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Wednesday, 21 January 2015

Inspirational pottery glazes

The special ceramic materials and process

Reactive coloured glazes
The first article of my series on glaze making identified the basic ingredients used in glazes as silica, felspar, frit, and clay and explained how a little science would help speed up your artistic flair!.This second of the series on glaze making discusses the non core ingredients of a glaze and explains how a little science in the form of good processing can ensure that you achieve the best result from your glaze making efforts.

The 3 Basic Questions

To make truly inspirational glazes we need to establish at least 3 things
1) What type of glaze we want to make?
2) What type of body it will be applied to? Stoneware?, Earthenware? Biscuit? Clay?
3) What firing temperature/cycle do we intend to use?
Once we have the answers to these questions we can begin to formulate the glaze.

The choice of non core glaze materials

Making a silky matt -what to add to your basic glaze

Silky Matt glaze

Let us assume we want to make a white silky matt textured glaze firing at 1150C on stoneware biscuit.
We have already identified silica, felspar, clay and frit as core glaze materials. This means that at least 2 of these materials are used in almost all glazes.
Using the same cake making analogy as before, these ingredients are considered the eggs, butter, flour and sugar equivalents of the glaze recipe!
The materials we might consider adding to these core materials could be:-
Limestone (calcium carbonate)
Needle-like crystals on glaze surface
Dolomite (calcium magnesium carbonate)
Zinc oxide
Alumina (aluminium oxide)
wollastonite (calcium silicate)
clay (alumino silicate)

These materials help to form crystals on the surface of the glaze on firing and thereby help create the matt (dull) texture. However these materials also influence how the glaze melts and bonds to the body on firing. It is an absolute necessity to match the glaze to the clay body or on cooling the glaze may just flake way like old paint! (Technically the glaze and body thermal expansion need to match so that the glaze is in compression after firing.) But lets not go too deeply into how we do that at this stage.

Alumina does not readily melt at this temperature so the likely (and easier) materials to use are zinc oxide, limestone, dolomite and wollastonite. Adding these materials in the right proportions to make a suitable glaze takes a lot of trial and error.

Developing the Glaze Recipe

If you start a with a basic 1250 C recipe as follows:

Base Glaze

Silica 38
Felspar 40
Clay 10
Limestone 12

and start replacing the silica and felspar with more fluxing ingredients that create crystals and allow you to fire at a lower temperature you might eventually reach a formula

Modified Glaze

Limestone 12
Dolomite 25
Felspar 20
Clay 25
Zinc oxide 4
Boron frit 9
Silica 5


Imagine how many test glazes you might need to make before arriving at such a detailed recipe? Note how different this is from the starting base transparent glaze! The material recipe is not the only part to consider when making glaze. The particle size of the glaze materials needs to be reduced to a fine powder by grinding with ceramic pebbles in water to less than 75 microns. Often, for best results, the mean particle size needs to be closer to 15 microns-the diameter of the finest human hair. This allows the glaze particles to react and melt during the firing process.

Firing the Glaze

The firing process is equally important in obtaining satisfactory and repeatable glaze results. Initially the firing cycle should remain constant as you develop your glaze recipe. A typical glaze cycle might be 150 C per hour ramp from room temperature to the peak at 1150 C, followed by a holding period (soak) at peak temperature of 1 hour, followed by kiln switch off and natural cool to room temperature.
Note that the cooling is often as important as the heating process when firing matt or crystal type glazes.
Clearly making glazes is quite a complex process. However, like the best potters, in time you will come to believe that this is what makes pottery glazing so interesting. You can never be sure what will come out of the kiln each day!
In the next of the series the use of ceramic colouring pigments to create even more interesting coloured pieces will be examined..

Pottery books of value

A great way of backing up your knowledge is to read some simple pottery making books. Some of the best books are those you keep going back to as your knowledge grows!

Thought for the Day

In pottery making it rarely goes exactly as you would like first time. Stay positive! Use it as a learning experience.
Henry Ford Quote:
"Failure is only the opportunity to begin again, only this time more wisely."
~ Henry Ford

Happy Potting
The potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters FriendGo now to sign up for my free newsletter.

Sunday, 21 December 2014

Why do I need frit in my glaze?

A glaze after firing can be considered to be a glass. Therefore it is not surprising that traditional glass making materials such as quartz, limestone, and felspar are commonly used to make glazes. However glazes which are intended for firing below 1150C need other elements such as lead or boron and alkalis to create the right properties. To enable these elements to be included safely they are often pre-melted with other glass making materials to form a FRIT.

What is a frit?


Borosilicate frit
Lead frit
China clay













A frit is a a pre-melted glass which is granular in nature. Once crushed and ground to a fine powder it provides the basis for all low firing glazes. Often a glaze contains merely one frit and clay. The clay is added as a suspending and binding agent to aid glaze storage, application and firing.

How many frits are there?

There are literally thousands of frits. The compositions have been researched and developed over a long time period to provide the right technical properties such as fusibility, thermal expansion and durability. A wide range of properties are needed to meet the wide range of bodies and firing cycles used by manufacturers and craft potters.In general terms frits are normally classified as transparent or opaque and then lead containing or lead free.For environmental/safety reasons lead containing frits are gradually being phased out and replaced with lead free alkali borosilicates.

Can I make my own frits?

It is possible but not recommended. Large scale frit manufacturers often make frit by a continuous method in a high temperature box type kiln using specialist refractory linings. These kilns produce tonnes of frit per day, meaning they can achieve consistent high quality output. For other than the most demanding glaze requirement, making your own frits is unlikely to be cost effective.

How many types of frit do I need?

This depends on the number of clay bodies and firing cycles you employ. As a basic requirement you need a transparent frit, an opaque frit and an expansion modifier frit. These should allow you to make transparent, opaque and semi opaque glazes suitable for conversion to coloured glazes by adding colouring pigments.

Why not use frits in all glazes?

In theory this is possible but the costs would be excessive. For example high temperature stoneware and porcelain glazes can be manufactured with lower cost glass making materials such as felspars, quartz, limestone and clays.

Where can I buy frits

Most pottery material distributors sell small quantities of powdered frit suitable for the studio or craft potter. Industrial scale potters may buy the frit direct from manufacturers such as Endeka, Ferro or Esmalglas.

Happy Potting
The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.      Go now to sign up for my free newsletter.

Monday, 17 November 2014

Opacification of Glazes

Glaze opacifiers are materials which when added to a glaze change the level of transparency of the glaze. Glazes are often described as clear/transparent or opaque. In order to achieve color tone and hide the body color it is often necessary to add opacifier particles which stay discrete in the glaze after firing. Materials which have a significant difference in refractive index from the parent glaze produce the most effective opacification. To the right you can see the effect of adding an opacifier to a transparent glaze. It becomes whiter and milky (opacified ) in nature and flows less.

Below you can see the effect of adding opacifier to a transparent glaze containing different colored pigments


Top = transparent glaze            Bottom = opaque glaze


Typical glaze opacifiers are zircon (zirconium silicate), tin oxide, and titania (titanium dioxide). 

Zircon opacifier powder

Zircon

Zircon is the preferred opacifier for glaze due to the low cost, inertness and stability in glaze. To achieve full opacification, the opacifier content and the particle size are important factors. For addition of zircon to the glaze mill during grinding a particle size of 95% less than 3 microns and a content of 5-10% ensures sufficient opacifier is dispersed to create optimum opacity.  

However an exceptional level of opacity can be achieved by pre-melting the zircon into a glass (called frit) and then using the frit as part of the glaze recipe. These frits are transparent before firing but crystallize zircon during the glaze firing process to give a high level of opacity. 

Tin Oxide

Tin oxide has historically been used as a glaze opacifier but its high cost has limited its recent use to low temperature majolica or special effect glazes. At a level of 5% in a transparent glaze a high level of opacity can be achieved. Its lower solubility in glaze compared to zircon means that lower levels of tin oxide can be used to create the same level of opacity as zircon.. Tin gives a slightly blue white tinge and also has a lesser effect on the glaze appearance than zircon which increases glaze viscosity during dissolution.One major drawback of tin is it reactivity with some oxide pigments. For example, with chrome oxide a pink discoloration may result from the formation of a chrome-tin spinel crystal.
Titania reactive glazes


Titania

Titania is also a very costly opacifier and is used primarily where reactive special effect glazes are required. Like zircon it has a higher solubility than tin in the glaze and even at levels of 4%  tends to give a yellow tinge to the glaze after firing. It readily reacts with other materials in the glaze to create many unusual crystalline phases on cooling.This is ideal for special effect glazes but undesirable for standard opaque glazes.




Happy Potting
The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

Go now to sign up for my free newsletter.


Wednesday, 10 September 2014

The Wedgwood Collection and the changing face of UK pottery

Wedgwood Museum

Staffordshire pottery manufacture changed forever in the late 1980's and early 1990's. The power of the supermarket buyers, the draw of low cost manufacture overseas and the changing markets in Europe all contributed to the massive decline in pottery manufacture in the UK. So many big names like Royal Doulton, Spode, and Royal Worcester, to name but a few, now exist merely as brands with the factories closed and demolished. Bleak as it was for the thousands of workers who lost their jobs during that time, many of those workers still talk fondly of the friendships, skills and pride of working in the hundreds of potbanks which existed at that time in north Staffordshire and especially Stoke on Trent.

Staffordshire Oatcakes
It was not just  the potteries that demised but also the services to those factories, the engineers, the material suppliers and sandwich and oatcake shops that provided breakfast and lunch to the hungry workers at break times.For the lucky few they found work elsewhere and the humble Staffordshire Oatcake lives on mainly through online marketing and Social Media.

Many of those pottery workers now rely heavily on their work pensions created during their employment at companies like Wedgwood. However Wedgwood pension trust has a big pension deficit that is causing  problems.

Over the last few years the importance of this pension deficit has been highlighted as the struggle to keep the magnificent Wedgwood Collection at its home in the Wedgwood museum in Stoke on Trent continues.

For those who have not seen the collection I urge you to do so. You will not be disappointed. At over 80,000 historical pieces, ranging from early experiments for new bodies and glazes through to modern production as well as rare manuscripts and letters, pattern books, works of art and photographs, covering the 250-year Wedgwood history, it is one of the most unique industrial archives in the world. The importance of the collection is explained in more detail in this Wedgwood video link.

The collection is under threat because a legal loophole allowed a Wedgwood previously in liquidation to place pension debt in the Museum trust. The trustees have no option but to try to pay off this debt by selling some or all of the collection.

We cannot lose this collection from the UK or allow it to be dispersed around the world to private collectors. It is OUR heritage and I urge you to donate to help meet the £2.74m shortfall still required to keep the collection together in the UK.

Thank you for your support.

Monday, 4 August 2014

Pottery Glaze Safety

Introduction

The safety of pottery manufactured by credible large scale pottery manufacturers particularly in Europe and the USA is a priority. However many smaller scale studio potteries have a lesser understanding of the leaching of toxic components from glazes. Why? Because it is difficult to understand and even more difficult to test accurately without specialist equipment.

However everyone involved with pottery manufacture needs to understand about glaze toxicity in the 'sue everyone' culture of today.

Changes to legislation in the USA in the early 90's provided the springboard for worldwide change.
A case of lead poisoning in the USA caused by pottery which did not conform to any legislation worldwide (YES NON CONFORMING WORLDWIDE!) led to big changes. The result of the political intervention that followed was to effectively ban lead glazed ware (under proposition 65) in California and many other US states.Whether this was a justified scientific solution to the original problem is still debated by many in the industry. However the result is here to stay.

Lead in Glaze

Lets start with some facts about lead :
  • Lead in large quantities is known to be toxic. Indeed the ancient Egyptians used it for homicidal purposes.
     
  • Lead in small quantities is known to be harmful. It can seriously affect the learning ability of small children and cause other harmful effects in adults.
     
  • Lead glazed pottery leaches lead when subject to strong acids. However this may be as low as parts per billion; less than might be found in drinking water!
     
  • Lead leaching from the glaze surface is not directly related to the lead content of glaze. Many other factors such as firing have an equal or greater influence.
     
  • Lead glazed pottery has been in existence for thousands of years
Clearly the picture for lead glazes is not good. However the technical understanding gained over many years of industrial research and manufacture allowed them to be used with relative safety. Nonetheless the pottery industry has moved forward.

 Environmental as well as political pressures has ensured that millions of pounds have been spent to research and develop unleaded glazes and colours by major manufacturers. Unleaded glazes and colours have now become the norm and are available for most types of pottery body.

But unleaded glazes are not without issues. Lets consider why:

Unleaded Glaze

Unleaded glazes are mainly glass, sometimes with a crystalline phase and are considered as long-lasting and indestructible. This is not strictly true as all glass leaches to some extent when it comes into contact with acid foodstuffs even water. In the case of acids,  contact with the glaze surface over a period of time can cause a much greater leaching effect. The intermittent use of alkali dish-washing agents can also dull the glaze surface leaving it more prone to acid attack. Early unleaded glazes were particularly prone to this type of attack by strong dishwashing detergents.

Some unleaded glazes contain elements such as Barium, Zinc and Cadmium which are also considered toxic when released in large quantities from the surface of the glaze. Indeed some countries legislate for this by imposing limits on the release of cadmium and zinc elements and other heavy metals in their metal release legislation or guidelines. The safe handling of barium carbonate and cadmium compounds in the manufacture of glazes is also a concern for glaze producers.


Coloured Glazes

Copper green glaze
Coloured glazes have long been known to give greater problems than white glazes in terms of toxic metal release. This is due to the often overlooked fact that the choice of pigment greatly influences the ease at which acids can attack the glaze surface. For example it is well established that the combination of copper and lead in a glaze gives significantly greater lead leaching than from the lead glaze alone.

Other colouring elements such as cobalt, manganese can act in similar ways even in unleaded glaze. Therefore it is important to know the effect of pigments and intrinsic durability of each glaze you make.

Toxic Metal Testing

Toxic metal testing of pottery intended for food contact is carried out by specialist testing organisations who are accredited to carry out standard tests such as EU 2005/31/EC or ASTMC738 in the USA. The most recent European limits for this test is specified in 2005/31/EC. To reduce cost and avoid testing of clearly unacceptable glazes quick tests can be used to screen out poorly durable or lead glazes. Cutting a lemon in half and placing it overnight on the glaze surface is one such quick test. Another is using a quick lead test such as lead inspector.

However it is recommended that all glazes produced for food contact use are tested by certified test laboratories to ensure compliance.

Summary

To produce safe ceramics that comply with current legislation it is important to understand the formulation and firing of glazes. It is  more complex than most people think and testing by a recognised testing laboratory is the only true way to ensure compliance. The problems of safety of  lead glazes are well known, and these glazes are being phased out and replaced with unleaded equivalents. However unleaded and coloured glazes are not totally free of issues and reference to the health and safety glaze documents of suppliers is strongly recommended.



Happy Potting
The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

Go now to sign up for my free newsletter.

Thursday, 19 June 2014

Pottery Materials-Potters You've Never Had it So Good!

Sources of materials

Materials dug from the ground have been the source of pottery making for thousands of years. However early potters relied heavily on local materials, especially the clays and sands. This was certainly the case in my beloved Staffordshire, England where local marl clays formed the major source of clay for early industrial potters.

Borax granules
Today materials are shipped around the world to areas of high industrial scale ceramic manufacture. For example the high quality white china clays from the south of England are often shipped to Asia to make the finest bone china and porcelain bodies. Similarly borax and boron minerals from Turkey and the USA are used in Europe to make glazes and frits for tile, tableware and sanitary ware manufacture.

However it is not just the source of available materials but also the quality and consistency that has helped modern potters and potteries to improve their quality, designs and efficiency.

Materials for body making

Body materials
The main materials used to produce a clay body are silica or quartz, felspar, and clays. These materials come in various purities, particle sizes, and qualities. To produce the finest white bodies, high purity low iron materials are used and these materials are sourced locally where available. However most areas of pottery manufacture do not have the required purity or consistency required from all their local suppliers and are therefore forced to consider other supplies. It is the consistency in purity, and control of of material properties that has made a significant difference to large scale pottery manufacturers. They can automate processes and improve efficiency to reduce costs. Materials are now stardardised to fit processing routes. For example 30 mesh and 200 mesh materials are common in the UK. These improvements in materials for industrial scale customers also has a knock on benefit for craft, hobby and studio potters who can also purchase these same materials from distributors and use them with greater confidence.

Materials for glaze making

Frit granules
Like body the main materials used for glaze making are silica, china clay, and felspar. However frits (fused glass fragments) and other minerals such as limestone, dolomite, wollastonite, zircon are often added to control the fired properties of the glaze. Expensive technical grade chemicals such as zinc oxide, barium carbonate,and boric acid are also used in small quantities in the glaze or frit to meet the most demanding glaze requirements. However it is in the use of boron products where industrial scale and craft scale differ the most. Large scale glaze manufacturers predominantly use frits as a source of boron, where craft glaze makers use boron products such as Gerstley borate. The benefits of using frits for industrial scale glaze manufacture are far reaching. The recipe of the glaze is designed to be lead free and often fine tuned to give the widest firing range and technical performance. Control of glaze slip properties, glaze application, and firing properties are of paramount importance especially in automated plants which are not suited to the use of borates in glaze. Slip control of the glaze involves controlling both the solids content of the slurry, the viscosity (fluidity) and the drying time by means of chemical additives.

This differentiation in supply is a benefit to craft and hobby potters who can choose a from a wide range of frits or borates depending on their scale of operation or cost needs. .

Materials for decoration

A whole range of  methods are now available for decoration of pottery. When colour was introduced to pottery in early years it was in the form of naturally occurring coloured minerals usually high in iron. Consequently the colour range was restricted to brown- red to grey blue in colour depending on the firing condition. However with the development of pigments from the eighteenth century many more colours became available with Chinese blue colour on white glaze becoming very popular.                                      
range of onglaze decorating colours


Today a full range of stable mixable ceramic colours are available ranging from bright yellow to blue to red. Together with a wider range of decorating techniques the modern potter now has the opportunity to design truly unique ware that is also repeatable.

Range of ceramic stains


To put this into perspective an anology might be to compare Fords "you can have it in any colour as long as its black" to the current range of colours and effects used to paint modern vehicles.

In a similar way to body and glaze materials, decorating colours such as onglaze have become standardised and more consistent in both their application and firing. Colours have been developed without lead and are available in the best medium (liquid) to allow optimum application whether that is hand painting, machine banding or screen printing. Introduction of digital printing has allowed digital images to be translated from computer to ware using ink jet decals or even direct printing in a similar fashion to paper printing. For the hand made purists in pottery computer support to design has not always been welcomed but it has extended the range of design to new levels.

In the future design will be pushed even further by 3D printing which is in its infancy in the pottery industry.

Summary

Clearly material technology and control in the pottery industry has progressed almost unnoticed since the early eighteenth century pottery manufacture. However it has progressed and the development of more consistent, environmentally friendly colours together with a wider colour palette has been of great benefit to modern potters, large and small..  The advent of digital printing in recent years and the potential of 3D printing is set to revolutionise the industry further creating even more design possibilities.

Yes Potters it is true! You've never had it so good!
Comments welcomed?

Happy Potting
The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

Go now to sign up for my free newsletter.



Wednesday, 23 April 2014

How to Make Coloured Pottery

The Appeal of Colour

Copper Ruby glaze
The addition of colour to pottery adds so much to its appeal. Whilst white pottery can emphasise the distinctive form of a piece it is often colour which catches the eye! Colour is often associated with a mood or feeling and this varies between cultures and countries. For example red is a very emotive colour and can mean anything from love and romance to danger and fire! Choosing the right colour to make or decorate your pottery however is a matter of personal choice and allows for much creativity and freedom of expression.

Colour can be added to pottery in many ways including body colour, underglaze, inglaze, onglaze and also as a component of the glaze itself.


In this article I will try to review the two main types of ceramic pigment ( raw oxide and ceramic stains) available and how they can be used to produce highly decorative pottery.

In the second of my series on glaze I described how to make a white textured stoneware glaze. 
Simply adding inorganic colouring oxides such as Iron Oxide to such a glaze produces colour but not always the desired colour!  Carry on reading to find out why!

Raw Oxides in Coloured Glaze


Top = Oxides in transparent glaze
Bottom = Oxides in opaque glaze
Copper oxide crackle glaze
It is common for raw oxide pigments to be used used in pottery making. Many studio and craft potters prefer to use cobalt oxide, chrome oxide, Iron oxide and copper oxide as colouring pigments.These oxides give blue, green, yellow-brown, and green-blue respectively on firing in or under the glaze. Often the fired colour of the starting oxide is not the same as the original oxide colour e.g. cobalt oxide changes from black to blue on firing in a glaze. However mixing of these oxides in a glaze, gives variable but often aesthetically pleasing artistic effects on firing. It is for this reason, and the lower cost involved that many studio potters often use these materials.

Iron Oxide and cobalt oxide in glazes
In using oxides as pigments It is important to match the pigment type and content to the glaze to achieve the most consistent results. In the example above iron oxide gives a yellow colour when added to a glaze in small percentage (eg 1%) compared to a brown colour in high percentage (eg 15%). In combination iron oxide and cobalt oxide often give grey or a black glaze colour (see example right). The difference in the colour between an opaque and transparent glaze containing the same pigment content is also marked. In the example above the same pigment content is compared in a transparent glaze (top) and an opaque glaze (bottom). A stoneware textured glaze will produce colour tones similar to those of an opaque glaze.


Organic pigments such as those used in paper printing are clearly not suitable and will simply burn away during firing

Ceramic Stains in Coloured Glazes



Top = Ceramic stains in transparent glaze
Bottom = Ceramic stains in opaque glaze

In contrast to raw oxide pigments, ceramic stains have been specially formulated to create a wide range of colour tones in glaze. In their manufacture they have undergone a heat process and a fine grinding process so that they are highly temperature stable and capable of being mixed together to generate intermediate colour tones. This property is highly valued by large scale manufacturers who need consistency of colour tones. However this all comes at a cost compared to raw oxides.

Onglaze and Inglaze Decoration


Pantone Mugs showing onglaze colours
It is common for high quality whitewares such as bone china and porcelain to be decorated with special colours called onglaze (low temperature) or inglaze (high temperature). These colours use a mix of special fluxes and the ceramic stains identified above to create a wide range of intense, durable colours and bond them to the already fired glaze surface.. Whilst a few studio potters try to make their own, they are best supplied by specialist manufacturers to ensure they meet current legislation and perform satisfactorily in use. These colours are often supplied as powders or pre-dispersed in a liquid allowing them to be applied by hand painting or screen printing. This type of colour is very versatile and is often used to make precision decorative decals for water slide application onto pre-glazed pottery. A new development called Digital printing now allows these decals to be personalised and produced in small quantities making it a cost effective method of decoration for craft and hobby use.

Summary

Clearly there is more to making coloured pottery than is immediately obvious. For those who want consistent colour that can be mixed to give intermediate shades then use ceramic stains either as a glaze component or in decoration products such as inglaze or onglaze colours. For those who want unpredictable but aesthetically pleasing results use oxides as a glaze component or under the glaze. Whichever option you choose using colour creatively will only add to the appeal of your pottery.

Happy Potting
The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

Go now to sign up for my free newsletter.