Glassware anneal

Because the thick glass (of a heat exchanger) that had been annealed several times is now confronting hot water, it is more likely to fail (corrode and break) than other borosilicate glassware. 

If you ever find a dry glass container on its heat source (the liquid within having boiled off), it is possible that the container may have developed thermal strain. In this condition, it is dangerous and likely to crack or break into pieces without any notice or warning. Unless you have the facilities to examine for strain or to properly anneal the potentially strained glassware, throw it away (see Sec. 1.1.13). 

The thermal expansion also depends on the composition and varies (from 3.3 x 10 per °C for ordinary borosilicate glass (widely used for laboratory glassware) to 7.2 x 10 per °C for speeial types used in graded seals. The annealing temperature, depending on composition, is from 510°C to 600°C. 

In Fig. 6.44b is an enlargement of Fig. 6.44a and shows the detail of a good type of internal seal. Figure 6.44c-h show faults to be avoided, such faults may cause the join to crack, sooner or later, no matter how carefully the glassware is annealed. The most dangerous faults are large variations in wall-thickness in the vicinity of the ring seal, and re-... 

The flame-annealing technique employed with this second end is important. It should be studied, understood and applied whenever this type of glassware is made. 

Many items of laboratory glassware can be made and repaired when reasonable competence has been acquired in the basic operations described in Chapter 6, and flame annealing is properly understood and can be effectively used. Although it is commonly thought that it is easier to repair broken glassware than to remake the item, this is not always true. In many cases repairs can be undertaken only by those who are capable of making the items in question. 

Buchner flasks are made with a substantial wall-thickness since they must withstand pressure differences of 1 atm. No attempt should be made to repair such flasks unless they can be oven annealed. Flame annealing is inadequate for such heavy-walled glassware the possibility of failure when evacuated is considerable and the consequences could be serious. 

It should be noted that the safety of the user is much more important then any small saving in time or money that may result from repairing damaged glassware. Unless the repair is neat, strong and annealed, then the glassware should not be re-issued for use in the laboratory. 

Cones and sockets should, after cutting, have a length of tube left, not less than the length of the ground zone. The heating should not extend to the ground zone. The finished glassware should be oven-annealed. 

There is always a safety concern in handling glassware. Beakers, flasks and any other items with chipped rims should not be used - mainly because the chipped region is sharp and presents a danger during use and washing up. If the item is worth saving then the chipped area should be filed smooth or annealed in a flame. 

We have often made reference to the necessity for annealing blown glassware before it is allowed to cool down to the atmospheric temperature. We may consider it in some detail at this stage, since its importance in the commercial factory is far more than in the laboratory, because of the larger sizes of glass, with thick walls, dealt with in these places. We may understand it better if we consider the necessity for the annealing, how it is done, and how its efficiency may readily be tested. 

Table-blown glassware of the laboratory may also be given an annealing with advantage in a small annealing oven heated by a gas flame. A convenient brickwork chamber for the laboratory or small workshop is one about 2 feet square in area and about i foot deep. It should be provided with a small chimney outlet with a sliding... 

FIGURE 6.23 Suggested schedules for commercial annealing of soda-lime-silica glassware. 

Annealing Thermal treatment to remove stress from glassware... 

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