Let’s start by looking at hide and skin in very general terms; what is the original purpose of this amazing material that we use to make leather from? Well, surprisingly, skin is a truly multipurpose material performing a whole host of functions that are essential for life. Among other things, it is a:

  • Packaging material – it is an elasticated, self-sealing bag that safely contains everything within whilst still allowing free movement. Life would be very messy without it!
  • Microbial barrier – it is a seal that prevents microbes entering and leaving the body. Without it a living creature would soon die from infection.
  • Temperature regulator – it keeps a creature warm when the weather is cold and cools it down when it is hot.
  • Umbrella – it offers protection from the elements; the sun and rain in particular.
  • Identity card – it provides outwardly distinguishing features that enable other creatures to clearly identify species, gender or age.
  • Camouflage – just as its shape and colouration provide a means of identification, it can also enable a creature to become barely visible in certain surroundings.
  • Diagnostic tool – many serious diseases not directly associated with the skin have symptoms that visibly manifest themselves in the skin, eg jaundice.
  • Water regulator – it provides a barrier that inhibits water loss and, therefore, stops the body becoming desiccated.
  • Energy store – the body uses the skin to store fats as a future energy source. This fat also aids insulation in cold climates.
  • Sensory organ – it houses nerve endings that enables a creature to feel things.
  • Vitamin factory – vitamin D is produced in the skin when it is exposed to sunlight. This vitamin is essential for the proper formation of bones.
  • Grip/release tool – in some animals certain areas of the skin grow ridges to provide grip, eg soles of the foot. In others, mucus is produced to make the skin slippery and difficult to hold on to.
  • Breathing mechanism – some creatures, eg frogs, absorb oxygen through their skin.

Apart from being multi-functional, the skin is considered to be the largest organ in the body.  Being able to make leather from it after it has performed all these wonderful functions is an added bonus!
So what is it in skin that enables it to perform such varied functions?  A simple analysis of well-fleshed cattle hide will  contain:
64%  Water
33%  Protein
02%  Fat
01%  Other substances, eg salts
Inevitably there will be some variation in such an analysis of the skins from different species, eg sheepskin contains more fat.
From the tanner’s point of view it is the protein portion that is important since most of the protein found in hide or skin is collagen which we use to make leather from. However, other proteins are also present including elastin, keratin and non-fibrous soluble proteins such as albumins and globulins. If we break down the protein portion of skin into its constituents we would find:
33% Protein – of which 29% is Collagen, 2% Keratin, 0.3% Elastin, 1.7% non-fibrous proteins1
Collagen forms the framework of skin; its unique structure enables skin to be very strong, yet remarkably flexible. These properties are passed on when we turn skin into leather.
The keratinous proteins (the hair and epidermis) provide colouration, protection and warmth. Blood vessels close to the skin surface and sweat glands also play a role in temperature regulation. The sebaceous glands produce oils and waxes that coat the hair as it grows thus adding protection from the elements. 
Elastin is a fibrous protein that helps maintain the elasticity of the skin and the soluble proteins form a gel-like substance between the fibres of the collagen matrix where they help maintain skin structure. Many of these features can be seen by looking at a cross-section of hide that has been treated with a dye that reveals their presence (see Figure, below).
So it can be seen that hide and/or skin is no ordinary material; it is a complex and highly organised mixture of a whole host of different substances and structures. Whilst not directly involved with turning hide into leather, each and every one of these substances and structures will have an impact on the character of the leather that the tanner produces. In the articles that follow in this series we will examine some of them in more detail, describe their impact upon leather and offer advice on how to deal with them appropriately.