LEATHER TANNING PROCESS
What is Leather?
Once upon a time many thousands of years ago, primitive man began to make leather. It would have been one of the very first manufacturing industries .Animals were hunted and killed for food, but before they were eaten the skin had to be removed from the animals. Sharp flints were probably used to peel the skin away from the carcass. The skins would then have been worn for warmth and protection from the elements and probably wrapped around the feet; as the first type of shoes. But the skins left like this would soon begin to decompose and rot away. They realised that drying the skins would preserve them, but the result was a very hard, inflexible and uncomfortable material. The skins would have to be softened. This would have been done by rubbing with fats.
The fat also had another advantage, it helped prevent the skins getting wet. This way the skins lasted even longer and with more bending and flexing of the wearer, the softer and more pliable they would become. Then some old chap made a fantastic discovery. By using water, various barks, leaves and berries, you could make a liquid containing vegetable extracts. It was a powerful solution, as was discovered when the skins were immersed in it. They became rot resistant and considerably softer than the dried skins ever were. The active agents in this liquid are called tannins. This was probably one of the first methods of tanning leather.
This process of tanning skins spread and was improved upon. By Roman times, armour, water containers, belts, straps, tents, boats, etc. were regularly being made from leather. By the middle ages, things began to be very well organised. Tanneries were set up, mainly concentrated into special areas. These areas had to have good sources of materials: - A supply of hides and skins - Plenty of water - Lime for softening and assisting with hair removal - Plenty of trees for the extraction of tannins from the bark. The processes that we use today are based on those discovered by our ancestors all those thousands of years ago.
Simply defined, tanning is a process by which the hide or skin of an animal cured through the removal of the flesh, fat, moisture and bacteria that cause putrification; thereby converting the hide or skin into a stable, pliable material called leather. Basically, there two kinds of tanning processes: vegetable tanning and chemical tanning. Vegetable tanning consists of soaking clean hides or skins in an acid solution prepared from the bark, roots, leaves, or nutshells of plants that are rich in tannic acid. Although dozens of different kinds of plant materials can be used in the manufacture of vegetable tanned leather, nineteenth century American tanneries commonly used only three organic sources of tannic acid: sumac bark, oak bark and hemlock bark. Sumac bark was used primarily by morocco makers for tanning goatskins while oak bark and hemlock bark were used for tanning other kinds of hides and skins. Many tanneries combined oak and hemlock bark to produce what is called union or union crop leather.
Chemical tanning or mineral tanning consists of substituting various inorganic substances for the plant materials used in vegetable tanning. Substances such as aluminum salts, ferric salts, chromium sulphate, and chromic oxides were used. Chemical tanning was introduced into a few American tanneries in the 1890s but it was not widely adopted for many years because the resulting leather was considered to be inferior to vegetable tanned leather.
The vegetable tanning process has been used for thousands of years, and the basic technology remained largely unchanged from the late medieval period to the mid-nineteenth century. The industrilization of the tanning industry in the second half of the nineteenth century included no remarkable changes in the tanning process but was accomplished chiefly by expanding the physical size of the tannery , decreasing the amount of time that was necessary to lay away hides or skins, and mechanizing virtually all of the traditional hand work of the tanner. The tanning processes, tools , and buildings utilized during the mid-eighteenth century would be immediately recognized by anyone working in a large industrial tannery of the late nineteenth century.
Although there is no evidence to indicate what kind of leather was produced at the first Philipsburg tannery, "union crop sole" was the only type of leather that was produced at the second Philipsburg tannery. Except for the location of the bark storage sheds, the pattern of processing steps at the first Philipsburg tannery (1870-1876) is unknown. However, the layout of the second Philipsburg tannery (1876-1912) is well documented and permits an opportunity to examine the tanning process at this late nineteenth century industrial site.
The tanning operations at the second Philipsburg tannery were arranged in a roughly circular pattern commencing and terminating at the railroad siding. Hides were unloaded from railroad cars into the hide house on the south side of the siding where the hides were probably cleaned and limed. These hides were then transferred to the beam house on the north side of the siding yard where they were de-haired, fleshed and bated. The hides were cut into sides before tanning, and the actual tanning took place in the tan yards. When tanning was completed, the hides were taken to the drying and finishing houses on Thirteenth (Water) Street where the sides were oiled, dryed, and rolled. After rolling, the sides were probably taken to be dried in the smaller dry house at the west end of the hide house, and then loaded directly onto the same railroad cars which had been emptied of hides at the hide house. The leach house and ancillary buildings (i.e., blacksmith and carpenter shops, storage and hair drying facility) were located outside this processing circuit at the east end of the tannery complex.
The following narrative and photo/glossary links will allow you to learn more about the details of the leather tanning process, its techniques, systems, and history of the industry and of the Philipsburg tannery.
The vegetable tanning process, whether undertaken by hand in a small colonial
tannery or in a largely mechanized industrial tannery, can be broken down into six basic steps: FLOW CHART
• Procurement and Storage of Raw Materials
• Cleaning and Preparation of the Hides.
• The Preperation of the Tanning Solution.
• Storage and Shipping.
Procurement and Storage of Raw Materials
A list of the basic raw materials required to operate a tannery is relavively short and consists of a steady supply of hides or skins, a dependable supply of tree bark rich in tannin , a plentiful supply of clean water, lime for dehairing hides, and oils for treating the finished leather. For an industrial tannery, access to a railroad and fuel for steam power were additional requirements.
Before 1800, most leather was manufactured in small tanneries where raw materials could be easily acquired from the local neighborhood. However, as settlement intensified along the eastern seaboard and forests were cut down, it became increasingly difficult for tanners to procure a steady supply of hides and skins from local slaughter houses and enough bark to operate a profitable tannery. Between 1830 and 1850, two major changes took place in the leather industry. First, tanneries began to move westward from the eastern seaboard into forested regions of central Pennsylvania and New York where oak and hemlock were plentiful. By the mid-nineteenth century, the construction of inland navigation systems and railroads made these inland sites readily accessible to all major eastern markets. Second, by 1850 new sources of hides replaced the uncertain supply from local slaughter houses. The commercial meat-packing industry in the midwest became a major source of hides while the import of foreign hides through eastern seaports increased dramatically. Assured a dependable supply of hides and tan bark, these new industrial tanneries grew to unprecedented size.
Bark storage consumed a vast amount of space at the Philipsburg Tannery Site. In 1879-1880, for example, the secondPhilipsburg tannery consumed 6,500 tons of hemlock bark and 750 tons of oak bark, all of which was stored on the premises. Three large bark storage sheds as well as a large bark pile were located west of the main tannery buildings along the railroad sidings. This area was also used for bark storage during the period of the first Philipsburg tannery. It appears that the lower railroad siding was built as part of the second Philipsburg tannery to facilitate the intra-site conveyance of tan bark by train from these bark sheds to the leach house.
There were no special provisions made for the storage of other raw materials, such as lime used in the dehairing process andoils used in the finishing process. Since the power used to operate the tannery was generated from burning used tan bark , it was not necessary to set aside special areas for the storage of fuel, furnace ash, or for spent tan bark.
Water, large amounts of which are used in the tanning process, was readly available from Cold Stream and was probably drawn directly from the stream as needed. There is no evidence Cold Stream was dammed or that water was diverted into reservoirs and stored for future use. It is also very likely that Cold Stream was used for the disposal of liquid waste from the tanning process.
Industrial tanneries generally purchased their hides in bulk lots directly from mid-western meat packers or foreign suppliers, often with the railroads themselves acting as the middlemen. Traditionally, hides obtained from slaughter houses were sold to tanneries with "old trim" , that is to say, with offal such as ears, tails, shanks, snouts, and some flesh attached. Hides generally arrived at the tannery after minimal preservation at the slaughterhouse. Hides from American meatpackers were usually salted while foreign hides were often dried or dried and salted. Occasionally, hides were also pickled. Borax was also suitable as a preservative for hides. Since slaughter house preservation was not particularly long-term, it was necessary to deliver hides to the tannery quickly as possible so that the tannery could to begin processing them before they spoiled. Foreign hides were generally more agressively preserved and required additional processing to soften and clean them.
By 1890, the demand for tan bark was so great, that tanneries rather than lumber merchants controlled most of the forests on the western slope of the Alleghenies. The gathering of tan bark was a seasonal activity. An entire year’s supply of bark was collected in late spring and summer. The bark was peeled from trees or logs, cut into four foot lengths and stored at the tannery in large bark sheds or ranges of large piles which were "self-roofed" so that rain did not leach the tannic acid from the bark. As needed, bark was conveyed to the leach house where it was ground and prepared as the tanning solution.
Cleaning and Preparation of the Hides
The way in which a hide was prepared for tanning partially depended on the way it had been preserved by the meat packer or foreign supplier. Dry hides could not be mixed with salted hides, and best results were achieved when hides were sorted by size and weight before they were put to soak. In any case, the first step in preparing hides for tanning was to remove all dirt and natural fluids. Once the hides were superficially cleaned, they were soaked in water to soften them and remove any preservatives such as salt or borox. When the hides were taken out of the initial soak, they were often divided lengthwise into sides . If the hide required further softening, it was worked in a hide mill, a device that resembled the fulling machine in a woolen factory.
After the hides were cleaned and softened, they were de-haired. The traditional method of dehairing was to soak the hides in a lime solution which swelled the fibers of the hide and loosened the hair at the follicle. During the liming process it was desirable to keep the hides in constant motion by reeling them from pit to pit, turning them, or dragging them back and forth through a long trough. By the late nineteenth century, depilation in a sweat pit had become more common as an alternative to liming. Once the hairs were loosened, the hide was washed in water and taken to the beam house .
The tradition method of removing hair, fat and flesh from a hide was to beam it by spreading each hide out individually on a slanted log or beam where the hair and flesh were scraped off manually using an array of different knives and scrapers. By the end of the nineteenth century, this process of dehairing and fleshing had been completely mechanized in most of the the large industrial tanneries.
Once the hide was de-haired and fleshed, it was bated to remove the lime from clean hide. Traditionally. the hide was rinsed in warm water and placed in a rotating drum containing a variety of substances such as dog manure, pigeon droppings, hen manure, molasses, or bran. However, no more than a thorough washing was often sufficient to remove the lime from hides which were intended for use as sole leather .
A beam house is depicted on the 1897 Sanborn Map of the tannery; although, it is not on the 1887 map. The 1887 and 1897 maps illustrate a one-story structure called the "hide house" that was located on the south side of the railroad siding between the finishing house and the tan yard. This building was probably the point at which hides were unloaded from the railroad cars. However, there were five large, circular vats on the south side of this building which indicates that some sort of processing was also undertaken at this locus, such as cleaning, soaking , and liming. It is unclear whether the activities in this area included beaming and bating.
A description of the second Philipsburg tannery written in the Philipsburg Daily Journal in 1877, indicates that hides were chemically de-haired then cleaned by hand on eleven beams. Whether mechanical methods of cleaning, fleshing, and dehairing were subsequently adopted could not be determined.
The leach house, where the tanning solutions were prepared, was a one-story structure located at the east end of the hide house at the terminus of the railroad siding. Two bark mills were located in the basement at the west end of the leach house. The boilers and furnaces were located adjacent to the north wall of the leach house. The juxtaposition of the power generation facilities and the leach house was purposeful because the second Philipsburg tannery had adoped the practice of burning spent tan bark as fuel for the boiler furnaces using the Hoyt Furnace method.
The second Philipsburg tannery utilized a highly mechanized percolation system to prepare tanning solutions. Each leach vat was equipped with an Allen and Warren Sprinkling System which delivered a spray of weak tanning solution to each leach vat which was filled with seven cords of freshly ground bark. The ground bark was delivered to each leach vat by a conveyor system which consisted of a wooden box measuring 12 inches wide and 8 inches deep through which a heavy iron chain was passed. This conveyor system, which extended for a distance of 120 feet, was suspended above the leach vats. As the chain moved through the box, the ground bark was carried along and deposited in each vat through a hole in the bottom of the box. Once the tanning solution had reached the desired concentration, it was delivered to the tan yard by a system of pumps and pipes. In 1877, the main section of the leach house contained twelve round leach vats, each of which measured twelve feet in diameter and seven feet in depth. A decade later, there appears to have been sixteen vats in the leach house .
The Preparation of the Tanning Solution
Tanning solutions were prepared by a process called leaching which is nothing more complicated than preparing a "strong tea" from a combination of ground-up bark and hot water. The process consisted of passing hot water through the ground bark in a manner which achieved the greatest density in the resulting solution. A leach tank was filled with fresh ground bark and hot water and allowed to stand for two days. At the end of two days, the tannic acid solution at the bottom of the vat was stronger than the solution at the top of the vat. The solution was manipulated through a series of the ten or twelve vats until the desired tannic acid concentration was achieved and was conveyed to the tan vats . The principal tool used in this step of the tanning process was the barkometer , which is tanning terminology for a hydrometer, and was used to measure the acidity of the tanning liquor.
By the third quarter if the nineteenth century, some tanneries had installed furnaces which were specially designed to utilize the spent tan bark as fuel in either a dry or wet state. The new furnaces solved the dual problem of fueling the furnaces and disposing of large amounts of waste products.
A double tanning process was commonly used for manufactuing sole leather. Initially, the hides were placed in a rocker that consisted of a rectangular tanning vat fitted with a frame made of a series of poles over which the hides were hung. This frame was pinned into the sides of the vat so that it could move in an arc of four to six inches causing the hides to slightly agitate the tanning solution while remaining submerged. The rockers were generally aligned in a series which ranged from a very weak tanning solution in the first vat to a very strong solution in the last vat. Green hides were initially placed in the weak tanning solution and moved to progressively stronger and stronger solutions over a period of approximately seven to ten days. In some tanneries, the hides were kept in the same rocker vat while the tanning solution was periodically replaced with a stronger one. After processing in the rockers was completed, the hides were put in handlers (vats in which the hides were laid flat in a strong tanning solution) for an additional ten to fourteen days.
From the rockers and handlers, the hides were taken to the tan yard where they were put up in the layaway vats . These vats were usually nine feet long, seven feet wide, and six feet deep. The hides were laid out flat, one by one, with a thin layer of ground bark between each hide. The vat was then filled with a tanning solution. Periodically, the tanning solution was drained off and pumped back into the leach house through a series of pipes where it was recharged. Meanwhile, the layaway vats were repacked and refilled with fresh tanning solution. This process was repeated four or five times over a period of several months. The hides were kept in the layaway vats for an average of three to four months.
In 1887, there were two tan yards, a large yard (located on the south side of Cold Stream), and a smaller tan yard (located on the north side of Cold Stream). The two tan yards were connected by a bridge over Cold Stream. Both tan yard buildings were one-story high, and probably consisted of little more than roughly constructed, enclosed sheds erected to protected the tan vats from the elements. In 1877, the tan yard contained a total of 210 vats, which included thirty rockers and one hundred and eighty lay-away vats.
The double tanning process for making sole leather was used at the second Philipsburg tannery. Sides (hides cut in half lengthwise) were placed in rocker frames which held sixty sides each. The rocker frames were then processed through a series of ten rectangular vats containing progressively stronger tanning solutions. It took about ten days to process a frame through the rocker vats, and at the completion of this process, the sides were placed in layaway vats, each of which held one hundred and ten sides. It appears that handlers were not used at the second Philipsburg tannery unless the observer failed to distinguish them from the lay away vats. The sides remained in the layaway vats for about one hundred days; they were removed from the lay-away vats and were taken to be dried and finished.
When tanning was completed, the hides were removed from the lay-away vats. If they had been tanned whole, they were split into sides, rinsed in the final tanning solution, and piled to drain overnight. The sides were then oiled on the grain side and hung on poles in a drying loft where the temperture was carefully regulated and no light was allowed to enter. Exposure to light at this point in the process damaged the color of the hide. The leather dried in about two days. When it had dried completely, it was taken to the finishing room where the sides were rinsed in water and rolled in a rolling machine. The wet sides were again hung on poles and dried for a second time. Once dried, the sides of tanned leather were trimmed, weighed, bundled and sent to market as "unscoured leather" .
When sole leather left the tannery, it was ready to be cut and assembed into shoes by the shoe maker. If further processing was required, the leather might be sent it out to curry shops for final finishing. Traditionally tanning and currying were considered separate crafts, and all leather was sent to the currier for finishing. In the late nineteenth century this distinction had blurred and tanneries were generally expected to produce finished goods on their premises.
Tanned hides from the Philipsburg tannery were apparently moved directly from the tan yard to the the finishing house where they were dried. trimmed, and oiled. Only the basement and the first floor of the finishing house were actually used for working the finished leather while the upper floors used for drying. The multi-storied structure would have had latticed floors and interior partitions so that heated air could flow evenly throughout the building. It is uncertain to what extent the finishing process at the second Philipsburg tannery had been mechanized. Apparently , the sole leather produced at the second Philipsburg tannery was shipped unscoured, directly from the drying house adjacent to the railroad siding.
Storing and Shipping
Sole leather was traditionally shipped to the manufacturer in bundles of sides consisting of the pack that was tanned in a single vat. However, by the end of the nineteenth century, some tanneries were required by shoe manufacturers to cut soles and insoles prior to shipment from the tannery.
No warehouse facilities were set aside for the storage of finished leather. After the tanning process was completed, the finished leather was bundled and loaded onto railroad cars for immediate shipment to market. There is no evidence that soles were ever cut on the Philipsburg Tannery Site.
Maps dating to 1887 indicate that a blacksmith shop and carpenters shop were located northeast of the main block of tannery buildings. These structures were presumably related to mainentance of the works. A hair drying facility was located on the south side of Cold Stream above the tan yards. By 1897, a grease house had also been erected at the northeast corner of the beam house. The necessity for erecting a grease house may indicate either a change in the condition of the hides which the tannery received or a change in the methods used to degrease and flesh hides in the beam house.