Case Studies of Surface Engineering
Glazing
Glazing is the process of covering a ceramic body with a thin layer of glass. For pottery, such as
porcelain, stoneware, bone china and terracotta, the thickness of the glaze is of the order of 100 ?m.
Advantages:
? glazes are impervious coatings (particularly useful for porous bodies such as terracotta);
? glazes have a high resistance to chemical attack;
? glazes add mechanical strength to ceramic bodies through the sealing of surface flaws;
? glazes are hygienic.
When choosing a glaze for a particular body the most important consideration is that of relative
thermal expansion.
We want ?g < ?b, so that the glaze is in compression after the ceramic has been cooled from the
firing temperature (which is typically > 1050 °C).
If ?g > ?b we get ‘crazing’, i.e., fine hair cracks in the glaze. These are useful in decorative pottery,
but not in tableware, because the cracks are potentially unhygienic
Other requirements:
? choice of transparency, e.g., transparent, opaque or matt;
? restrictions on toxic compounds, e.g., PbO. Lead is introduced into glazes through lead
bisilicate rather than lead oxide; lead oxide is readily soluble in stomach acids.
Nowadays, glazes on bone china are virtually all lead-free. The glazes of choice are alkali
borosilicate (ABS) glazes, typically containing significant amounts of CaO, Na2O as well as Al2O3,
SiO2 and B2O3. This change in practice from the former use of lead-containing frits has been driven
by legislation from the U.S.A. – the bone china market is very dependent on the U.S.A. for sales.
Lead-based glazes are still used in the hotelware and porous earthenware market sectors. The
advantages of lead-based glazes are that they melt at relatively low temperatures, flow easily and
impart shine to glazes through the increased refractive index of the glaze because of the presence of
the heavy lead ions.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
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