When glass is blown into any shape, it has first to be heated to its melting point. After forming, there can be irregular wall thickness. Thin sections will cool and shrink faster than thick sections and internal stresses are set up. When these become greater than the breaking point, cracking or shattering will take place. Sometimes these internal stresses are just short of the breaking point and the piece will remain intact for years, then suddenly fracture.
So on finding your newly dug bottles leave them outside for at least twenty four hours in order for them to acclimatise, on finding them it is also well worth remembering to pack them well before transportation. (I always take a few old milk crates).
NEVER use warm water to clean them, I have only done this once and learnt my lesson, fortunately it was nothing too rare (a Beavis Patent).
Generally remove all loose dirt by scrubbing in cold water using scourers and bottle brushes before the cleaning begins. Mix a solution of 2 lbs. of Soda Crystals with 2 gallons of hot water. Once cooled, carefully place the bottles in the solution and leave for at least 2 days. Get yourself some bottle brushes, a good selection can be found at home-brewed beer shops. Re-scrub as above and give a final rinse in cold water and dry.
The majority of glass bottles recovered from old dumps were manufactured from soda glass - a mixture of soda-lime and silica. Soda and lime are water soluble substances and when glass items are buried for long periods under adverse conditions - i.e. dampness combined with alkalinity - the two soluble constituents become leached from the surface, leaving a film of insoluble silica. This film gives the surface an appearance of being frosted or etched, a condition usually referred to as 'sickness' among collectors.
This is very difficult to remove, below are some tips but they are dependent on the severity of the "sickness" and where it is.
Pour enough concentrated bleach into the bottle to cover the bottom and seal the top, I use tin foil and an elastic band and leave for up to 2 days. The fumes from the bleach loosen the stain and make it easier to remove the staining, make sure you clean the bottle thoroughly afterwards in cold water using a bottle brush.
Use "magic balls" which are copper alloy ball bearings which are rotated in the bottle with a cold water / soda crystal solution.If this does not have the desired effect then seek the advice of a professional bottle cleaner who may use acids.
