article seems to have been largely ignored , certainly South of the border.

If I break down the process into its different components we can see the advantages of their methods.

The substrate that the Calotype was produced on was fine writing paper. The qualities required were that it be smooth , fine and uniform in  texture . With a strong sizing and one that didn't absorb water in the numerous washes that the Calotype undergoes , one free of chemicals often included in the manufacturing  process . Paper made from rag cloth was best but many papers contained fragments of brass and other metals from buttons that hadn't been removed from the rags prior to milling. Most English papers were gelatine sized which was beneficial.
The paper mills included all kinds of secret ingredients in manufacturing , these included whiteners such as glass and chloride of lime as well as Poisons  such as arsenic.  Some times iron fragments could be seen in the paper , these seem to have come from the machinery in the milling process. Then as now the number of papers that worked with the finely balanced chemistry of the Calotype process are few . The incorrect choice of paper led to many failures and as Adamson put it  "Many incipient photographers have thrown up the art in despair" . Dr John's recommendation was a "Yellow post "bearing the water mark Whatman Turkey-mill .This was also favoured by Talbot who no doubt recommended it to the photographers of St Andrews . Playfair certainly used it and many examples of the Adamson and Hill partnership bare that water mark .The other paper frequently recommended bore the mark  R Turner Chafford Mills or in latter years R Turner Patent Talbotype paper . There was also a paper by Nash which Adamson describes as a thick post and recommended for printing whilst others recommended paper by Nash for the negative . Many manuals fudged the issue by just recommending gelatine English papers and others Brewster included who must have known which papers to use were very non specific  almost as if they wanted you to fail. I suspect that Brewster was guarding the interests of his friend in this respect . Analysis has shown that Hill and Adamson used as many as three different varieties of papers during the course of a day . Different papers have different speeds of exposure and some give better mid tones and others stronger blacks or better whites . I often use more than one because you never quite know if it is going to fail . There is no way of telling until too late .

Another factor which according to the London Photographer and chemist  Chas Long , led to as many failures as improper paper was impure chemistry . The chemicals had to be hand made and the way in which they were made differed from chemist to chemist indeed some individual chemists varied the way in which they made their chemicals from week to week ! A reliable source was vital modern day manufacturing tolerances were non existent  . From plain paper to Calotype to print is three days work . The partnership was prolific in both quantity and quality . My own feeling is that Adamson must have sourced his chemicals and not made them himself. Unless that was his winters work. And I wonder if he procured them from Smith and Govan the chemist three door down from his brothers house and whos album of Calotypes has several fine prints by Hill and Adamson.

Having procured both paper and chemicals of the correct quality the next stage is to Iodize the paper. The chemicals required were Potassium Iodide and Silver Nitrate. Potassium iodide isn't light sensitive , Silver Nitrate is light sensitive but only moderately  Combined they are exquisitely sensitive certainly in comparison to the other photographic processes of the day . The chemical reaction is that the two chemicals combine to create Silver Iodide and Potassium Nitrate.  The creation of Silver Iodide in the pores of the paper is the aim of the process . It is not water soluble once formed.  The Potassium Nitrate must be washed out because if it remains then the paper becomes insensitive . Any potassium iodide not converted must be removed too. At that time Potassium Iodide was very expensive in 1849 for instance it was 3 shillings an ounce , the same sum would buy a quarter of a tonne of coal and the amount in the Iodizer was nearly a shilling. Failure was expensive .

Talbot's Iodizing process involves  three stages , coating the paper with silver nitrate and allowing it to dry , dipping it into Potassium Iodide and then washing it . The first stage is simple and using a brush to apply the chemical is very economical . Because the Silver Nitrate is light sensitive a  dark room is best used . Talbot recommended doing this during the evening and by candle light . Later Calotypists recommended yellow Calico coverings over the windows and Dr John recommends red curtains giving a red light to work by. The paper was left to dry spontaneously  , drying by the fire could easily lead to problems.

The next stage was a bit more problematical. Talbot stated a vague 1 or 2 minutes dip in a bath of potassium iodide . Bingham recommended 30 seconds , Cundell states a few seconds . Llewellyn gives no timescale at all and indeed states that the process can be performed in day light . Too short a time and the Silver Iodide isn't formed . Aware of all the conflicting advice my own experiments have shown that 2 to 3 minutes is best.

Adamson overcame the vagaries of the above by adopting a simple method of preparing the paper involving a single wash . In short 25 grains of silver nitrate are dissolved in one ounce of distilled water , to this 3 ½ drachms of potassium iodide are added . This turns the water bright yellow . The solution is then stirred and as if by magic it clears after about a minute.  This solution is then brushed on to the paper and one ounce is sufficient to coat about 20 half plate size pieces . The paper is allowed to dry and then washed. This was devised by the young William Holland Furlong who arrived in St Andrews as an assistant to the Professor of Chemistry Arthur Connell in 1841 aged 14.  A letter detailing his method was presented to the Literarary  and Philosophical society of St Andrews by John Adamson in April 1843 . It certainly seems to have not crossed the border in to England . The English single wash method is far more convoluted and was devised by the Chemist C J Jordan in 1848 although it took another 5 years for it to become popular.

The last stage of the Iodizing is critical this is the wash and it is absolutely vital to remove the potassium nitrate and any residual potassium iodide. Talbot's just advises to dip in water , Thornthwait recommended holding the paper by its two corners and moving it briskly to and fro in the water , Cundell recommended floating it for 10 minutes . Bingham recommended passing each piece of paper through 4 four large vessels of water and Adamson – well he recommended a two hour wash . Thomas Sutton writing in the mid 1850s recommended 24 hours . My own experiments have shown that two hours is the minimum with a modern paper but the longer the better. I have no doubt that the papers of the 1840s responded differently but remember Hill's words about the process being liable to go wrong.

The resulting paper is almost insensitive to light . It can be kept for years in a dry dark place in a portfolio. I have used paper Iodized two years ago with success.  Further more Adamson states that the paper can be improved by being sunned – exposed to the sun for ten minutes prior to sensitization.  This was discovered by Robert Adamson and yes it works – of course , giving more contrast to the image.

Having laid a perfect coat of a primrose coloured Silver Iodide into the pores of the paper the next stage is to sensitize or "excite" the paper . In Talbot's patent this consisted of two parts . The Aceto Nitrate and a saturated solution of Gallic Acid . Aceto Nitrate is composed of Silver Nitrate and strong acetic acid dissolved in distilled water.  The Silver Nitrate is the light sensitive component and darkens when exposed to light , the acetic acid needs to be pure and as strong as possible or as Adamson puts it pure crystallisable acetic acid , in modern terms glacial acetic acid , one in which ice like crystals form at low temperatures.  It's purpose is to restrain the silver nitrate and preserve the white tones of the Calotype in other words it gives contrast . You can use a few more drops of Acetic acid if as Adamson puts it "The whole surface darkens before the picture is sufficiently strong "during development. With the modern Canson Marker this is certainly the case . I add 2 drops of Acetic acid per 10 mill of sensitizer .

The other component is a solution of gallic acid . This is a very small amount less than 0.4 of a gram in 100mill of distilled water in metric . Talbot's instructions use Gallic acid as both a sensitizer and a developer . The Gallic acid in the sensitizer is to give it additional speed in other words make the exposure as fast as possible. Gallic acid whilst being absolutely vital to the Calotype process is a very unstable compound . The sensitizing Solution of Gallo nitrate of silver has to be mixed immediately prior to use and applied quickly . The solution quickly changes colour and becomes useless. Gallic acid by itself will blacken at the slightest impurity,  my own experience has been to see a solution of gallic acid blacken when a bead of sweat dropped in to it or a cats hair fall into it from my clothing.  The working environment needs to be spotless !

The two chemical solutions are mixed together in equal proportions and applied to the paper which is then dipped in water for an unspecified length of time . If you were to apply  this solution undiluted you would be dismayed to see the paper blacken with out exposure to light . Of course the reason that it needed to be dipped in water (distilled ) was to dilute this solution,  it was far too strong. In a letter to Talbot  on the 5th October 1841 Brewster wrote that John Adamson had been working like a horse at the Calotype and had noted that the sensitizer was  too strong .  The solution to this was to dilute the mixture so that it didn't need to be dipped . The St Andrews Calotypists  were using a diluted sensitizer as early as 1842 probably from advice by Talbot to Brewster but equally possible from John Adamson.   In a letter to Talbot pleading for advice written by Sir Hugh Lyon Playfair on the 15th August 1842 he describes the process the St Andrews Calotypists were using and the sensitizer is equal parts of Aceto Nitrate and Gallic acid  diluted with the same bulk of water . In this instance he used a drop count , 20 drops of Aceto Nitrate , 20 drops of Gallic Acid and 40 drops of Distilled water.  At this point in time the Adamson brothers were starting to enjoy success with the process. The need to dilute the solution was picked up by Cundell and Bingham  and all the 18 50s English  processes. The Adamson solution was to dilute the sensitizer still further a dilution of 8 times its bulk was their bench mark but up to 25 times depending on the paper and how long it needed to be kept for. My own experience is that for instance Ruscombe Timothy 2 paper which is a traditional gelatine sized rag paper needs the stronger dilution (8 times) whilst the modern Canson Marker graphic paper needs to be diluted 25 times. Both will keep all day .

However every variation before and most after 1849 persisted in using gallic acid in the sensitizer  and this was the Achilles heal of the process. In the heat of summer the English Calotypists failed regularly . The Adamson brothers had found that if you omitted the gallic acid then the process becomes far more assured and reliable . If you wanted a bit more speed then a few drops of gallic acid were allowed for but really this is the Calotype version of Russian roulette and rarely makes any difference. This is such a simple solution and I can think of no logical reason why the English Calotypists persisted in prescribing gallo nitrate of silver . Their Continental counterparts had already dropped it from their processes , but that is another story,

Talbot's method of application was the camel hair brush but repeated use of the same brush will almost certainly result in contamination  . Adamsons solution was to use cotton wool and then discard the swab after every application.  Cundell  and Bingham recommended floating the paper on a glass plate coated with sensitizer  but this requires some dexterity and introduces more things to go wrong.
Having covered the paper with sensitizer it is allowed to rest for 30 seconds and then the superfluous solution is blotted off with clean blotting-paper. 

Now light sensitive the paper is placed in a dark slide ready for the camera.
Exposure is from 20 seconds to three or four minutes but like many descriptions  of the day the size of lens and the aperture is not stated .  Exposure time is a matter of keeping careful notes and trial and error . Of course there was no such thing as a light meter and it is likely that they made a test shot to gauge how accurate their educated guess was . Dr John doesn't say which sort of lens he was using , at first I thought it was a portrait lens which have greater apertures but  I now think that it was a Landscape lens .  

After exposure the dark slide was conveyed back to the dark room for development  . Here the method is the conventional gallo nitrate of silver as described by Talbot . It is applied with cotton wool and the paper has to be quickly washed over . The picture soon appears and when it is sufficiently strong it is removed to a bath of clean water where it may remain excluded from light for some hours until it is ready to be fixed. Development is a matter of experience and judgement . Many of the contemporary descriptions for development persisted with Talbot's use of a heat source to help the Gallo nitrate bring out the picture , these vary from holding in front of a fire , using the heat from a steam iron or holding over a bath of steaming water. Of course these methods only lead to more complications and probably stemmed from the use of cold cellars as dark rooms . Adamson as usual keeps the process simple.

Interestingly they also allow for use of a full strength aceto nitrate solution to sensitize the paper with a subsequent development in just gallic acid . This works but only with certain papers and the longevity of the paper before it spoils is questionable. It would be ideal for fast portraits as the paper becomes very sensitive. It is mentioned in the description almost as a foot note indicating it was not recommended in most circumstances. But in the right hands definitely an option.

The method in France during the 1850s was to float the paper in a dish  gallic acid , this gives a very even development but for a reason I cannot explain it takes far longer for the image to reach the desired intensity . I use this method often , the main hazard being the discoloration and exhausting of the gallic acid . Certainly the method of application makes a difference . My own recent experiments  coating the paper with cotton wool have shown that on an overcast day in St Andrews the exposure can be reduced from 6 minutes to 90 seconds with a Victorian landscape lens at approximately F22 which on a bright sunny day with strong shadows then this could be reduced to 23 seconds and with Hill and Adamsons  Davidson lens which was fixed at F10 then 6 seconds is possible! It must however be pointed out that the light In St Andrews is exceptional , the town being on the coast and the sea acts as a mirror.

The water bath after development is interesting,  this is the only description that prescribes such a long wash to remove the gallo nitrate. Often it is just a couple of dips in water that is prescribed.  Again attention to detail. The gallo nitrate must be removed before fixing or stabilising and this is not emphasised in most early accounts .

Finally we come to the fixing procedure . In order to preserve the picture it is essential to perform this correctly  . If you study the Calotypes and prints of the Adamson and Hill partnership  and those of Dr John and his later protégée Thomas Rodger then you can see that they clearly understood and had mastered the procedure. They are as rich in colour as the day they were made and in part this is why they are so special . Certainly it is very difficult for the printed page to replicate how rich the original  print is.

Talbot's original recommendation was for Potassium Bromide . This does not fix the image but stabilises it. The yellow silver Iodide is not removed but is converted to silver bromide with an excess of bromide . This may explain some of the yellow Calotypes that are still in existence.  Although other reasons are the hue of the paper (a yellow post) or damp from incorrect storage over the years or indeed aging of the paper. Talbot favoured this method because if the bromide fixed Calotype faded because of repeated printing exposures it could be revived by re development. Of course he knew this but you won't find it in any of the manuals of the time. So of course the early Calotypists were horrified when their hard  earned Calotypes faded not realising that there was a possibility that they could revived.

The preferred method of the Adamsons was to use Sodium Thiosulphate or hyposulphite of soda as it was known then . This chemical which at the time was expensive and difficult to obtain ,  works by dissolving the unexposed Silver Iodide which can be then washed out. The consequences of incomplete fixation is a darkening of the image , the consequences of incomplete washing is that the Sodium Thiosulphate  will continue attacking the Calotype and it will fade .
 Adamson recommended to " place it in a common earthenware dish of sufficient size , pour over it a few ounces of the solution of hyposulphite of soda  , and then heat the dish over a fire until the yellow colour is removed , which will happen before the liquid approaches a boiling temperature.  Another negative may be whitened by the same solution . The hyposulphite of soda should now be removed by placing the negative in water for twelve hours or by repeated washing " The key is the removal of the yellow iodide of silver from the pores of the paper followed by the wash.
No other account of the fixing process recommends such a thorough washing of the paper.
Analysis of the Adamson and Hill Calotypes by Eremin, Tate and Berry's has shown that until 1845 the preferred method of fixation was with Potassium Bromide after that date it was Sodium thiosulphate  . This may have been due to the availability of the chemicals or because the understanding  behind the use of Sodium thiosulphate  was incomplete.

So there we have it ,  Robert Adamson knew that by paying attention to every detail of the process ....

That he could simplify the Iodisation process with a single wash and that a long wash afterwards was required to remove the Potassium nitrate.

That the Iodized paper could be sunned to increase the contrast.

That Talbot's sensitizer was too strong and could be diluted by up to 25 times.

That gallic acid in the sensitizer was unnecessary and was the cause of many failures.

That additional acetic acid was sometimes needed with some papers to control the speed of development

That after development a thorough wash was required to remove the gallo nitrate of silver .

That if this was done then the Calotype could be stabilised with Bromide.

And that if Sodium thiosulphate  was used as a fixer then a very long wash afterwards was necessary.

What we don't know is how achieved such an astonishing work rate in a city not reputed for its sunshine . How many images was he responsible for independently ?

How did he make his prints so rich ? Again this is something that requires strong sunshine .

Like Hill I think that Robert Adamson was the most successful manipulator the art has seen and that together with  John Adamson they were responsible for many improvements in the chemistry of the process.

In another letter to David Roberts , Hill wrote in a flourish of brovado " Now there is my naked bosom in the matter , which I hope will not make you rate me an absolute and arragant as  Petruccio , making you exclaim " Why ! This gallant will command the sun" . And it certainly does appear that he had this ability such was his genius . But equally so does his partner and great friend the understated and all too often under estimated Robert Adamson. The Calotype and the Salt print need sunlight to excell without good sunlight they only produce weak images ,how exactly they produced such an amazing portfolio will probably remain a mystery.

(1) Sara Stevenson The Personal Art Of David Octavius Hill.


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