Jul 03

Glass Enamel Leaf PinsImage 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.

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