Last update: November 29, 2010 02:24:03 PM E-mail Print

 

A COMPARISON OF LEATHER PROPERTIES OF SKINS FROM

TEN DIFFERENT SOUTH AFRICAN SHEEP BREEDS


M.A. Snyman

Grootfontein ADI, P/Bag X529, Middelburg, (EC), 5900

C.A. Jackson-Moss

International School of Tanning Technology

P.O. Box 2085, Grahamstown, 6140

 


1.  INTRODUCTION

Skins contribute significantly to the value of slaughter animals. It is therefore essential that the true value of the skins of different breeds be known to ensure that producers receive the optimum remuneration for their product. The aim of this study was to evaluate tanning quality of the skins of different South African woolled, mutton and dual purpose sheep breeds.

 

Afrino skins are classified and sold as Merino skins in the industry. Afrino breeders, however, feel that Afrino skins could be of a better quality than Merino skins and should be classified as such. Although it is believed that Damara and Van Rooy skins have similar tanning qualities than Namaqua Afrikaner and Dorper skins, no scientific results are available to confirm this. Dorper breeders and producers also claim that skins of hair type Dorpers are of a better quality than those of wool type Dorpers. There are, however, no scientific proof for or against this claim. According to Persian breeders and producers, wool-on tanned speckled Persian skins are quite popular overseas. They are therefore interested in having a comparison made between Black/Red-head Persian skins and Speckled Persian skins. The tanning properties of skins of ten different sheep breeds were evaluated at marketing age of the different breeds.

 

2.  MATERIAL AND METHODS

2.1  Breeds

Skins of the following breeds of the extensive sheep grazing areas were evaluated, namely Merino (wool), Afrino (mutton & wool), Hair type Dorper (mutton), Wool type Dorper (mutton), Namaqua Afrikaner (fat tail mutton), Damara (fat tail mutton), Blackhead Persian (fat rump mutton), Speckled Persian (fat rump mutton) and Van Rooy sheep (fat tail mutton). Dormer sheep (mutton & wool) were included in the trial as representative of the coarse woolled breeds, in order to be able to correctly classify Afrino skins.

 

2.2  Skins

After slaughtering, the skins were preserved with medium coarse salt on a kg salt per kg wet skin basis. Each skin was identified and the skins were send to the Tannery Division of the Leather Industries Research Institute (LIRI) in Grahamstown for testing.

 

2.3  Tests

Eight skins of each breed were evaluated. Two skins were processed with the wool on, and six were tanned through to nappa leather for final testing and visual evaluation.

 

Prior to the processing of the skins, samples were removed from the neck and butt region of skins of each breed in order to carry out histological studies.

 

The following properties were evaluated on the nappa leather :

 

After processing of the skins to undyed crust, the skins, with the exception of the Merino skins, were shaved down to a substance of 0.7 - 0.9 mm. The Merino skins could not be properly shaved due to the ribbiness of the skins. After shaving, the skins were dyed and five skins from each breed were sampled in the butt region and physical tests carried out on the samples.

 

2.4  Description of the different tests done on the processed leather

These tests give an indication of the strength of the leather as well as the amount that the leather could be stretched before the upper grain layers crack, which will cause damage to the surface of the leather.

 

Tensile strength

This is the strength of the leather when placed under a force. A small sample is cut out in the shape of a dumbbell. This is placed into an Instron machine. The sample is held firmly in two clamps. These two clamps move apart at a steady speed of ± 100 mm/min. As they move apart, the force required to stretch the leather is measured automatically. At some point, the leather sample breaks. The force required to break the sample is called the tensile strength of the leather and is measured in Newtons or Mpa. For each test, samples cut along as well as across the length of the skin (from head to butt) were tested.

 

Extension at grain break

This is measured during the tensile strength test described above. At the point of breaking, the leather has also been stretched. The percentage stretch is called the elongation at break and is expressed as a percentage.

 

Extension at grain crack

This is also measured during the tensile strength test described above. Very often the grain or top layer of the leather breaks or cracks before the whole cross-section of the leather sample breaks. The Instron machine picks up this change in the sample, and also registers a reading. This point is known as the elongation at grain crack and is also expressed as a percentage.

 

Lastometer

Unlike the tensile strength test where the leather sample is pulled from side to side, the lastometer is a test where a small ball is pushed from underneath the sample. The sample is cut out in the shape of a small circle. This is clamped into a holding device and the ball is then pushed upwards into the sample. The distance that the ball can travel before the grain or top layer of the leather cracks, is given as the test result and is expressed in mm.

 

Slit tear strength

This test is also carried out on the Instron machine. A small rectangular piece of leather has a small slit cut into it. This sample is held at its base by a clamp. Another clamp is inserted through the slit and as it moves, the slit is pulled apart. The point at which the slit tears, is called the slit tear strength and is measured in N/mm.

 

3.  RESULTS AND DISCUSSION

3.1  Histology

Although it is evident from the histological examination of the skins from each breed that there are variations in each skin and that each breed is unique, certain comparisons can be made. All the different breeds exhibit certain commonalities. The wool follicles extend to a depth of at least 50% of the skin thickness, the corium minor makes up approximately 50% of skin thickness and the fibre bundles in the corium major are typically of a horizontal nature (See Figure 1).

 

 

Figure 1. Histological examination of a skin sample

 

Of major importance from a leather point of view is the degree of fat within the skin. The Sudan IV stain is used to stain fat, and if the different breeds are compared, it is evident that the Merino, Van Rooy, Afrino and Namaqua Afrikaner have significantly higher fat contents in the skin than the Persian, Damara, Dorper and Dormer, which appear to have very little fat within the skin structure.

 

Within each breed, a difference in wool follicle density is evident from neck to butt. The histological examination indicates that there is obviously a difference in fibre thickness, as there are differences in wool follicle size.

 

3.2  Grain surface

A close examination of the grain pattern of the skins of the different breeds shows that each breed has its own characteristic pattern, although some are extremely close as would be expected (Figure 2). This pattern arises as a result of the different density distribution of the wool and the differing thickness of the wool fibres itself. It would actually be possible to identify the different sheep breeds from an examination of the grain pattern.

 

 

Figure 2. Grain pattern of Damara, Merino and Afrino leather

 

As expected, the grain surface of the Merino skins showed severe ribbiness (See Figure 2). Ribs are characteristic of Merino skins and contribute to their poor economic value. Some of the Afrino and Dormer skins also showed the presence of small ribs, but these were less pronounced than those on the Merino skins.

 

3.3  Physical tests

The physical test results are summarised in Table 1 for leather produced from skins of the ten breeds. In Table 2, the ranking of the different breeds for each trait tested is presented, together with symbols indicating which breed differs from which in each test. From these tables it is evident that Merino leather performed significantly poorer than most of the other breeds, while Damara leather was significantly stronger than some of the other breeds tested. A discussion of the different tests’ results follows.

 

Table 1. Physical test results (SE) of leather produced from different sheep skins

Breed

Tensile strength :

along (Mpa)

Tensile strength :

across (Mpa)

Extension at grain Crack : along (%)

Extension at grain Crack : across (%)

Extension at grain Break : along (%)

Extension at grain Break : across (%)

Slit tear strength : along (N/mm)

Slit tear strength : across (N/mm)

Lastometer (nm)

Afrino

18.13

(2.55)

16.67

(2.72)

62.33

(7.27)

66.33

(4.91)

74.67

(6.66)

92.00

(6.28)

53.20

(4.19)

53.53

(7.29)

12.40

(0.48)

Damara

22.56

(1.98)

19.26

(2.10)

80.60

(5.63)

74.40

(3.80)

89.00

(5.16)

82.40

(4.86)

44.98

(3.24)

44.56

(5.64)

12.14

(0.37)

Hair Dorper

18.72

(1.98)

12.86

(2.10)

70.20

(5.63)

72.60

(3.80)

82.20

(5.16)

85.60

(4.86)

47.24

(3.24)

41.70

(5.64)

11.52

(0.37)

Wool Dorper

14.84

(1.98)

13.02

(2.10)

72.80

(5.63)

61.20

(3.80)

83.00

(5.16)

77.20

(4.86)

43.22

(3.24)

36.96

(5.64)

11.62

(0.37)

Dormer

12.85

(2.21)

11.48

(2.35)

60.50

(6.30)

63.00

(4.25)

75.50

(5.77)

89.75

(5.44)

47.85

(3.62)

50.40

(6.31)

11.80

(0.42)

Merino

11.86

(1.98)

8.26

(2.10)

44.20

(5.63)

63.60

(3.80)

54.00

(5.16)

84.00

(4.86)

37.62

(3.24)

47.56

(5.64)

13.12

(0.37)

Namaqua Afrikaner

19.44

(1.98)

13.24

(2.10)

57.00

(5.63)

71.80

(3.80)

66.20

(5.16)

84.80

(4.86)

45.14

(3.24)

44.34

(5.64)

12.12

(0.37)

Blackhead Persian

20.86

(1.98)

18.44

(2.10)

65.40

(5.63)

67.20

(3.80)

74.20

(5.16)

82.40

(4.86)

42.54

(3.24)

46.56

(5.64)

12.44

(0.37)

Speckled Persian

21.34

(1.98)

16.94

(2.10)

63.60

(5.63)

72.40

(3.80)

77.20

(5.16)

82.40

(4.86)

44.70

(3.24)

42.16

(5.64)

13.10

(0.37)

Van Rooy

21.18

(1.98)

14.64

(2.10)

67.60

(5.63)

68.60

(3.80)

72.60

(5.16)

89.20

(4.86)

47.86

(3.24)

44.32

(5.64)

12.74

(0.37)

Average

18.18

(0.65)

14.48

(0.70)

64.42

(1.86)

68.11

(1.26)

74.86

(1.71)

84.98

(1.61)

45.44

(1.07)

45.21

(1.86)

12.30

(0.12)

 

 

Table 2. Ranking (1 = Best, 10 = Poorest) of different sheep breeds on physical test results of leather produced from their skins

 

Breed

Tensile strength :

along (Mpa)

Tensile strength :

across (Mpa)

Extension at grain Crack : along (%)

Extension at grain Crack : across (%)

Extension at grain Break : along (%)

Extension at grain Break : across (%)

Slit tear strength : along (N/mm)

Slit tear strength : across (N/mm)

Lastometer (nm)

Afrino (a)

7

4 f

7

7

6 f

1

1 f

1

5

Damara (b)

1 d,e,f

1 c,d,e,f,g

1 e,f,g,i

1 d

1 f,g,h,j

7

6

5

6

Hair Dorper (c)

6 f

8 b

3 f

2 d

3 f,g

4

4 f

9

10 f,i,j

Wool Dorper (d)

8 b,h,i,j

7 b

2 f

10 b,c,i

2 f,g

10

8

10

9 f,i,j

Dormer (e)

9 b,g,h,i,j

9 b,h

8 b

9

5 f

2

3 f

2

8 f,i

Merino (f)

10 b,c,g,h,i,j

10 a,b,h,i,j

10 b,c,d,h,i,j

8

10 a,b,c,d,e,h,i,j

6

10 a,c,e,j

3

1 c,d,e

Namaqua Afrikaner (g)

5 e,f

6 b

9 b

4

9 b,c,d

5

5

4

7

Blackhead Persian (h)

4 d,e,f

2 e,f

5 f

6

7 b,f

7

9

6

4

Speckled Persian (i)

2 d,e,f

3 f

6 b,f

3 d

4 f

7

7

8

2 c,d,e

Van Rooy (j)

3 d,e,f

5 f

4 f

5

8 b,f

3

2 f

7

3 c,d

Superscripts a - j : The specific breed differed (P<0.05) from those included in the superscript for a specific trait

 

Tensile strength : along

Damara leather was the strongest and Merino leather the weakest. The “wool” skin leathers tend to be weaker that the “hair” skin leathers, i.e. Merino, Dormer, Wool Dorper and Afrino leather all had a lower tensile strength than Hair Dorper, Namaqua Afrikaner, Persian, van Rooy and Damara leather.

 

Tensile strength : across

Damara leather was also the strongest and Merino leather the weakest when tested across the skin length. Merino leather was weaker than those produced from Afrino, Damara, Persian or van Rooy skins, while Damara leather was stronger than those of the Dorper, Dormer, Merino and Namaqua Afrikaner.

Extension at grain crack : along

Again Damara leather was superior to Dormer, Merino, Namaqua Afrikaner and Persian skins. Merino leather performed poorer than those of the Damara, Dorpers, Persians and van Rooy.

 

Extension at grain crack : across

Damara leather showed the longest extension before the top layer of the grain cracked, while leather from Wool Dorper skins cracked after the shortest extension. There were, however, no big differences among the breeds.

 

Extension at grain break : along

Merino leather performed poorer than those from all other breeds, except the Namaqua Afrikaner. Damara leather was stronger than Merino, Namaqua Afrikaner, Persian and van Rooy leather. The other breeds did not differ from each other.

 

Extension at grain break : across

There was no significant difference among the breeds for this test.

 

Slit tear strength : along

Afrino leather showed the biggest resistance, while Merino leather performed poorer than Afrino, Hair Dorper, Dormer and van Rooy leather. There was no significant difference among the other breeds.

 

Slit tear strength : across

Afrino leather performed best and Wool Dorpers poorest. Again there was no significant difference among any of the breeds.

 

Lastometer

Merino leather could be stretched the furthest, and hair Dorper’s the least. Hair and Wool Dorper and Dormer leather could be stretched significantly less than Merino, Persian and van Rooy leather.

 

3.4  Breed comparisons

Hair vs. Wool Dorper

Leather produced from Hair Dorper skins could be extended more than Wool Dorper leather before the top grain layers crack or break when tested on samples cut across the length of the skin. For all other physical tests performed, there were no significant differences between Hair or Wool Dorper leather.

 

Comparison of Afrino vs. Merino and Dormer

Leather from Afrino skins differ significantly only from Merino leather for three of the tests performed. Afrino leather was significantly stronger, could be extended more before the grain cracks and a higher force was required to tear the leather in the slit tear strength test compared to Merino leather. Furthermore, Afrino leather did not differ significantly from that of any of the other breeds tested and were more comparable with leather produced from Persian and Dormer skins than Merino skins as far as the physical tests are concerned. The appearance of small ribs on some of the Afrino skins would, however, offset the superior physical leather properties. There is a large variation among Afrino sheep with regard to the appearance of small ribs on the skin.  It is difficult in practice to classify Afrino skins separate from Merino skins, as it is not always possible to observe small ribs from the flesh side of raw skins. 

 

4.  CONCLUSION

From the results of this study it is evident that, with the exception of Merino skins, there is  very little difference among breeds with regard to the quality of the leather produced from their skins. The lack of sufficient numbers of skins from Damara, Namaqua, Van Rooy and Persian sheep, contribute to the fact that Dorper skins are the most popular and sought after in the industry. It is further obvious that the practice of classifying Afrino skins as Merino skins, is not correct. A viable and practical method of identifying and classing Afrino skins separate from Merino skins in the industry, should be investigated.