Should I make my own Pottery Glazes?

Coloured glazes used by Tinkling toadstools

This question has caused a lot of debate among potters. It is a similar question to "should I make my own bread, cakes or pastry?" Clearly there is no right and wrong answer. People who bake their own bread and cakes do so for numerous reasons. Whether it is to reduce costs or make it fresh or make it taste better; all are valid reasons. However most do it because they like it and the self satisfaction it gives them to create something of value.

 Similarly, the potter will often claim that making glazes gives him the same type of satisfaction, is cheaper and gives him more scope for creativity! But lets look at the issues involved in making glazes in detail to see if his claims are true:-

 Costs

Glaze materials

There is little debate that buying the glaze materials to make the glaze can reduce costs significantly. Common materials such as clays, felspars, silica, frits and pigments are all readily available from a number of suppliers. However, often they are not in the ideal form to produce a glaze without additional processing. Most need at least a number of processing steps to make a satisfactory glaze. These steps include, preparing a recipe, grinding or mixing with water, sieving, magneting, testing, and controlling the glaze slip consistency using additives. The time and knowledge to correctly process and test the glaze are mostly overlooked when cost comparisons are made. For those with time to learn about glaze materials and the process this is not an issue. However for many hobbyists who are time poor it clearly does not make sense economically.They are better using reliable commercial glazes.

Creativity

Unique copper ruby glaze

Without doubt making your own glazes is a creative exercise. Again using the same cake making analogy as before, you have complete control of the make up of the glaze and how it will look after firing. Just like cake recipes there are thousands of recipes for glazes each giving its unique colour, texture and appearance depending on how its fired. This uniqueness is what many studio potters strive for.  By varying the pigments and make up of the glaze in theory you can innovate to your hearts desire! Therefore creativity can surely be improved by making your own glazes!

Technical restrictions

In practice, however,  there is considerable science behind making acceptable glazes:

• The glaze needs to technically match the body or the glaze may just peel off like old paint. 

• There is a limit to the range of compositions that can be made- for example some recipes will just never make glossy glazes.
  
• The firing cycle and atmosphere have as great an affect on the glaze appearance as the recipe.

The fired glaze surface of functional tableware needs to pass the requirements of legislation e.g. toxic metal release into foodstuffs.

So if you want to be creative with glazes you also need to have some technical knowledge. Potters without a scientific background may struggle to understand these technical parameters and hence making glazes could ultimately become more trouble than its worth.

Health and Safety

Unleaded Glaze

In recent years this issue has grown in importance for potters making their own glazes. Lead, for example, is being phased out in commercial glazes and its use and safety has been severely questioned especially in the USA. Other materials such as Barium compounds need to be used within strict guidelines for safe use. Even pigments such as cobalt and nickel oxide have been questioned over their safety. Clearly there is a lot to learn technically to comply with health and safety during the making of glazes. These issues are understood by commercial glaze manufacturers who can provide up to date data sheets on the glazes they manufacture and supply.

Equally important is an understanding of the legislation that applies to the fired product to meet the demands of the end market. Functional tableware has to meet much more legislation than ornamental ware in the various countries around the world.

Summary

In summary making your own glazes safely and successfully is much more difficult than at first appreciated. Technical and health and safety issues are not easy to overcome. However for a potter or hobbyist with a scientific understanding he can indeed be more creative and lower some material costs by making his own glazes. For the uninitiated it will be a long road of trial and error and frustrations. For those determined to try I recommend you do some reading and preparation online or in the wealth of books available before you begin. 

Good Luck and Happy Potting

The Potters Friend

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

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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.

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

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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.