Floater leathers require a loose grain11 January 2005
When producing floaters tanners use every opportunity to achieve something they normally try to avoid during the production of thick uppers: a loose grain. Ironically, this is often found to be as difficult to achieve as it is to promote grain tightness. Loose grain occurs when the two layers of the hide with totally different structures, the papillary layer and reticular layer, no longer interconnect as strongly as before. For normal upper leathers this junction is strengthened or supported by retanning. From now on we will refer to these layers as the 'grain layer' and 'flesh side', respectively. Neutralisation plays the first important role in the retanning because a high pH weakens this grain/corium junction. However, worst of all are the adverse effects caused by the wrong choice of oil and incorrect quantities applied. Deeply penetrating and highly stable oils soften and weaken the junction and, in extreme cases, make the layers 'float' on top of each other. The grain layer has a softer and more pliable structure than the flesh side. The grain layer can move freely above the flesh, being 'loosely held' compared with standard leathers. This causes something like 'bubbles' which are developed as a bold milling grain pattern. Shaving removes some of the flesh side and the more shaved the leather, the less pronounced this bubbling effect becomes. For a strong milling pebble, the grain layer needs to be attached to a sub- stantial flesh side. This emphasises the contrast between the structures, which manifests itself on the surface as a milling grain. So, the possibilities of making floaters from thin leathers are limited. When heavy hides are used most of the flesh side will be removed once a substance of below 1.2mm is reached. This is the reason why, we believe, it is impossible to make floater leathers on wet-blue below a certain thickness; there needs to be a substantial amount of flesh still available. Theoretically, it seems to be one of the easiest things to make floater leathers of uniform appearance from batch to batch. In practice it is not. To understand the theory is helpful, both for achieving our goal and to follow the problems which can occur. Successful drum operations depend on the neutralisation, a minimum of retanning and specific fatliquoring. Good floater leathers are characterised by: * a milling grain of uniform pebble size in all parts of the leather * a milling pebble which is as bold as possible * as little weight as possible for hides of substance of 2.2mm or thicker Common to all production methods is the specific method of neutralising. As we mentioned above, through means of pH adjustment it is possible to weaken the junction between grain layer and flesh side to such an extent that fatliquors will be deeply deposited and can form a lubricating layer, on which the grain and flesh can move freely. For floater leathers, sodium formate and ammonium bicarbonate are commonly combined to achieve a pH greater than 6.0. Ammonium bicarbonate has a quicker and deeper penetration than sodium bicarbonate and, for this reason, it has to be preferred although sodium bicarbonate will also work when no ammonium bicarbonate is available. Although indicators may show the same high pH through the cross section after 60 minutes, this is no guarantee that the complex processes of the neutralisation are fully completed. Only time can bring this about. For this reason it is important that leathers of 1.6mm or thicker are left in the bath overnight until equilibrium is reached. The current popularity of floater leathers is partly due to the deteriorating quality of raw hides in general. Thus, heavily milled, bold-pebbled articles convey a natural leather feeling while at the same time they effectively hide visible damage. However, printing is often necessary to reduce visibility of more severe damage. Another reason for printing lies in the fact that even under the best of circumstances it is impossible to produce identical milling patterns even within the same batch. To minimise the risk of irregularities, most floaters are nowadays printed prior to milling. No specific retannage is needed to assist in print retention. The only purpose of printing is to improve the uniformity of the milling pattern, but the less visible the print is, the better. Retanning is left to a minimum and used to improve on filling and handle. No firming retanning agents or vegetable extracts should be used and not only do they significantly alter the handle they will also yield a more irregular milling pattern. A small percentage of very soft and good filling phenolic products, the same as used for upholstery and garment leathers, can be used for improved handle, if required. They will also tighten and fill the belly parts which naturally tend to become looser than with standard upper leathers. Melamine-based products or other filling agents should only be used with great care because they may easily firm up the leather too much. Stronger retannages only come into play when very empty hides are used, or when the leathers have been relimed or strongly bated for a better opening-up of the structure. By working in such a manner it becomes possible to reduce the relatively large amounts of fatliquor needed for floaters and to minimise a feeling of heaviness which such leathers can have. On the other hand such leathers often lack fullness and for this reason retanning is needed. This may in turn create the necessity to use additional fatliquor. For some time large quantities of the fatliquors used for floater leathers were based on sulfited marine oil products. Used as the single product they gave reliable results, soft and well-milled leathers which lacked nothing in fullness. A known disadvantage is the heavy appearance and over-fatliquored feel often obtained when they were used in larger quantities. Another unpleasant effect was the development of smell with time. Because of these side effects most tanneries now prefer to use them only in very small quantities or not at all. Today, specific modern marine-based products without these side effects are available, and synthetic fatliquors or blends of natural and synthetic oils can fully match the properties of classic marine-oils. Fatliquoring floater leathers always contains the risk of excess amounts of softening agents and difficulties with fixing all these oils. When the leathers are toggle dried the fatliquor migrates to the surface and causes darkening of the leathers and a greasy surface. Offering the minimum possible amount of fatliquor and maximising its effects is, therefore, even more important. To achieve this it is important to remember the following: * even and deep distribution of a uniformly emulsified fatliquor * improved uptake of this fatliquor * strong and lasting (irreversible would be ideal) fixation. Unfortunately, even with the best available fatliquors it is not possible to fully achieve this. Only the addition of certain softening polymers will ensure the level of penetration, uptake and fixation needed to make a milled article of the requested uniform appearance. 25-40% of such polymers can be added to the fatliquor mixture. The amount of fatliquor can be reduced by the same percentage. It is always better to process whole hides instead of sides to gain an even milling pattern in the backbone and butt area. However, doing so may not fit into everyone's production methods. The final crucial point is the drying method. Of all the different drying methods, toggle drying is preferred when the milling pattern needs to be as bold and pronounced as possible. Drying in this manner maximises the weakening of the junction between the upper and the flesh sides, causing the desired effect. Vacuum drying, with subsequent hang drying, results in a smaller milling grain. Leathers dried in this manner have not been subject to the stretching strain put onto the fibres, which results in a livelier, lighter and somewhat softer handle. Wet milling to a near-dry state with subsequent hang drying is another option. In some cases this has proven to be the right method to achieve both a bold pebble and the liveliest feel.