Leather sampling methods

The complete and systematic examination of leather, including physical and mechanical tests 1 and chemical analysis, is carried out principally for tannin- or chrome-tanned leathers. The methods to be used in this examination arc given below and are preceded by the procedure to be followed in sampling. 

A method of horizontal attenuated total reflectance (HATR) by Fourier transform infrared spectroscopy (FTIR) is considered more useful for infrared spectroscopic analysis of leather samples. By this method, the sample is simply put on the flat plate of ZnSe crystal or KRS-5 crystal of the HATR accessory. The Fourier transformation of the interferogram is converted by the computer into a plot of absorption against wave number that resembles the usual IR spectrum (William and Fleming, 1998). 

Cut three specimens from the leather sample by the sampling method in Section 3.4.1. Each specimen consists of approximately 5 g of leather. Cut the specimen to small pieces of around 1 cm in diameter. Soak the specimen in a 250 ml flask with distilled water that is 20 times the weight of the specimen (ASTM D2810,2001b). Stopper the flask and shake thoroughly. Keep it in a conditioned room (temperature 23 1 °C) for 6 h. Remove the leather specimen from the flask and measure the pH of the solution with a pH meter that has been calibrated against standard pH solutions. Standard pH solutions (pH 4.0, pH 7.0 and pH 10.0) are conunercially available. 

More and more products made from synthetic materials substituting for leather have been introduced into the market. S ynthetic leather substitutes are usually nonwoven fabrics coated or laminated with some kind of polymer. It is very often desired to identify if a product is a genuine leather or a synthetic substitute. The following four test methods can be used to identify the samples. 

Cut a small piece of the sample and bum it in the fume hood. Leather bums without flame but releases a smell of burning protein. However, the synthetic leather bums without smell of burning protein. The bum test is simple and quick. It can be used as a preliminary test and confirmed by the other methods. 

Based on the absorption bands of leather listed in Table 3.1, it can be determined whether the tested sample is a genuine leather or synthetic polymer. However, care should be taken with the spectra of nylon, silk and wool which could have many of the absorption bands listed in Table 3.1, because they have the structures of amide bands. They can be distinguished from the genuine leather by the other three methods described in this section. 

Leather is a natural product and is subject to extensive variability. The physical and chemical properties vary considerably depending on the location from which the leather test sample is taken. The standard test method from the American Society for Testing and Materials (ASTM D2813, 1997) ensures random sampling of finished leather and fabricated leather items for physical and chemical tests. Test specimens should be cut from only one side of the backbone with their long dimension perpendicular to the backbone line. Test specimens should be taken from different parts of the shoulder, belly and tail of the leather. The number of specimens taken depends on the reliability of the test results, the deviation and the error of the testing procedures and should be recorded on the test report. Physical tests of leather and leather products, unless otherwise specified, should be performed under the standard atmospheric conditions of 50 4% relative humidity at 23 1 °C. 

Chromium analysis can be performed on the ash sample from leather or can be perfonned on the leather directly. ASTM D2807 is a method for testing chromium by leather digestion (ASTM D 2807-1998). In this method, weigh 1 g of leather to the nearest 0.0001 g and cut it into small pieces 0.5 cm in diameter. Transfer the specimen to a 250 ml flask and add sequentially 20 ml of concentrated nitric acid (HNOj), 15 ml of perchloric acid (HCIO ) and 10 ml of sulphuric acid (H. SOj. 

Cut-resistance is an important property of leather against mechanical forces, especially for products that are more at risk of cutting such as gloves, shoes or sport goods. The test method is to measure the force needed to cut through the sample at a distance of 20 mm (International Organization for Standardization, ISO, 1999). 

MALDI has also been used to identify the origins of four animal hair samples from the accoutrement of the 5300-year-old Tyrolean mummy, called Oetzi [174]. The analytical method involved the determination of proteins by analyzing peptides directly derived from tryptic hair digests without any separation or enrichment. Using this method, two samples obtained from Oetzi s coat and a sample from his leggings were found to be made from sheep hide, while the upper leather of his moccasins was from cattle hide. 

To determine the method used in mineral-tanned leathers, it is necessary to test the sample for certain inorganic compounds. The most common mineral tanning compounds are chromium salts sometimes aluminum salts are used, more rarely, iron salts. They may also be used in combination. Spot tests can be used for the detection of these inorganic tanning agents. The sample required is so small that it may be taken from manufactured products (shoes, small leather articles) without visible damage. 

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