Dear Lorraine,

I know you are fascinated with the smell of mackintoshes so I thought I would recount some of my experiences in the rubber industry with particular reference to odour.

I am now retired but I worked in the latex foam industry for over 40 years making mattresses and sheeting. Although synthetic latex now dominates the industry, natural latex really is a wonderful material to work with.

Natural latex is the sap from the tree Hevea Brasiliensis. In its raw state it is dilute and very susceptible to bacterial action so ammonia and small quantities of other chemicals are added as preservatives. The latex is centrifuged to about 62% total solids content making it effectively a dispersion of rubber particles in water with the approximate composition 60% cispolyisoprene rubber, 2% proteins and 38% water. It possesses a characteristic odour but this is obviously masked by the ammonia. (In fact there is an odour test to test for putrefaction whereby the ammonia is neutralised with boric acid and the latex is destabilised and the particles coagulate into a rubbery lump. In the absence of putrefaction the rubber has a pure, clean, fresh almost sweet rubbery odour.)

To manufacture foam rubber mattresses the latex is blended with about 10% of additives which comprise soap, sulphur, accelerators and antioxidant. The additives possess a mild, musty sulphurous odour which slightly alters the odour of the latex. The compound is foamed to the correct density and poured into warm moulds. When the lid closes a small amount of foam is squeezed out between the lid and the pan and this is where you are reminded of the fascinating properties of natural latex. The spew starts to set from the warmth of the mould and forms a wonderful soft semi-elastic rubbery type of foam, and because the gelling agent has neutralised the ammonia, it possesses a clean rubbery smell slightly modified by the sulphur and accelerators.

The moulds pass into a steam oven where the rubber is vulcanised in steam for about 30 to 40 minutes to improve the strength for mattress to be removed from the mould. The mattress is then thoroughly washed in water to remove reaction by-products that are believed to be responsible for some of the odour.

The foams are then dried in a hot air oven at about 120°C where a degree of post vulcanisation occurs. As vulcanisation proceeds via a complicated mechanism involving sulphur and sulphides, the odour can slightly change depending on the temperature and time in the oven. The final product, therefore, possesses a faint pleasant rubbery odour, which has been modified by the sulphurous vulcanisation reaction. The odour then seems to change slightly if the product is allowed to stand for a month or so. The mattress possesses a deep resilient luxurious feel characteristic of natural rubber.

I believe waterproof fabrics are manufactured either from latex or rubber solution in solvent. The odour characteristics should be similar to those of mattresses except that the washing stage is omitted. As you are aware a mackintosh gives natural rubber every opportunity to demonstrate its wonderful luxurious and olfactory properties!

Eric