Glass Corrosion

Glass Corrosion

What is Glass?

Glass is quite a remarkable substance, being neither a liquid nor a solid, sharing the properties of both: The structure of a liquid but the hardness and rigidity of a solid. Glass is formed by heating a mixture of predominately silicon dioxide (about 75%) and metal oxides (sodium, calcium, aluminium etc) into a viscous state, it is then cooled into a rigid state at a rate to prevent the formation of a regular crystalline structure. During the solidification the metal ions (Na2+, Ca2+) sit within the silicon dioxide network of chemical bonds, disrupting the ordering of the network thus producing a disordered structure akin to a typical liquid structure. The type and amount of added metal oxides determines the properties and the end-use of the glass. For example, adding Boron instead of calcium ions produces glass much more resistant to heat – Heat-proof cookware.

Corrosion

Glass is inert and resistant against a range of substances commonly found around the home, it is therefore often used to store food and liquids. However, this inertness is not indefinite, even to the presence of water. Glass in fact undergoes a constant reaction with water or substances dissolved in water, known as reactions in an aqueous solution or hydrolytic reactions. For example, a common sight with all old windows in places like churches and stately homes is the ‘surface haziness’ of the glass, this is evident on the exterior surfaces and is the result of carbon dioxide and sulphur dioxide gases dissolving in water, forming acids. The now acidic solution reacts with the glass and leaches the metal ions out (ion exchange process). Strong changes in temperature accelerate this process. On the flip side, glass exposed to high pH in solution (alkali) results in the breakdown and dissolution of the silicon network – the decomposition of glass.

Because glass is sensitive to pH changes in the presence of water, the best defence against glass corrosion is to keep glass objects dry and out of humid environments. Of course, this is impractical for glasses that need regular washing, but cleaning does effectively remove surface contaminants that could accelerate corrosion.

Damage to glass can arise from a number of different ways and for different reasons. For glass objects commonly used in the household, namely glasses and other utensils, the objects are often subjected to alkaline conditions in elevated temperatures in a dishwasher. Generally though, damage is not caused by the dishwasher, but is made visible in the process – even if washed in just water and after a few cycles. The following table summarises this.

 

Starting Point  Type of Damage  Causes  Remarks 
Manufacture  Scratches, pinholes, liquid-type appearances, cracks Local strain on glass  Strongly enhanced by machine dishwashing breakage of glass pieces
Use  Scratches, damage caused by vibrations  Among others, incorrect packing in the dishwasher tray  Enhanced especially by temperature changes between the wash and rinse cycles
Automatic dishwasher with low-alkaline dishwasher agent  Iridescent, opalescent colours  Silicate deposition in a thin layer  Glass becomes darker, but still transparent

 

The initial stages of damage to glass appear as localised iridescent patches that turns opaque and spreads as the damage advances. The rate at which this occurs depends on the following factors:

  • Temperature – High temperatures increase the rate, but changes in temperature can cause local stresses within the glass structure, leading to failure.
  • Water Hardness – Very soft water increases the rate compared to hard water.
  • Quality of the manufactured glass – Inconsistent annealing and working of the glass object can result in inhomogeneities of the glass surface, resulting in microscopic pitting. Dishwashers quickly make these defects obvious.

 

Protection against Glass Corrosion

While there’s little that can be done to combat poor quailty glassware, and absolutely nothing that can be done to combat corrosion that has already been established, it is possible to take a few measured steps in cleaning glassware that should guard against corrosion. As we have seen the dishwasher can dramatically accelerate corrosion or simple highlight the presence of defects, which then lead to corrosion. So, logically, the avoidance of using a dishwasher is the first step certainly for the most delicate glassware. It is recommended that delicate glassware should be handwashed, using only a neutral detergent in warm water. The glassware must be rinsed afterwards in clean water at the about the same temperature – delicate glassware must never be subjected to sudden cooling or heating.

If, for whatever reason, the glassware has to be put through a dishwasher, then the wash cycle should be as cool as reasonably practical (40-45oC) and allowed to cool gradually afterwards. This is typical if the glassware items have been used to serve food and are heavliy soiled because of it. If this is the case then the items should be pre-washed by hand, soaked for a few minutes in warm water and gently rinsed before loading into the dishwasher. This step allows the more difficult soiling on the glass to be soften and dislodged, prioir to removal by the dishwasher. At around 40oC most enzymes in detergents function quite slowly, usually requiring at least 55-60oC, so pre-washing and soaking allows the dishwasher to perform to the standards that are expected.

Some dishwashing detergents, not all, have ‘glass protection’ additives which contain a number of different ions, most important are: Calcium, Silicon and Zinc, but could also contain magnesium and aluminium ions. These additives have been shown to dramatically reduce the onset of corrosion – But corrosion that has already set in cannot be reversed.

If you ever notice the rim of glassware, you’ll see that corrosion tends to, or at least appears to, start at the rim. This corrosion is the result of an increased proportion of ions and inherent strain on the glass at that point. The introduction of ions within the detergent as additives tends to reduce the onset of corrosion.

 

 

Ref: Hauthal G. Hermann, Household Cleaning, care, and Maintenance Products, 2004

Rafael Cobos
rafcobos@futurecleansystems.com