Funderburk, Kit. A Guide to the Surface Characteristics: Kodak Fiber Based Black and White Papers. (Second edition)/Chapter 3: Baryta Coating for B&W Papers
From George Eastman House : Notes On Photographs
 Table of Contents / Acknowledgements /Introduction / Chapter 1: Kodak Alphabet Soup: What's in a name? / Chapter 2.1: Papermaking for B&W Papers / Chapter 2.2: Papermaking for B&W Papers / Chapter 3: Baryta Coating for B&W Papers / Chapter 4: Making the Fiber Base B&W Surfaces / Chapter 5: Can a difference be seen? Can a difference be measured? Does it make a difference? |
| © 2009 Kit Funderburk Reproduced by permission of author |
Contents |
Baryta coating comes to Kodak
When Kodak started making photographic coatings on paper in the early 1880s, the Raw base was supplied with the Baryta coating already applied. In 1897, Kodak decided to undertake the baryta coating part of the paper support production. Baryta coating production, with 12 employees, started at Kodak Park in 1900 pre-dating the manufacture of photo raw base at Kodak by about 15 years. During the early 1900s, baryta coatings were applied to raw base that was still imported from Europe.
The original two coating machines were also imported from Europe. Both were brush coaters, a process in which the coating material was applied to the surface of the raw base and then smoothed out by oscillating brushes made from badger hair. In 1909, the operation was moved to a different building and expanded to eight brush coaters. In 1918, the operation moved again, this time to the same building that was constructed for the paper mill expansion. Company history was that when the construction firm in charge of the new building went on strike George Eastman hired their foreman and formed his own company to complete the work. This construction company later became a division of Kodak and continued to carry out capital projects for the company.
The expertise required to start a baryta coating operation was imported as well when George Eastman hired Dr. Reinhold Becker of Mannheim, Germany to set up the operation. Dr. Becker’s first employment contract was negotiated directly with Mr. Eastman in 1898 and stated that it would be Dr. Becker’s responsibility to supply all the baryta coated paper that was required.
In 1903, Dr. Becker’s contract was renewed making him the superintendent of the Baryta Coating Department. Historical records from this time indicate that the baryta operation was very secretive and the formulas were closely guarded. This was not surprising since a clause in Dr. Becker’s 1903 contract stated that he could not “disclose or divulge any information, formulae, machine, patents, or inventions of his department” to anyone other than George Eastman or other specifically designated persons. Dr. Becker’s contract was renewed several times and he remained with the company until about 1935. [1]
What is Baryta and How is it Made?
The term “baryta” was derived from barite or barytes, the crystalline mineral ore of barium sulfate. In the context of photo paper, baryta (or baryta coating) was used to describe barium sulfate pigment which was coated along with a gelatin binder and other additives onto raw paper base.
Barium sulfate was prepared by a precipitation process and the resulting wet barium sulfate paste was referred to as blanc fixe. The precipitated barium sulfate was a very white pigment which was chemically inert and had excellent resistance to color change from heat and light. The main function of baryta coating for photo paper was to provide a smooth, white layer to cover the paper fibers and prevent the emulsion coating from penetrating into the raw base. As such, the baryta coating was applied only to the side of the paper that was to be coated with the emulsion.
Kodak initially imported all of the blanc fixe from Europe. When this supply was cut off by the U.S. entry into WWI, a facility was set up at Kodak Park to precipitate barium sulfate from the reaction of barium chloride and sulfuric acid. The process used started with dissolving 1100 pounds of barium chloride in 400 gallons of hot water in a 1000 gallon tank. This was done by suspending 11 flannel bags each containing 100 lbs of barium chloride in hot water. After being left overnight to dissolve, sulfuric acid was added from drums hoisted over the rim of the tank. After reacting for 2-3 hours, the acid was drawn off the top and the tank was then topped off with water to bring the total volume to 1000 gallons. This was repeated four times. Then the batch was further diluted into 3 smaller tanks. After mixing overnight, the batch was run through a filter press to concentrate the blanc fixe. The entire process took more than 2 days to complete.
This process was used until 1927 but it was recognized that the blanc fixe was inferior in quality to that previously obtained from Europe. Therefore, importation was resumed and the blanc fixe plant at Kodak Park was idled. The blanc fixe from Europe was shipped at 28% water content but dried out during shipping to about 25%. During the depression years, large inventories of imported blanc fixe accumulated. Over the long storage time, bacterial action degraded the blanc fixe, causing photoactive sulfide compounds to form.
Therefore, in 1930, a new production facility for blanc fixe was installed to replace the earlier system. This time the process used barium chloride and sodium sulfate. The barium chloride was purchased but the sodium sulfate was manufactured at Kodak Park by reacting niter cake (sodium nitrate) with soda ash (anhydrous sodium carbonate). The barium chloride was first stirred into water then filtered to remove barium carbonate that formed from the minerals in the water. After the barium chloride and sodium sulfate reaction, more soda ash was added to hydrolyze excess sodium sulfate to form sodium hydroxide which reacted with the iron from the niter cake so that it could be removed.
This process was used until 1954 when the blanc fixe plant was again rebuilt and the precipitation process was carried out with barium carbonate, sulfuric acid, and a small amount of hydrochloric acid. Barium carbonate was purchased in 2000 lb. boxes under specifications that allowed no more than 12 parts per million (ppm) iron and 4 ppm of reducible sulfur. The hydrochloric acid was also a commercial product, the by-product of the manufacture of triphenyl phosphate; sulfuric acid was supplied by the Kodak Park acid plant. In the first of two reactors, the barium carbonate and acids were blended at a pH of 4.0 along with a sufficient flow of water to produce a final output of 10% solids of barium sulfate. In the second reactor, a small amount of additional sulfuric acid was added to achieve conversion to a purity of 99.9% barium sulfate at a pH of about 1.5. Following the reactors, the slurry was washed and processed through two stages of centrifuges, with an intermediate screening, to arrive at a final concentration containing about 43% water.
The barium carbonate process, which proved to be more economical and much easier to control, was used until about 1996 when the blanc fixe facility was shut down for the last time and the conversion was made to purchased barium sulfate. From 1954 to 1996, there were several changes made in the source of supply of barium carbonate as the market for this raw material fluctuated. The basic process remained much the same during this time though the equipment was continually upgraded.
The demand for baryta coated paper grew quickly following the start-up in 1900 and by 1927 the operation had expanded to 10 coaters. By 1935, the number of coaters had increased to 12. In the mid-1930s, the air knife coater was invented by S. D. Warren Company and quickly found application at Kodak, eventually replacing all of the brush coaters. In the air knife method, the coating material was applied to the surface of the sheet in the same manner as in brush coating. Following the application of coating, the sheet wrapped around a backing roll where a sharp jet of air smoothed the coated surface. A unique feature of both brush and air knife coating was that the coating layer applied was relatively uniform in thickness and followed the contour of the paper surface on which it was coated. This was important since an uneven coating might cover up or obscure any texture feature in the raw base.
It was normal practice to add milk to the baryta coating to act as a defoamer. Unpasturized milk was delivered from the Big Elm Dairy in 8 gallon ceramic crocks. The crocks were hauled to the machine and the milk was dipped out and added to the coating through filter bags. The handling of the crocks resulted in cracks to the rims which caused cuts and became a serious safety issue.
The milk was also the source of a quality issue as it sometimes caused grease-like spots in the coating due to separation of the butterfat. A technique utilized by one employee to prevent this, obviously in the name of quality control, was to skim off the cream and drink it. Apparently several employees must have done this since there are reports from the time of undulant fever caused by drinking unpasteurized milk.
The new air knife method had the disadvantage of creating more foam in the coating than could be controlled by the addition of milk, so isobutyl alcohol became the standard defoamer. The use of alcohol would not have been possible without the conversion, in 1929, to stearic acid sizing in the raw base. The rosin sized raw base used previously was not resistant to alcohol.
The blanc fixe was the starting point but there was obviously more to the baryta formulas. Gelatin, a colorless, water soluble protein obtained from animal hides and bones, was the sole binder used for baryta coating. The source for gelatin was from outside manufacturers, from a gelatin manufacturing plant in Kodak Park, or from Eastman Gelatin Corporation in Massachusetts which was formed in 1930 from the purchase of a gelatin manufacturer. Over the years, several attempts were made to replace the gelatin with other natural or synthetic binders but none were successful.
In preparing a typical coating mixture, the blanc fixe was first mixed with water to make it free-flowing. Then dyes and Optical brighteners (if being used) were added to produce the specific tint desired in the final product. In the early days, dye was added to only a portion of the blanc fixe in order to make a concentrated dye batch that was later blended with un-dyed batches to obtain a variety of tints. When applying multiple baryta layers, the top baryta layer sometimes had a different amount of dye than the other layers.
Gelatin, after soaking in water and melted to 160o F, was then added at a precise amount, most commonly in a 9.6:1 barium sulfate to gelatin ratio but for some specific grades at a 14.0:1 ratio. Next, the gelatin hardener(s) were added over a 10-15 minute period to prevent localized coagulation. The original material used for hardening was formaldehyde. Chrome chloride was used starting in the mid-1930s; for some formulas, a mixture of chrome chloride and formaldehyde was used. Following this, isobutyl alcohol was added along with other additives (surfactants, spreading agents, etc.) as required.
In the early 1900s, after mixing together the blanc fixe, gelatin, and additives, the formula was ready to be coated. Colloid mills were used at some point, probably about the 1930s, to further emulsify the mixture to reduce the particle size of the barium sulfate. The colloid mills did their work by the high hydraulic shear created between a rotor and a stator. The colloid mills were later replaced by homogenizers which also reduced the particle size but the shearing force was created by high pressure pumping through special valves. The batch was then placed in a holding tank for delivery to the coating machine.
The Baryta Coating Process
The coating process began with the delivery of rolls of paper to be coated to the unwind section of the coating machine. The rolls of paper would have been raw base receiving the first baryta coating or baryta coated paper returning for application of additional baryta layers or gelatin overcoats. Gelatin overcoats were sometimes used as protective layers on top of the baryta coatings or to prevent penetration and/or interaction between the baryta layers and subsequent emulsion coatings. In a small number of cases, gelatin was also coated on the raw base to prevent penetration of subsequent coatings or as an alternative to a baryta coating for textured papers where the heavier baryta layers might cover up the surface characteristics. In the later case, the products were often referred to as having been “baryta” coated since they were coated in the baryta operations even though there was no barium sulfate present.
The first unwind stations had limited capability for splicing a new roll to an expiring roll. Depending on which side of the paper was to be coated, the coating machine sometimes had to be shutdown in order to make a splice. Later, the machines were equipped with unwinds which allowed for roll transfers without shutting down regardless of the winding orientation of the starting roll.
It should be noted that up to at least the 1930s, and probably for much longer, the rolls of raw base paper did not come directly from the paper mill. After the required testing and quality inspections, as well as any rewinding necessary to remove defects or to improve roll condition, the raw base rolls were stored to season for at least several weeks prior to coating. The most likely explanation for this aging process was to allow time for volatile components to dissipate. At this same time, pulp was stored for 2 years also presumably to allow for dissipation of unwanted organic compounds. After melamine formaldehyde was introduced as a wet strength agent in the raw base in the early 1940s, it was well documented that formaldehyde continued to volatize from the raw base for long periods of time. It is not clear when these storage times were reduced but over the years it became increasingly important to reduce inventories and this led, at least in part, to the rapid turnaround time between papermaking and baryta coating. In the late 1990s, rolls from the paper mill would have been baryta coated almost immediately after production.
From the unwind stand, the paper to be coated was transported to the coating station. The baryta coating was applied to the paper by transfer from a roll rotating in a pan filled with the baryta formula. The original applicator rolls were felt covered. With this process, the coating speed was limited by the uniformity of the transfer of the coating formula to the paper. The replacement of the felt covered rolls with smooth, metal rolls in the early 1950s improved the transfer and allowed the speed to be increased. These applicator rolls were wider than the paper to be coated so wipers were used on the edge of the roll to remove excess coating and to leave an uncoated edge on the paper which was later trimmed off.
Kodak Historical Collection #003, Department of Rare Books and Special Collections, University of Rochester Library.
Kodak Historical Collection #003, Department of Rare Books and Special Collections, University of Rochester Library.
All 12 of the baryta coating machines originally had loop dryers (also known as festoon dryers) in which the coated web was transported through a heated drying tunnel while suspended from wooden sticks with the web hanging in a “loop” from each stick. The wooden sticks were carried along by a chain drive to the end of the drying tunnel where the web was “de-looped.” The wooden sticks were very popular for tomato stakes for the home gardeners and some never made the return trip to the other end of the machine.
Kodak Historical Collection #003, Department of Rare Books and Special Collections, University of Rochester Library.
De-looping was a speed limiting step as taking up the tension in the web and accumulating the sticks was a source of edge tears so the coater could not be run over 200 feet per minute (fpm). This was not an issue with the brush coater speeds since the coating speed was much less than 100 fpm. With the air knife coater, it was possible to increase the speed to about 125 fpm, and with changes from felt application rolls to smooth metal rolls the air knife coater speed could be increased to 175 fpm. By the late 1950s speed was increased to over 200 fpm with light coat weights.
The loop drying process did not have much control and the main focus was simply to dry the paper as thoroughly as possible. This caused a number of accommodations. Paper that received the heaviest coat weights could only be produced between June 15 and September 1 due to high humidity affecting the drying. Also, it was normal to coat the first layer at night and then let the paper “rest” until the next day for the following coating layers. The later addition of a conditioning section to the machines made it possible to control the amount of moisture in the coated paper. After the coated paper was thoroughly dried, the conditioning section added damp, hot air in order to meet the moisture content requirements for the paper (usually about 5-6%). The length of the loop section and conditioning section was about 200 feet total. However, since the paper hung in loops the total length of paper in the drying sections could be as much as 3000 feet.
After coating and drying, the baryta coated paper was wound up into a roll of coated paper (paper width varied slightly but on average was about 43 inches wide). The early winders could only accommodate relatively small rolls of about 18 inch diameter. In the 1950s, some of the winders were improved so that 2 rolls could be combined in order to produce one roll of about 32 inch diameter. Most often the baryta coated paper would be put through the machine again in order to apply another baryta layer. In the early 1900s, as many as 6 separate baryta coatings were applied with the combined weight of the coating totaling about 60 grams/m2. The total coat weight and the number of passes through the machine were dependent upon the product; the smoothest, double weight papers having the heaviest coverages and the rough textured, single weight products having the lowest coverages.
Kodak Historical Collection #003, Department of Rare Books and Special Collections, University of Rochester Library
Kodak Historical Collection #003, Department of Rare Books and Special Collections, University of Rochester Library.
Over the years, as both the raw base and coating operations improved, coat weights and the number of passes through the coating machine were reduced. Even so, in 1957, 70% of the baryta coated products still received at least 2 passes through the machine and it was 1975 before some of the single weight baryta coated papers would be reduced to only one layer. Reductions in coat weight were most often taken in steps as can be seen in the following example for the support made for Velox Unicontrast F (smooth, glossy, single weight):
| Year | First baryta coat grams/m2 | Second baryta coat grams/m2 |
|---|---|---|
| 1950 | 30 | 30 |
| 1960 | 24 | 28 |
| 1965 | 22 | 23 |
| 1975 | 33 | 0 |
The next step in the baryta coated paper process was calendering. This step was similar to the calendering step on the paper machine except that here the calendering operation was separate from the coating operation and 2 different types of calenders were used. The first type was a supercalender with alternating steel and cotton filled rollers. A supercalender was used to impart a high degree of finish to the coated paper. The application of load on the metal rollers caused a depression in the cotton filled rollers. During rotation, the cotton started to flow as it tried to return to its normal state. This plastic flow caused a relative motion of the metal roller surface against the filled roller imparting a polishing action to the surface of the coated paper. The second type of supercalender, identified as matte calendars, had the same alternating metal and cotton rollers but the metal rollers were less highly polished and were used on the rougher texture or matte papers.
In the paper mill, all raw base received some amount of calendering. Some of the raw bases were later supercalendered prior to baryta coating. For the coated papers, depending on the specific product application, calendering might be done after each, some, or none of the baryta coatings. Generally, the smooth, glossy grades received the highest amount of calendering with the rougher textured grades receiving less (so as not to destroy the texture).
Kodak Historical Collection #003, Department of Rare Books and Special Collections, University of Rochester Library.
In about 1962, calenders with all metal rollers (like those in the paper mill) came into use for baryta coated papers as a means to produce more uniform sheet thickness. By 1972, more baryta coated papers were steel-steel calendered than were supercalendered, a treatment reserved for only the smoothest, high gloss papers. Beginning in the early 1920s, several of the B&W paper supports were printed on the backside (non-baryta side) with an identifying logo. The first product printed was Velox but over the years the papers for other product lines were printed with distinctive logos including Velox Rapid, Kodabromide, Unicontrast, and Illustrator's Azo as well as others.
From at least the 1960s, the printing process used was Rotogravure but based on some early accounts it is likely that the first printing was done by the Offset process. Records indicate that the earliest inks were formulated from lamp black and linseed oil.
Another operation performed on some baryta coated paper was embossing. This will be discussed in more detail in Chapter 4.
While not used for traditional baryta coated paper, another coating method was used in the 1940s and 1950s to produce waterproof papers primarily for the military. These papers were made by Hot melt coating cellulose derivatives on the raw base. The first waterproof papers were made in the early 1940s by hot melt coating cellulose acetate butyrate. This paper was replaced soon after the war by cellulose acetate butyrate coated from solvent rather than by the hot melt method.
The Rise and Fall of Baryta Coated Papers
There were many more baryta products produced than there were raw bases since it was common to use one raw base for several different products. In 1930 there were 30 different designs used for raw base. In this case, design includes the specific combination of fiber, chemicals, physical properties, and process parameters used to produce the raw base. While the differences were sometimes very subtle, each raw base was distinctly different and intended for a different use. However, the number of unique papers increased to 80 different papers after baryta coating. In one case, a single raw base -- a double weight, white tinted raw base -- was used to produce 7 different baryta coated papers, each with distinctive properties for different product applications.
In 1968, the number of distinctive raw base papers had increased to 72 which were turned into 150 different baryta coated papers. The increase in number of papers since 1930 was due to new sensitized paper products being added, such as instrumentation, graphic arts, document copying, and industrial papers. In addition, the numbers also included color products that were still produced on baryta coated papers. In fact, the new papers mentioned above had outpaced in volume the more traditional fiber base B&W papers commonly used in what came to be known as the commercial and professional markets. In addition, the rapidly increasing demand for color products outpaced all of the products and the growth of resin coated papers for many products meant that by the early 1970s about 70% of the products were resin coated rather than baryta coated.
From 1912 to 1924, production of Kodak baryta coated papers more than doubled in volume and from 1924 to 1936 increased by an additional 20%. The peak however came just before the end of WWII with an additional increase over 1936 of an amazing 300%. This volume decreased after the war but rose again to the same high level during the Korean Conflict of the 1950s and continued at that level on into the late 1950s. However, by 1985 the volume of products, including both resin coated and baryta coated papers, was 11% lower than the 1944 peak and even then most of the products were resin coated papers, not baryta.
In 1972, of the original 12 baryta coaters, only 9 were still in operation. The first of the coaters to be taken out of production was shut down in the late 1930s. Another was taken out in the late 1950s/early 1960s and a third was dismantled in the late 1960s. In addition, of the 9 coaters still remaining, only 5 were configured as loop dryer machines and used for baryta coating. The other coaters had been converted to flat, air impingement dryers, and were not used for making traditional baryta coated papers but were producing specialized coatings including anti-stat, anti-curl, lithographic, etc. By 1974, only 6 coaters remained, 3 loop dryer machines doing baryta coating and 3 flat dryer machines making other products.
The last of the loop dryers was taken out of service in the mid-1980s and the baryta coated papers remaining were transferred to the 3 flat dryer coaters. Baryta papers that had been converted to single baryta layers in the 1970s were converted back to double layer coatings (two - 17 gram/m2 coatings). When the last of the baryta coated grades were made at Kodak Park in 2000, only 2 baryta coating machines remained and they were run only on a partial schedule.
- ↑ Reinhold Becker Papers D.354, Rush Rhees Library, University of Rochester, Rochester, NY