Cleaner, greener ovens

Materials World magazine
,
3 Jul 2015

Mark Devey, Chairman of the Vitreous Enamellers’ Society, looks at the latest trends in oven cleaning cycles in the appliance industry.

Vitreous enamel (VE) is well known for its unique properties, including resistance to graffiti, abrasion, acids, alkalis and detergents. It is also flameproof and colourfast, which is why for many years it has been the coating of choice on architectural panels and signage used on the London Underground, as well as on road tunnels around the world.

Because of its chemical resistance, VE is also used as a coating for silo tanks, flue pipes, chemical vessels, valves and hot water tanks. In addition, there is still a huge market for vitreous enamel on appliances such as cookers.

The consumption of vitreous enamel on the outside of cookers has reduced in recent years, with glass hobs and stainless steel finishes becoming fashionable. Many parts such as cooker side panels, doors and fascia panels, which were traditionally coated in VE, now tend to be coated in epoxy paints. Only where heat resistance and scratch resistance is essential, such as for coating ovens, roasting dishes, grill pans and pan supports can we be sure to find VE.

Normal groundcoat enamel oven

This is the traditional type of enamel finish. It is normally found in dark colours, black, brown and dark blue and has either a solid finish or includes a white fleck, which is titania-based and reduces the wear of the wirework on the vitreous enamel during the life cycle of the appliance.

The basic oven will be coated in acid-resisting groundcoat enamel. The enamel formulation is an alkali-borosilicate glass including transition metals, such as cobalt and manganese, to provide the mechanism for the vitreous enamel to chemically bond to the steel substrate.

The formulation is developed to meet the firing conditions of the production plant where it is processed, typically in the temperature range of 800–850°C. Normally, the final formulation will include a soft, medium and hard firing constituent frit to widen the firing capability of the finished enamel. The enamel application is either made in a wet form, typically by dipping or spraying, or the coating is made using electrostatic powder. In the case of wet application, the enamel is subsequently dried before firing. In the case of powder application, the coated oven is simply fired. In both cases, a continuous furnace is traditionally used.

It can be very difficult to clean this type of enamel when food soils the surface during cooking. The enamel can only be returned to pristine condition by using proprietary strong oven cleaners, which many would prefer to avoid.

Self-cleaning ovens

These ovens are coated in a matt black or grey VE, known as catalytic or self-cleaning enamel, which has been available in the industry since the 1960s. The back, sides and roof of the oven are normally coated, but not the base. This is usually a normal groundcoat enamel, from which spills cannot easily be cleaned. In many cases, the self-cleaning oven is fitted with loose liners coated in the self-cleaning enamel and fitted to a conventionally enamelled oven.

The catalytic coating has a large and porous surface area, so is not a traditional vitreous enamel finish – the objective is to disperse the residues produced during cooking. The coating contains high levels of transition metals, manganese, cobalt, copper and iron, which are held together in a glassy matrix. The catalytic coating is either wet sprayed or applied in powder form, most typically over a fired vitreous enamelled groundcoat and then fired in a continuous furnace at 820–840°C. The catalytic coating absorbs the splashes of food residue which will then burn off during the normal cooking cycle at 200–260°C to leave a residue of ash.

Pyrolytic ovens

These ovens have been manufactured by a number of the leading brands in Europe for many years. The oven is coated with a heat- and acid-resistant enamel. When the oven requires cleaning, the appliance can be put onto the pyrolytic cycle. During this cycle, the temperature of the oven is increased to around 500°C for 1–2 hours. The cycle decomposes food spills by gaseous degradation and oxidation, so the grease and food residues turn to ash, which can then be easily removed.

Special high-temperature enamels are required for this function, as well as enhanced insulation to ensure no external surfaces become dangerously hot. The appliance also requires a safety lock to ensure that the oven cannot be opened by mistake during the high temperature cleaning cycle. 

The specially developed pyrolytic enamels must have increased heat resistance as the cleaning cycle is close to the softening point of the enamel coating. These groundcoat enamels are formulated to give good chemical resistance by including a combination of TiO2 and ZrO2, smelted into the formulation. The application follows the same methods for conventional enamels, but the firing temperature is normally higher – between 850–880°C.

New steam or water cleaning ovens

This is a newly patented enamel coating for cavities and bakeware, which has a natural and green cleaning system, consuming very little energy. The drawback of the existing pyrolytic oven technology is the high-energy use during the cleaning cycle. The downside of the catalytic self-clean oven is that over time the porous surface tends to clog and become less efficient.

With the new steam cleaning ovens, the enamel surface can be returned to pristine condition after a short cleaning cycle, without the use of chemical cleaners. 

The cleaning cycle uses typically 500ml of water, which is added to the base of the cold oven. The oven is then heated to only 90°C for 30 minutes and, after being left to stand for 10 minutes, the residues can be easily removed with a damp sponge.

This new vitreous enamel development is formulated for application either in wet form or as an electrostatic powder, and is fired between 820–850°C.It is an inorganic system that is VOC-free and has the scratch resistance of conventional enamel.