Image by Rigadoon Glass via Flickr
This is an article that I have copied from the www.lillie-selkirk.co.uk site. If you are looking for an AGA cooker in Scotland or a Rayburn Cooker Scotland then this is the company to contact.
I therefore quote from their article which describes their AGA enamelling process which I presume means the AGA enamelling process at their plant in Telford in Shropshire:
AGA Vitreous Enamel
Thursday 7th February 2008
Many customers are amazed by the enamel finish on the front and top of their Aga appliances and want to know more about how it is created. Below is an in depth guide as to how the final product is made:
Production
Vitreous enamel is a process of putting a coloured ‘glass' finish onto the outer surfaces of an appliance. The resulting surface is hard, scratch and stain resistant (to varying degrees) and imparts a smooth easily cleaned surface.
Pastel and light colours are somewhat ‘softer' and are therefore slightly less resistant to scratches and stains.
Many years ago enamels used to contain lead, but whilst these gave a good surface finish, they were extremely soft and could even be stained by hot water.
All enamels used by Aga are acid resistant, conforming to current British Standards and are also governed by COSHH regulations.
The process of vitreous enamelling castings is as follows.
1. Machined castings are received from our foundry.
2. After checking, the components go for annealing. This involves processing in the furnace at a temperature approximately 30 degrees higher than that required for enamelling. Annealing relieves casting stresses, burns off surface impurities and slightly softens the metal.
3. Following annealing, the castings are shot-blasted and then inspected and any blemishes filled. The fill paste is a ceramic water based substance.
4. Prior to the mid -fifties ‘direct-on' enamelling was used, but changes in the actual casting metal constituents made for poor appearance and surface finish. A water-based ground coat is therefore applied by hand spraying, after which a drying period of around 1-hour is carried out in an oven to remove the water and prevent it attacking the metal underneath the enamel coating. It also prevents pockets of water turning to steam pressure during firing. The ground coat is approximately 7 thousands of an inch thick and generally improves adherence and appearance.
5. After drying, the ground coat is fired to bake it on, at a temperature of approximately 800 deg.C, depending on colour, for 45 minutes. It is allowed to cool naturally, following which a check is made of the enamel thickness.
6. The topcoat is then sprayed on, normally two coats for an Aga, but a third is sometimes applied. Enamel is then dried for about 3 hours (slightly longer for Red) then fired at around 770 deg for 45 minutes. This process is repeated for further coats. All components are then subjected to quality checks for colour etc. Temperatures in the furnace are critical within a few degrees, as is the length of firing time, as overheating can affect colour shades. The colour matching of doors to front plates is generally done from stock but some special matching by temperature can be done.
The enamel process will cause castings to ‘grow' by around 2mm per 30mm length, dependent on the number of times that it has been fired.
Click here for more information about how to clean your Aga from my site.
Quality
The finish of vitreous enamel is subject to the requirements of British Standards 3831:1964, from which the following extracts are taken.
‘Some minor faults may be permitted if they are not visible from the normal inspection distance of 3 feet. On the other hand, certain blemishes, which are so obvious as to spoil the appearance of the complete appliance from the users point of view, may, as indicated in the following clauses, constitute non-compliance with this standard.
The quality of the finish shall comply with the following requirements:
1. Cracks: The enamel surface shall contain no cracks.
2. Flaking: The enamel shall not have flaked off any enamelled surfaces.
The quality of the finish may show the following imperfections subject to the conditions stated:
(i) Hair lines and strain lines: Hair lines or strain lines of ground coat showing through the top coat with no break or crack in the enamel are permitted provided that they do not detract from the general appearance of the appliance.
(ii) Tears (beads) : Tears (beads) are permitted provided that they do not detract from the appearance or function of the appliance in service.
(iii) Runs (drain lines): Runs (drain lines) are permitted provided that they do not detract from the appearance or function of the appliance in service.
(iv) Pinholes: There shall be no holes in the enamel surface, which can be shown to extend to the base metal. For pinholes, which do not penetrate the ground coat the requirements given in sub-clause, (vii) below for specks shall apply.
(v) Blisters: Blisters are permitted provided that due to the particular design or fabrication of the article they are unavoidable, and do not detract from the appearance or function of the appliance in service.
(vi) Depressions and raised areas: Smooth, well covered depressions or raised areas in the enamel surface are permitted provided they are small and widely spaced.
(vii) Specks and inclusions: Specks showing on or through the enamel surface are permitted provided that they are not concentrated in one area and are not greater than 1/32" in diameter and provided that they do not detract from the general appearance of the appliance.
(viii) Orange Peel (ripple) Orange peel (ripple) is permitted provided that it does not detract from the general appearance of the appliance.'
An ‘orange peel' finish, although perhaps less attractive, is thinner and is more hard wearing and scratch resistant.
End of article
Well there you go the way it is really done properly. No wonder my AGA cooker looks so good.
