170 Years of Porcelain Insulator Production Technology


One would have to think hard to find anything that was leading edge technology more than a century ago and today still widely considered state-of-the-art. Glazed electrical porcelain is such a product. Yet while looking much like what was crafted by hand decades ago, the porcelain insulators produced today share little in common with their relatively crude ‘ancestors’. Constant refinements in composition, processing and testing have made modern porcelain insulators at HV substations impressive by any standard. In spite of inroads by more technologically advanced alternatives, this remarkable durable material has succeeded in remaining the dominant insulator technology applied at substations the world over.

Electrical porcelain has been successively refined over decades to allow application even under the most demanding electrical and mechanical requirements.

Electrical porcelain has a rich history going back to the time when small-scale pottery firms first began making telegraph insulators. These were crude, threadless pieces – produced in small volume compared to alternatives made from glass, which were cheaper and also regarded as superior.

Painting from former Frauenthal Keramik in Austria captures role of manual labor in porcelain insulator production decades ago.


As electrical distribution began to develop in the 1880s, larger and better quality insulators were needed to carry the voltages of overhead power lines. Industry pioneers worldwide began to experiment with different mixtures of clay that would yield insulators with better electrical as well as mechanical performance. Soon, porcelain began to replace glass for most distribution applications due to perceived superior performance in both insulation and strength.

Painting at Norsk Teknisk Porselen in Norway.

As voltages continued to increase and insulators of growing dimension became necessary, the attributes required of clays became ever more demanding. Focus was increasingly placed on strength as well as on high plasticity and good drying behavior, with minimal presence of undesired organic matter. Other key parameters included fine grain size and low residue content, allowing the porcelain to be shaped into huge pieces without deformation and to be fired with no release of gases that might result in porosity in the body.

Controlled dosing of ingredients for porcelain mass.

Today, the ceramic bodies of porcelain insulators are prepared according to strictly followed recipes which involve a compromise in the relative amounts of different ingredients to meet goals of long service life, ease of production or some optimal combination of low cost with sufficient required performance. For example, a typical formulation for the porcelain mass will contain varying proportions of ball clay, kaolin (for strength and plasticity), feldspar (a flux that helps sintering in the kiln), and fillers such as quartz, alumina or calcined bauxite (intended to impart additional mechanical strength). A variety of secondary materials are also used to facilitate processing including water and additives such as binders – all of which are burned off in the kiln during firing at temperatures from 1200 to 1300°C.


At the same time, growing attention has been placed over the years on ‘microstructure’ of raw materials in order to avoid unwanted microscopic interfaces. In particular, experts believe that quartz crystallites found in some ceramic aggregates may feature critically ‘oversized’ particles that can modify and shrink during firing. Resulting microcracks can then propagate and become areas of inherent weakness in the body during dynamic mechanical loading or even under changes in ambient temperature. The larger the quartz crystallites, the bigger these crack and the sooner the insulator is at risk of failing.

Unwanted quartz crystallite separated from vitreous phase in C-130 alumina porcelain body. (Courtesy of Johannes Liebermann).

Given the above, methods have emerged in recent years to allow producers as well as buyers to monitor microstructure of the porcelain mass to ensure freedom from crystallites formed during production. Apart from materials testing to measure physical attributes such as density and mechanical strength, structural analysis is done using x-ray diffractometers. These allow the quartz content of different alumina porcelains to be conveniently and easily compared on charts.

The basic production technologies used for porcelain insulators have not changed appreciably for decades, although associated equipment such as ball mills, extruders, lathes, dryers and kilns have been progressively improved or enlarged for higher productivity or reduced energy costs.


The classical wet process is still the more common manufacturing technique and sees the porcelain mass pressed and shaped while having relatively high moisture content. The main advantage is lower investment cost and that that the production environment has much less of the dust caused by turning dry cylinders. The more recently developed isostatic production process, by contrast, requires expensive upstream equipment to mechanically compress the spray-dried ingredients into porcelain cylinders under extremely high pressure. The main benefit is reduced processing lead times, which, in the insulator business, can prove the key decision-making criterion among certain buyers.

Porcelain can be formed and shaped into huge variety of different housings

Many in the high voltage industry have come to regard electrical porcelain as a basic commodity. However, the truth is that there is a wide range of different qualities available in the marketplace – many of which might still pass basic test standards. This means that users may need their own criteria and methods to assess and qualify who should be on their preferred ‘short list’ from among hundreds of potential suppliers worldwide. Buying on the basis of price alone is seldom a realistic option. For example, in some applications, such as on distribution lines or for station posts at lower transmission voltages, small surface defects or blemishes to the glaze might have little impact on functionality or long-term service performance. However, for more demanding applications, such as HV wall bushings or breaker housings, even minor defects are considered a source of potential problems and therefore viewed as unacceptable. Here, potential suppliers must be carefully reviewed and regularly monitored to ensure continuous compliance with rigorous quality control procedures.


One of the drawbacks of electrical porcelain production using the classical wet process is relatively long production lead times. These are dictated by a series of relatively labor-intensive steps needed to progressively remove moisture once the ceramic body has been mixed into a homogeneous, watery slurry. Given this limitation, a more modern approach has been developed which sees spray-dried ingredients of the ceramic body compressed under great force into dry cylinders, ready for immediate turning.

Porcelain production using classical wet-process begins with mixing raw materials with water inside ball mill to produce homogenous slurry.
Removing excess water by filter pressing of cakes that result from screening of slurry coming from ball mill.
After filter pressing, cakes are pressed into cylinders that will be aged and fed into horizontal or vertical extruders under vacuum to produce cylindrical pugs used for shaping.
Pugs still have too high moisture content initially to be turned and must be left to dry, either through assisted electrical heating or in climate-controlled drying chambers.
To reduce production lead times dictated by need to progressively remove moisture from pugs using the wet process, isostatic pressing was developed decades ago. Here, fine mixtures of ceramic body ingredients are homogenized inside huge spray dryers before being funnelled and compressed into cylinders under high pressure, ready for immediate turning. High investment cost, specialized technology needed and environmental controls needed to limit worker exposure to fine dust all keep this process limited to only a small percentage of all suppliers.
Positioning cylindrical pug for machining on automated lathe. Recovery of cut porcelain mass aims at reducing waste and improved productivity.
(top) Dried pugs from wet process production turned on automated lathes into required shed profiles. Process combines skill with high machine investment. (bottom photo) Automated turning of pugs produced from isostatic process requires controlling dust in environment.
Cleaning and inspecting machined insulators prior to glazing.
After drying and quality control inspection, shaped insulators ready for glazing,
done by vertical or horizontal immersion in a bath or by spray, using variety of
proprietary production technologies.

One of most critical production steps, firing is energy intensive and requires unique skill in optimally loading kiln cart and also instrumentation to monitor and control firing cycle and temperatures at different locations inside kiln. Special very high kilns allow firing of long or jointed insulators for UHV applications.
Final production steps include cutting and grinding, cementing of flanges and tests for cantilever and pressure withstand.
Inspecting internal concentricity of fired porcelain.


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