The Donald Burton Prize Essay, 1969
In recent years there has been seen in the shoe industry the expected movement towards mass production. This has been made difficult on account of the very wide range of shoes which the manufacturer must produce, with differing sizes, widths, and styles, and because of the problems arising with the major raw material, leather. Alongside this increase in mass production techniques the growing over both the shoe and leather trades of synthetic materials has been noticeable. While it may be said that there is an undoubted need for other shoe making materials than leather, if only to meet the ever increasing demand, both in lasting by the for shoes, it is equally true that much of the interest in synthetics is owed to the possibility of greater ease in production with such materials.
This can be said for almost all goods for which leather is used. In the age of mass production raw materials are required which are consistent and standardised in as many ways as possible, including cost, and which can be satisfactorily handled by unskilled and semi-skilled labour. Standardisation, and definition, of the quality of leather is particularly important. Many of the synthetic substitute products have manufacturing properties which demand much more care on the part of the shoe manufacturer than is required by leather. The point here is that with synthetic materials the shoe manufacturer knows that if he can adjust his production to suit the new material then he will be able to achieve a simplified production situation. For even if the individual properties of the synthetic material may be poorer than leather the synthetic material has the vital quality of consistency.
Uniformity in the raw material is called for in every property – thickness, size, shape, colour, fat content, chrome content, type of finish, thickness of finish, and many other physical and chemical properties. Some are more important than others, according to the nature of the production to be used. This means that while compulsory labelling of the leather qualities does not seem to be a practical or valid objective at the present time as far as the purchaser of the finished shoe is concerned, it is a very valid demand for the shoe manufacturer to make. For example, the shoe manufacturer will require knowledge of fat content for processes involving injection moulding and in processes where the leather is to be glued. Chrome content is an important factor in vulcanisation processes, and to avoid damage and consequent waste the shoe manufacturer must be assured a minimum chrome content. The nature of the finish is important with regard to withstand heat and the various solvents which a shoe manufacturer may use, both in lasting by the automatic process and in subsequent finishing. These and other properties of the leather must be known to the manufacturer if he is to be able to use the leather with confidence.
Regular size and shape may also become a prescribed requirement for leather. Upper leather is sold by area in sides, and thus the shoemaker is buying such areas as the neck, the belly, and the butt all at the same price. Since these parts are not all of the same value to him some segmentation may be desirable. This would cause the tanner to change his rounding, probably before tanning, and release such parts of the hide as the belly for tanning for a different purpose, if one could be found. Regular shaped pieces of more even qualities would have many advantages to the shoe manufacturer. Most of these would come in cutting where less skilled operatives could be used, cutting time would be saved, and cutting losses would be reduced.
In the shoe industry itself the advances in manufacturing techniques have been very rapid. Cyclic mulling , automatic lasting, injection moulded soles, direct moulded soles, cemented and vulcanised soles are just some of the new processes which have been introduced and all have put new demands on leather.
Very many of these new processes make great demands on leather with regards to resistance of both leather and finish to heat. Steam used in mulling and lasting, heating in involved in direct and injection moulding, friction ironing, sole edging, stitch indenting, thread removal by yellow flame singeing, and the removal of wrinkles, all expect and require considerable stability to high temperature. In many cases this thermal stability is required alongside conditions of high pressure and moisture content and so many other properties of the leather are simultaneously tested.. The leather will also be required to withstand a high degree of mechanical stress, to withstand organic solvents, and to be capable of use with various cements.
One of the first processes which the leather comes into contact with in shoe making is mulling. As is the case in almost every department in the shoe factory attempts are being made to drastically reduce times of processing. The introduction of cyclic mulling is typical of this. Mulling itself is a process of moistening the leather to ease lasting, the moistening being done either by hanging the leather in a humid atmosphere or by treatment with steam. Peculiarly enough this is a process which has been found to be of far less value with most synthetic materials than with leather. The finish of the leather must be able to withstand rubbing in this wet condition. Mulling done in a damp atmosphere can be a lengthy process. Cyclic mulling will reduce this time to only a few minutes. Here, the leather is alternately treated with,hot and cold moist air. This alternate heating and cooling may be continued three or four times, and the leather will rapidly take up a great deal of water. This can be done either by passing the leather through a number of chambers or by changing the conditions in a single chamber. The nature of the finish may have a significant effect on the leather’s capacity to absorb moisture. The tanner has to be aware that this sort of moisture treatment, especially when combined with stretching over the last, may lead to extension of the grain and loss of embossed patterns. Another mulling process which may be used is instant mulling, which involves blowing steam at the toe of the shoe while it is on the last. This helps to avoid cracking. 1. G. Butlin’ has described the Temperature Gradient Conditioning process. “This consists of pressing the leather between two plates, a cold one, the heat sink, at about 20 degree C, and a hot one, the heat source, with a temperature slightly over 100 degrees C. Between the hot plate and the leather is a porous pad which contains a metered amount of water. The principle is similar to that of a “trouser press”. The temperature of the hot plate has tro be adjusted in accordance with the tannage the leather has received. Where lower temperatures have to be used, for example with semi-chrome vegetable or alum tanned leathers, the time dwell has to be increased.
Preforming is another process which is better done on leather than on synthetic substitutes. It, like mulling, involves the use of beat and moisture. Here parts, or sometimes the whole, of the shoe upper are shaped before lasting. Preforming of the back of the shoe is not a particularly new idea but forepart preforming is comparatively recent. The type of tannage which has been given to the leather will affect the temperature at which preforming can be done. This process allows for easier lasting, especially when stretch factors are considered, and savings in the amounts of leather used. It may, as with mulling, lead to the partial removal of embossed patterns. The finish of the leather will affect preforming, and there is a danger that the surface of the leather will wrinkle.
Lasting itself makes the greatest demands on leather. J. S. Maddams’ mentions some of the leather properties which must be watched in this process: plasticity, resistance to grain crack, tear strength, and lateral contraction. It is during lasting that the most severe stretch and stress is applied to the leather. Basically the process involves bringing the leather to the shape of the last and then assisting it to retain its shape. Modern developments have largely been approached with the purpose of shortening the period of time during which the leather is on the last. When being placed on the last the leather will be stretched in some places to 25-30% of its area. The leather will in other parts be forced to shorten its own length to conform to the shape of the last. Most of the modern lasting processes involve the application of more heat to the leather than in the past. An extreme of this is Dry Heat Lasting where the leather is placed loosely on the last and then treated with air at about 200’c, until it shrinks on to the last.
In the past retention of shape has been obtained by leaving the damp leather on the last for a long period. To shorten this period it is necessary to apply heat. This is the Heat Setting process. The lasted shoe is treated with air over 85’c, often up to 120’c. This can be done with air which is dry or moist. The wet air is better able to relax the fibres in their, new position. Here the leather must withstand high temperature and moisture, and these demands are made not only of the upper leather but also of the lining Jeather. It must be noted, however, that the heat is only applied for a short time and does necessitate all leathers being able to withstand the boil. The greater worry to the shoe manufacturer, indeed, is the softening temperature of the finish. By changing the elastic properties of the leather it is clear that the prt:cess of Heat Setting involves some form of leather degradation to achieve its purpose.
Methods for attaching the sole have moved away from stitching to cementing, vulcanising, or moulding. This involves a shift from one of the properties, stitch tear strength, in which leather very often shows considerable superiority over synthetic materials, and it is a shift which thus affects not only the shoe industry but also many other manufacturers who have previously used stitching in the production of leather goods and now find it more convenient to glue. With these new sole attaching procedures has come the ancillary process of roughing, removing the grain surface to obtain a better bond. The leather, of course, must have a finish which permits and aids roughing. The type of tannage the leather has received may affect the roughing properties. Roughing is not an easy process and it is one which both the tanner and the shoemaker would like to avoid. The upper is inevitably weakened, the leather may become very firm and liable to crack after penetration by the cement, and there is a great danger of roughing rather too large an area of the leather. The purpose of the operation is primarily to clean the leather surface, aid the penetration of the right sort of adhesive, and thus provide a better bond. The nature of the exposed nap is important as if long straggly fibres are predominant the bond will be weak. An impregnated finish may be of assistance here in giving a better bond and perhaps permitting the bonding of unroughed leather. A leather with a high fat content is more difficult to rough than one with less fat. By cleaning the surface for cementing it is clear that excess tans and grease will also be removed. This latter is very important as an excessive grease content can have further detrimental effects than the increased roughing difficulty already mentioned. With cementing any excessive fat is liable to weaken the bond. This problem is primarily a consequence of the free fatty acids present with the fat itself. The amount of fat permissible depends on the type of bond, and on the type of leather. The fat affects both the initial bonding base and-after migration-the bond itself. It must be said that very often faults in bonding are a consequence of not using the most suitable cement. With such a wide range of cements available it is normally possible to find a satisfactory cement for any leather.
The use of one Way Sole Attaching Adhesives and of Activating Cements is now common. This means heating the shoe bottom to temperatures of about 170’c to provide the heat for activation, although a very much lower temperature can sometimes be used. The use of heat activated adhesives allows all the component parts of the shoe to be gathered in one bay before lasting and allows for more efficient use of time on the last. Much of the on-last drying time normally required is removed and the operation is cleaner and easier than Using such cements as neoprene.
With vulcanisation of the sole the time of production is again decreased by using extra pressure. Thus the leather must withstand high temperature to avoid damage, but it is probable that chrome content is a better indication of ability to take the heat than shrinkage temperature. A chrome content of at least 4% seems necessary to be safe from possibility of burning. The type of retannage and the finish both have a close bearing on this process.
Nip line failures, where the roughed upper meets the heated welt plate, have caused most concern in the Direct Moulding of soles. Here the leather has to stand combined pressure and heat. A considerable amount of trouble can be caused at the nip line by lack of uniformity in the thickness of the leather. Where one part of the upper is thicker than the rest there will be great danger of failure. Failure may also occur where there is a thick seam. In this process the amount of fatliquoring the leather has received is important. Excessive grease may prevent the escape of moisture on heating and so impair the efficiency of the process. Faults at the nip line may appear during manufacturing itself or may not show until they give way in use. Heat damage is often shown by cracking when the leather is hand flexed.
Injection Moulding on the making last is used for PVC soles. The leather has to be roughed and is placed in the moulding machine while still on the last. This has found special acceptance in the manufacture of children’s shoes. There is a demand for leathers which can be used for injection moulding without roughing. Upper leather with a poor finish may damage in this process if the PVC escapes beyond the edge of the sole. The excess PVC will then be trimmed off and ‘flash’ will appear when the finish comes off with this trimming. The damage can be reduced by careful choice of finishes for the leather, but it is greatly increased by careless or excessive application of cement to the roughed upper.
With regard to soling materials, it is interesting to note that a higher proportion of shoes, especially women’s shoes, are produced with leather soles in the United States of America than in the United Kingdom. The explanation sometimes given for this is that the American tanner has made greater efforts to provide a sole leather suitable for the shoe manufacturer. It is not altogether certain that the various other materials used for soling have as many points of superiority over leather as is generally thought, and in the 1965 John Arthur Memorial Lecture Herfeld3 outlined many advantages which leather has over rubber for soles. One of the most important of these is the more suitable degree of extensibility which leather has. Rubber has high degree of extensibility and consequently a high degree of permanent stress, at any load. This high stress of rubber is a hindrance to good shoe manufacture as it leads to deformation of the shoe. Because of this rubber is often less durably applied to the last. This does not solve the problem, however, as gentler lasting will also lead to loss of form in the shoe during wear. With leather this problem does not arise and leather soled shoes keep their shape for a much longer time.
The use of synthetic materials for soles has brought other problems to the tanner. Especially where light coloured crepe or PVC soles are used there is a danger of dye bleed from the upper into the sole leading to discoloration of the sole. Sometimes this is caused by organic solvents and softeners used in the manufacture of the shoe and sometimes the pigments in the leather finish may be soluble in the soling polymer.
Migration of colour into the sole is not the only colour difficulty. Often the wet conditions of manufacture will lead to a loosening of the dye. This is particularly hazardous with the increased production of unlined shoes. Careful choice of dyes can overcome this problem.
At the completion of the lasting and soling operations the last stage of shoe manufacture is finishing. This title shadows a large number of operations but as far as leather is concerned the greatest worry must be the amount of touching up which must be dune to put right finishes damaged during production. With the new manufacturing processes that are being introduced comes a need for leather finishes better able to withstand high temperatures, moisture, and pressure. Improved finishes would not only help the shoe manufacturer but also the shoe purchaser who does not want to have to spend a lot of time restoring and touching up the finish on his shoes after wear. The shoe manufacturer must have some idea of the constitution of the finish or he may apply solvents to the leather in the finishing shop which the finish cannot take.
It is in the finishing shop that any creases in the upper will be removed by ironing. A fairly high temperature is used for this and plastic finishes may spoil with the beat. Thus the softening temperature of the finish must be sufficiently high, or if for any reason it is low the shoemaker must be informed. Dust and dirt picked up during manufacture will be removed.
It is clear that the tanner must have regard for the final consumer-the man who buys and wears the shoes, the immediate customer-the shoe manufacturer, and the competition-synthetic substitute materials. In this essay the first of these is largely to be ignored, but the last cannot be. For with the advent of these new materials it is no longer the case that they are always expected to meet the same requirements as leather. The injection moulding process is an example of one where the process gained favour by being suitable for use with non-leather uppers which could not be used with vulcanising and direct moulding. Other processes, like dielectric heating and vacuum forming are largely designed with plastics in mind. Consequently many of the properties, like stitch tear strength, where leather generally shows itself superior have become less important and the leather is expected to display its qualities in new directions. In many of the new shoe manufacturing machines the leather is stretched more severely than it was by older methods, and it is often heated and compressed more severely, commonlv having at the same time a higher moisture content than formerly. Regularity of the leather properties stands out as the immediate objective for the tanner, if he wishes leather to remain in the race towards mass production. With new machinery the flatness of the leather becomes important and the various procedures of leather drying can aid this. For mass production other properties, such as colour, must be standardised.
However, it is clear that by the very nature of the material tanners will never be able to achieve a totally consistent and regular product and that it will never be possible to standardise to within the limits of tolerance often expected of many plastics. It is equally true that with care, and indeed often with less care than is required with the substitutes, leather can be used in most modern shoemaking processes with complete confidence. The most relevant factor is the long-time bugbear of society itself–communication. Any user of leather must be aware of the qualities of the materials with which he is working. Small changes in the tannery can cause considerable problems for the manufacturer. but if he is told of the occurrence and likely effect of major changes such troubles can be reduced or avoided.
Advancement and the introduction of new processes always involve many difficulties and increased and novel demands being laid on raw materials. More than most raw materials leather has the versatility and adaptability to enable it to adjust to meet most requirements. It also has a character and beauty which is a guarantee to an exciting and vigorous future.
The Donald Burton Prize Essay, 1969