What, would you say, might be one of the most common causes of disagreement between tanners and manufacturers in the buying and selling of leather? Any ideas? What if I was to say that the most common cause of disagreement is the variations in the colour of the leather. Specifically, the failure to reach a target colour or the drifting of the matched colour from the agreed sample. Does that sound like a familiar scenario, regardless of which side of the fence you are?
Something that is not openly discussed is the frustration that manufacturers must deal with when it comes to colour drift inside a manufactured batch. Similarly, the variability of colour on one hide or side results in increased cutting wastage, particularly on aniline and semi-aniline leathers. Three cases of colour variation exist and will be looked at later:
Intra-hide/skin variation (on one hide or skin)
Intra-batch variation (in one batch the hide or skins vary)
Inter-batch variation (in between batches of hides or skins)
Fundamental cause
The basis of uneven colour comes down to:
Uneven dye binding sites
Dye penetrates more in localised areas
Dye penetrates less in localised areas
The dye chemistry plays a great role in all three of the reasons given above, as does the variability in the leather substrate, which comes down to uneven chemical preparation, or uneven binding of post tanning or tanning chemicals.
Most dyes used in the leather industry belong to the anionic dye class - specifically the acid dyes, direct dyes, and the pre-metallised dyes. Anionic (negatively charged) dyes will bind to cationic (positively charged) binding sites. In proteinaceous material, the cationic binding sites are the amino groups, the cationic chemicals (metals, cationic polymers), see Figure 1.
Figure 1. Cationic binding sites a) Amino groups (lysine and arginine) b) Metals (like the hypothesised dehydrated chromium tetramer linked to carboxylic groups c) 2-cyanoguanidine (dicyandiamide linked to aspartic acid)
The amino acid side chains, namely lysine and arginine that contain amino groups (Figure 1a - lysine) can be positively charged at low pH values. Histidine, the other cationic amino acid, can also ionise at low pH. If the pH does not get controlled at the end of the process to the same pH level, then the colour will never be consistent. A consistent pH value will ionise all available cationic amino acids to the identical level every batch, allowing the dye to bind to the sites in a consistent, even manner.
If the cationic groups added, in the form of metals or cationic retanning materials then the availability and distribution of those groups is critical. Uneven chromium content is devastating to even, bright dyeing colours. A tanner producing chromium-tanned leathers that vary (even by fractions of a percent) will result in batch to batch differences. Surface loaded cationic groups will also cause problems, especially if there is uneven penetration of the binding groups.
Physical barriers can also cause dyes to unevenly distribute, these could be physico-chemical (e.g., fat hydrophobic areas), or it could be physical barriers (e.g., heavy retan loading forms an impermeable wall).
Natural colour, skin pigmentation, that mimics the overlaying hair colour – or is the natural colouration of the skin layer (e.g., snake colour patterns) can cause dye unevenness problems. Pickle-free modern processing is causing an increase in the number of colour variations seen arising from the natural pigmentation – since pickle can often bleach the hide/skin and thus eliminate the natural colour interference. Black hair root (root fragments trapped after poor unhairing) is often contributing to the dirtiness in the final colour of the leather.
The last fundamental concern, by the tanner, should focus on the microscopic structure of the leather substrate. If the leather is unevenly structure due to fibre position or fibre breaks (e.g., sueded grain).
Intra-hide/skin variation
As outlined above, hide/skin inconsistencies are generally the most common causes of intra-hide/skin variation. A list, but not an exhaustive list, includes:
Natural skin limitations (e.g., coarse goat grain vs. smooth calf)
Sueded grain (including false backs)
Bites and rashes
Natural grease (could come from flesh layer)
High flesh content could influence penetration of all preparatory operations
Natural pigmentation (could be trapped hair)
Contamination (grease, iron – blood and metal, calcium, magnesium, resins from sawdust)
Epidermis (unremoved, scud)
Hide/skin structure (ribbiness, cartilage, high density – shell)
Coarse hair follicles (e.g., mane will give a darker colour)
Scratches (open – darker, closed – healed/lighter)
Limeblast or lime speckle (occurs on leather on the top of the pile waiting for fleshing)
Uneven tan deposition (cloud-like patterns) caused by uneven fixation like the drum stopping or adding basifying agent too fast
Uneven deposition of fatliquor or retan (precipitation or emulsion splitting)
Poor soaking/liming can cause wrinkling of grain causing uneven fibre splitting
Improper deamidation (removal of amine groups off asparagine and glutamine) gives fewer binding sites
Most of the prevention of these problems is the correct identification of the cause and then ensuring correct process control takes place. Some of the limitations will be natural characteristics and should be viewed as an aniline leather limitation (unless a spray dye touch-up is used)
Intra-batch variation
Figure 2 shows the typical problem seen in an intra-batch colour variation. The leather presents itself as a colour attempting to meet a single colour sample match. However, a single or multiple process parameter(s) have resulted in the colour, within the batch, present as a wide diversity of colours. Tanners would then sort the material into several categories, like the nine shown in Figure 2. Hopefully the Dark Dark (DD) category, seen in Figure 2 is lighter, or at least matches the customer target. All colours lighter than the target can be finish darkened up to shade. Tanners should always aim to dye lighter than the target or will face the spectre of having to try and bleach colour out or use a pigmented finish that allows lightening.
Figure 2. Shade variation often seen in intra-batch colours.
Nine categories are shown (D = dark, M = medium, L = light)
The cause of intra-batch variation is mainly attributed to uneven tanning agent content, or due to natural skin fat variations. It is also possible that one or two hides/skins could be drier than the others and would then not take up the post-tanning chemicals like the other damp material found in the pack.
Material that has not been limed or bated (read as not opened up) can also give differential uptake compared to the others. Another non-exhaustive list could read as:
Tan content differs from piece to piece
Moisture content differs from piece to piece
Opening-up from piece to piece varies
Surface area to volume ratio vary from piece-to-piece
Inter-batch variation
The final pack-to-pack variation is generally attributable to process condition fluctuations. In other words, if the chemical and physical parameters are not identical then the leathers are not going to be similar enough to ensure the fundamental conditions mentioned above are identical. These process parameters would include:
Post tanning pH, temperature, time, mechanical action is not identical
Dye pH, temperature, time, mechanical action is not identical
Fixation pH, temperature, time, mechanical action is not identical
Post tan and dye drum dimensions are not the same (speed, width, furniture, size, goods-to-liquor ratio)
Drum operator attention to detail vary
Drying conditions vary
Environmental conditions vary (air temperature, pressure, humidity vary)
Conclusion
This article has shown where most dye colour drift problems arise. A leather buyer and a tannery can work in partnership to ensure that colour differences can be minimised and hopefully eliminated with careful attention to detail. It is very frustrating where the colour drift source is higher up in the supply chain. For this very reason, the key to colour problem minimisation is through networking and partnership.