The Manufacture of Bone China
In the simplest of terms, all pottery bodies are formulated from clay, a glass-forming material, and an inert filler (rather like the carbon black filler to rubber for car tyres).
The first essential constituent of all pottery bodies is clay. The word covers a wide spectrum of quarried materials whose principal chemical constituent is kaolinite, a form of aluminium silicate. These clays are degradation products of granite. Geologically, china clays are the oldest and under the electron microscope appear as quite large plate-like particles, the older the clay the larger the plates. They are usually white in colour when fired. Ball clays are much younger clays, and whilst still plate-like their particle size is much smaller. They are more plastic than china clays and when semi-dry or totally dry they are stronger than the coarser china clays. They are invariably associated with higher contents of impurities, and unfortunately this makes them coloured to some extent on firing. In the UK, thousands of years of degradation of the granite of Bodmin Moor in Cornwall has given rise to the china clay deposits near St Austell. These are probably the finest in the World for quality pottery production. Ball clay deposits are more common, and in the UK the best of them for pottery production are quarried at the foot of Dartmoor in Devon.
In commercial manufacturing, the clay fulfils several all-important functions. Firstly it provides plasticity. We’re all familiar with the potter throwing his vase or whatever on a potter’s wheel. He can do this because clay becomes plastic just like butter when mixed with just the right amount of water, normally somewhere around 30% by volume. If you add more water, it start to get runny, and add more water still and we find that not only do the particles of clay form a stable suspension without settling out, but with some basic technical control, this clay suspension will also hold other particles in suspension too. It is these rheological properties of clay that allow us to make a controlled fluid mix of the raw materials in water which we call casting slip. We pour this casting slip into porous moulds to form hollow articles such as, say, a teapot. Then thirdly, clays are vital for their kaolinite which reacts chemically during firing with the glass-forming constituent normally present in pottery bodies to form a eutectic-like glass.
The Glass Forming Material
This is usually some form of feldspar, a potassium and/or sodium-rich aluminosilicate. In the UK we use Cornish Stone which like the clays, are also formed through the gradual degradation of granite, and unsurprisingly are to be found in Cornwall in association with the china clays.
A filler does what it says – it fills the voids between the other materials, and here we must aim for adhesion by the glass formed during firing to give cohesion and strength, compatible thermal expansion so that no stresses are set up during cooling, and the all-important combination of low cost and availability. Ground quartz as flint is ideal.
Traditional English Earthenware
Is perhaps the easiest to understand first. This is made from clay, cornish stone and quartz, very approximately in equal proportions. The clay content is a mix of china clays and ball clays, blended by each Factory to satisfy the needs of its production processes. On firing to 1120-1180°Centigrade, the kaolinite in the clay reacts with the feldspars of the stone to form a glass. Some quartz is also dissolved, but most remains, converted to the cristobalite form of silica. The resultant biscuit is porous (10-15%).
Porcelains differ from earthenware in that after firing they are vitreous (non-porous) and tend to be slightly translucent. They are made largely from china clay, feldspar and quartz, but compositions vary widely throughout the World. Some porcelains, perhaps most, are first fired to only 900°-1000°C, followed by glazing with feldspathic glaze and firing at 1250°-1350°C to generate the material we know as porcelain.
Is so-called because it is composed of calcined bone (45-50%), china clay (approx. 25%) and cornish stone (approx. 25%). First produced by Josiah Spode II around 1810, this is a uniquely strong body, particularly white in colour with a very attractive translucency. It is vitreous. The body has a relatively high thermal expansion, and this enables it to be glazed with glazes that soften at around 800°C. In turn, this enables the use of on-glaze decorations that sink into the glaze to particularly rich and attractive effect. It is the undisputed King of pottery bodies. But bone china manufacture is not easy, and requires great attention to raw material control, delicate care of handling in the clay state because of its low strength, tight firing temperature control on each firing. Its beautiful whiteness too is a handicap in that every minute speck of impurity and minor flaw stand out prominently. All this adds up to high cost.
Bone China Manuafcture
This is difficult. It is difficult at all stages. But the final product article is beautiful, and capable of being far more beautiful, more vibrant, and stronger, than one made of any other pottery body.
I quote from an article I read recently
“True bone china is a special type of porcelain that is more translucent and stronger. Instead of feldspar, bone ash is used as the flux. The process requires more careful attention than normal porcelain. The vitrification range is narrow, kilns need to be fired carefully. Ware warps so badly that it must be fired without glaze and supported in setters during the firing (it is then glazed and fired again at a low temperature). Forming methods have to be adapted to the very low plasticity of the bodies. Glazes have to be adapted because the ware has zero porosity. The whole process is quite foreign to what workers in an average pottery would be accustomed to.”
In a factory, bone china body is prepared in the Sliphouse. Here the dry raw materials are mixed together. The china clays are first fed into water in a mixing vessel called a blunger, then the calcined bone and china stone. All these materials are pre-checked in the laboratory for their individual particle sizes, several physical properties and firing characteristics, all of which play have an important bearing on the making characteristics of the clay body, and of the success of subsequent firing. After mixing to a tightly controlled composition, and blunging for several hours to both mix and de-aerate the slip, this slip is then passed over cleaning magnets and sieves (lawns) into a holding stirring vessel called an ark. From there it is fed into a filter press where much of the water is squeezed out under pressure, yielding slabs of clay body termed filter cake.
For plastic clay making of flatware such as plates, saucers, etc., and for any jolleying of cups or bowls, the filter clay is fed through an auger-like extruder called a pug. The shredding, vacuum de-airing and screw action of the pug confers plasticity to the clay as it is finally extruded as a long sausage to be cut to length and fed to the making operations.
For slip casting, the filter cake is re-blunged with water to which a deflocculant, normally sodium silicate, is carefully added until suitable rheological specification for casting is obtained.
Plates and saucers are normally made upside-down. On a jigger, a slice of clay is squeezed out on a revolving flat surface by a flat spreader tool to a pancake shape. The maker then picks this up by hand and throws it down over a mould that shapes the face of the required plate, doing this very skilfully to ensure that no air is trapped between the clay and the mould. This mould is then placed on a second revolving head, and a shaped tool is brought down to bear on the clay pancake and squeeze and spread it out to form the back of the plate. This older traditional method of flat-making has been superceded by roller-head making. In this, the slice of clay is placed straightaway on to the surface of the plate mould which forms the plate face. Then a revolving and heated head of steel around 3-5cm thick, carefully machined on its underside to the required profile of the back of the plate, comes down and squeezes out the clay in one movement. Some modern factories are now using dust-pressing techniques to form their plates.
The making process for hollow articles is determined by their shape. If it is possible to make them in a mould from which, after the forming process, they can be taken out by merely turning the mould upside down, then items such as most mugs and most teacups are either jollied (a process akin to jiggering for plates) or, more likely these days, formed on a suitable roller-head machine.
Some cups and mugs are designed with complex shapes, and like teapots, etc., these have to be made by slip-casting. In this process a casting slip, a slurry of the base raw materials in water and with carefully-prepared rheological properties, is fed into a hollow plaster mould which may be of several parts. Water is sucked out from the slip by the plaster, and a cast, a skin of the body material, slowly builds up on the inner surface of the mould. When this cast is of the required thickness, the caster upturns the mould and pours away the fluid slip remaining inside the mould for re-use. The cast remains clinging to the inner surface of the mould. The waste ‘spare’ at the top of the item – say the top edge of the mug - is later knifed off and the mould set aside to dry a little more. As they dry, clays shrink, and so shortly the cast releases itself from its mould and it is possible then to remove it. It is extremely delicate at this stage, and it is very easy to exert very slight pressure with a finger, say, and distort the barely leather-hard article. If so, the piece is ruined.
Handles and spouts are cast separately, cut to the shape of the mating surface of the parent article, and attached using a touch of casting slip as the ‘glue’.
Towing, Fettling & Sponging
When a plate or simple mug is released from its mould, its edges are roughly shaped. And a more complex item such as a teapot which may have been made from several component pieces stuck together, with each of these perhaps having been formed in multi-part moulds, there are obviously seams running all over it. So before firing these rough edges and seams are removed with various knives and sponges. This rounding off of the edges on plates is ‘towing’. Fettling is the removal of seams, etc., and spongeing with natural sponges is the final smoothing of the article’s surfaces. The object of all this smoothing, whether by hand or machine, is to round of all edges and smooth any minor clay roughness so that its state is perfect before firing. Remedial treatments after firing are much harder and more expensive to do.
The first firing of bone china ware is to 1215°-1230°Centigrade, and the hard strong product is then in its ‘biscuit’ state. The accuracy of firing is critical. Firing to some 5°C over the target temperature starts to cause over-firing typified by the onset of bloating bubbles, a centimetre or more in diameter. There’s no cure. If fired some 5°C too low, vitrification can be incomplete. In this case it is shown by an obvious reduction in translucency, and the article starts to lose its ‘ring’ if tapped. It starts to feel ‘dead’. Short-fired ware has to be re-fired. If this is ignored, then apart from having the low translucency, the article will have reduced strength and be prone to the dreaded ‘spit-out’ on subsequent firings when possibly some very expensive decorating processing will be totally lost at great cost.
After its biscuit fire, bone china is hard and non-porous and is rarely decorated at this stage as the decoration doesn’t attach to the biscuit surface easily. This is unlike earthenware and porcelain which are normally quite porous after their first firing, and this porosity ‘sucks’ on any decoration readily.
Glazes are clear glasses. Their composition firstly has to be designed to confer the correct thermal expansion to be a little greater than that of the underlying biscuit body. In this way, on cooling down after the glost firing, the glaze shrinks a little more than the body and is in compression when cool. Failure to appreciate this in days gone by is the cause of so many antique pottery pieces being crazed. Glazes also have to be free of lead. Lead oxide is an excellent glass-former, but unfortunately the open crystal structure of glazes and glasses allows this lead to be leached out by food acids. This cause of lead-poisoning was not really appreciated until the 1950’s-1960’s.
Glaze is applied by immersing the article in a suspension of the powdered glass in water which has a little china clay added to assist suspension. The rheology is controlled by addition of small amounts of calcium or barium chloride which acts as a flocculant, binding particles together, coagulating them. Otherwise, the glaze suspension is sprayed on, by hand or machine on a belt system.
Bone china is glost-fired to around 1100°Centigrade, some 100° lower than the biscuit firing. Prior to placing the articles, the foot of each article of high quality china is carefully wiped clean of glaze as this would otherwise stick to any underlying surface. This is not always done at the lower end of the market. On such pieces, particularly plates, one can detect three blemishes on the back of the plate rim. This is where the plate has been rested on a ‘pin crank’ for its glost firing.
In Glaze Decoration
This the term applied to decorations composed of very stable colouring oxide pigments which are fired on at temperatures of 1000°-1060°C, some 100°-40 below the glost-firing temperature. The range of such colours is restricted, but they are very stable chemically and are therefore used particularly in demanding situations – such as for catering tableware which is daily subjected to commercial dishwasher washing. Such bone china catering tableware can be significantly more attractive than its porcelain opposition, and of course its much greater strength can be a great advantage in such establishments.
On Glaze Decoration
Undecorated bone china is unmatched for its pure whiteness, strength and translucency. But its capability for gorgeous rich on-glaze decoration sets it far apart from any other pottery body. Ceramic colours are finely powdered coloured glasses. They are nothing like the normal organic or carbon-containing inks used for general printing. These coloured glasses in their rich colours are applied in a host of different ways and finally fired on to the glaze surface of the bone china article at temperatures around 800°Centigrade. At this temperature, they fuse into the surface of the glaze. And of particular importance to the bone china decorator is the fact that there have been developed now a wide spectrum of colouring oxide pigments that remain stable at this firing temperature. This range has its limitations. We certainly can not reproduce the wider range of the common computer monitor or inkjet printer. There are quite large gaps in the available range in the red area of the spectrum, for instance, that are still beyond us. But the range is much wider than is available to the decorator of porcelain whose glazes have high siliceous contents and are much harder as a result.
Most decoration of commercial bone china is via lithographing. In this, the colours are printed by silk screen on to a sheet of pre-gummed paper, rather like a kiddy’s transfer that we all used to stick on the back of our hands. In ceramic printing, the ceramic colours (powdered coloured glasses suspended in an oily medium) are printed one by one, with careful drying between, until the full decoration is down. After the volatiles have evaporated off, the powdered glasses are left as a fairly fragile porous deposit, so these are covered and bonded by a printing of covercoat, a shellac-like material. Later, the lithographer can dip the decal or transfer into water briefly to soften the gum. This releases the decoration of powdered glasses bonded by the covercoat layer from the paper, and can be slid off and on to the bone china article. After careful squeegeeing down on to the glaze, the decal is left to dry before being given its decoration firing to 780°-850°C. Two, three or even more decorating treatments, each with its separate firing, may be required for some elaborate and rich decorations.
Direct silk-screen printing is a technique whereby the article is seated on a chock and the printing is applied directly to it as it revolves under a small screen. This is useful for the decoration of small areas with one colour on, say, mugs for business promotional use.
Digital printing is a relatively new technique that has been added to the decorator’s armoury which is quite useful for some applications and small runs. It is a development of the conventional laser printer where the usual toners are replaced by very finely milled and pre-treated glass powders of their cyan, magenta, yellow and black ceramic counterparts. Depending on the designs being printed, a red colour is frequently substituted in place of the magenta to give a secondary alternative colour range. Superb colour registration is achieved, making it particularly suitable for some photographic subjects.
Gold and precious metal decorations: In ceramic decoration, gold is mainly applied as a suspension of gold metal particles in an oil or paste. At a content of around 6% metal, gold can be processed into sulpho-resinate solution, and when this is fired on to an article, via a decal or via brush application it comes out of the kiln looking bright and shiny – Bright Gold. At lower gold concentrations, these solutions generate lustres on firing, but whilst attractive, these are fragile and not sufficiently sturdy for much commercial use.
At metal concentrations between 6 and 20%, using progressively more actual gold metal powder in suspension, we have the range of self-burnish golds which we can exploit in a variety of applications.
At higher concentrations, the gold coming off the kiln appears dull and old gold in colour – that is, until it is burnished. Rubbing the gold, gently at first, causes the surface particles of to join up and form a bright skin. Further burnishing makes the deposit very stable, and characteristically beautiful, and treasured.
John Chown B.Sc., A.I.Ceram., F.I.D., F.R.M.S..