DAWIE DU TOIT
DAMARA STUD
REPORT
CONTENTS
A COMPARISON OF THE LEATHER PRODUCED FROM THE SKINS OF TEN DIFFERENT SOUTH AFRICAN SHEEP BREEDS.
BY
Dr. M A Snyman (Grootfontein ADI, Middelburg, Eastern Cape) &
Dr. C A Jackson-Moss (International School of Tanning Technology, Grahamstown
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 is known to ensure that producers receive the
optimum remuneration for their product. Afrino skins, for example, are
classified and sold as Merino skins in the industry. Afrino breeders,
however, feel that Afrino skins are of a better quality than Merino
skins and should be classified as such. Although it is believed that
Damara, Namaqua Afrikaner and Van Rooy skins have similar tanning
qualities to 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 is 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.
A project in which the tanning properties of skins of ten different South African woolled, mutton and dual purpose sheep breeds were evaluated, was done by Grootfontein Agricultural Development Institute,
in co-operation with the Tannery Division of the Leather Industries Research Institute (LIRI) in Grahamstown.
2. MATERIAL AND METHODS
Skins of the following breeds of the extensive sheep grazing areas were evaluated, namely Merino (wool), Afrino (mutton & wool), Hair 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 (fat tail mutton). Dormer sheep (mutton & wool) were included in the trial as representative of coarser wool breeds, in order to serve as a comparison for Afrino skins.
2.2 Skins
The skins were evaluated at marketing age of the different breeds, as most skins are available in the industry at those respective ages. After slaughtering, the skins were preserved with medium coarse salt on a kg. salt kg per kg wet skin basis. Each skin was identified and the skins were sent to LIRI for testing.
2.3 Tests
Eight skins of each breed were evaluated. Two skins were processed with 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.
The following properties were evaluated on the nappa leather:
* Grain surface of the skins
* Physical properties of leather produced from the skins.
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 test 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 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 dumbbell. This is placed in 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 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 cross-section of the leather sample breaks. The Instron machine picks up this change in the sample, 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 Wool on tanned skins
The fleece of skins from Dorper and Namaqua Afrikaner sheep tends to
become matted due to loose hair present in the fleece in the raw skin
when tanned with the wool on. These skins can be combed out manually to
achieve an acceptable appearance, however, it is rather time consuming.
Skins from the other hairy breeds (Damara, Persian, and Van Rooy), as
well as the wool breeds (Merino, Afrino, and Dormer), can be tanned
successfully with the wool on. Skins from Speckled Persians yield very
attractive wool on tanned skins.
3.2 Histology
The histological examination of the skins from each breed is shown in
Figures 1 – 10. Although it is evident that there are variations in
each skin and that each breed is unique, certain comparisons can be
made. All the different breeds exhibit certain commonalties. .The wool
follicles extent to a depth of at least 50% of the skin thickness, the corium
minor (grain or top layer) makes approximately 50% of skin thickness
and the fibre bundles in the corium major (layer underneath the
top layer) are typically of a horizontal nature.
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 red, 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 difference in wool follicle size.
3.3 Grain surface
The grain pattern of the skins of the different breeds are shown
in Figures 11-20. A close examination of the figures shows that
each breed has its own characteristic pattern, although some are
extremely close as would be expected. 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 even if the breed was not known.
As expected, the grain surface of the Merino skin showed severe ribbiness.
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.
3.4 Physical tests
The physical test results are summarized 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. The results obtained from four of the tests carried out are
illustrated in figure 21 for the ten breeds. From the tables and
Figure 21 it is evident that that Merino leather performed
significantly poorer than most of the other breeds, while Damara
leather was significantly stronger than some of the breeds tested.
A discussion of the different tests’ results follows
Tensile strength : along
Damara leather was the strongest and Merino leather the weakest.
The "wool" skin leathers tend to be weaker than the
"hair" skin leathers, i.e Merino, Dormer, Wool Dorper and
Afrino leather all had lower tensile strength than Hair Dorper,
Namaqua Afrikaner, Persian, Van Rooy and Damara leather.
Tensile strength : across
It is interesting to note that the results differed between tests
done on samples taken along or across the length of the skins. Damara
leather was the also the strongest and Merino leather the weakest
when tested across the skin length. Merino was weaker than those
produced from Afrino, Damara, Persian, or Van Rooy skins, while
Damara leather was stronger 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, Dorper, Persian and Van Rooy.
Extension at grain crack : across
Damara leather showed the longest extension before the top layer
of the grain cracked, while leather the leather from the Wool Dorper
skins cracked after the shortest extension. There were, however, no
big differences among the other 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 the best and Wool Dorper 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
3.5 Breed comparisons
Hair vs. Wool Dorper
Leather produced from hair Dorper skins could be extended more than
Wool Dorper leather before 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 and
wool Dorper leather.
Blackhead vs. Speckled Persian
There were no significant differences between leather produced from
either Blackhead or Speckled Persian skins for any of the physical test
carried out.
Comparison of Afrino vs. Merino and Dormer
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. As there was a large variation within the Afrino breed with
regard to the extent of ribbiness of the skin, it would be 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.
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. Skins from all these breeds, again with the exception of Merino skins, are well suited to be processed into clothing leather. The lack of sufficient numbers of skins from Damara, Namaqua Afrikaner, 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.