Food Packing

22) GENERAL PRINCIPLES OF FOOD PACKAGING: PACKAGING MATERIALS
1. Glass Pliny the Elder recorded that Phoenician sailors came ashore and, not finding any local materials suitable for supporting their cooking pots above the sandy shore, used lumps of trona (natural soda or sodium carbonate) that formed part of their ship’s cargo. As the trona was heated in the fire it combined with the sand to give a material that flowed. This story may not be entirely accurate but points to the fact that glass as a man-made material has a long history and some of the oldest dated Middle Eastern pieces are over 4000 years old; a deep blue charm is estimated to date from 7000 BC. Egyptians and Romans fashioned a wide range of intricate and colored glass objects. A highly developed industry existed in Syria in 1500 BC. In medieval times the centre of the glass blowing expertise was Venice (1200-1600 AD). The serendipitous preparation of glass by the Phoenician sailors is basically the same as that used today. A mixture of purified sand is heated with sodium and calcium carbonate together with some sodium sulphate. The gases evolved help to stir the mixture. The addition of calcium is necessary to make the glass insoluble in water - simple sodium glass is water soluble to give a very viscous liquid known as water-glass (used as an egg preservative in WW2). For those who like the chemistry: Na2CO3 + SiO2 -> Na2SiO3 + CO2 CaCO3 + SiO2 -> CaSiO3 + CO2 Na2SO4 + SiO2 -> Na2SiO3 + SO3 Glass made as above is known as soda-glass. Replacement of sodium with some potassium gives a harder glass that is familiar as window and bottle glass. The molten glass is made by a continuous process and is floated on a bath of pure molten tin, with which it does not mix, and cooled to give flat smooth sheets. The scale of production is enormous - in 1939/40 a glass melting plant poured out a 51" wide sheet of glass without interruption for 600 days - each day 3.25 miles of glass poured from the plant, which...