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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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.

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How to make a Pottery Glaze

The Glaze Recipe There are literally thousands of different types of glaze depending on the clay body used and the firing cycle and condition. A virtually unlimited number of colours, textures and designs are also possible. Applying more than one glaze to the piece just multiplies the possibilities. Each glaze needs its own specific recipe, making process and testing for optimum performance. However for ease of understanding I shall give the recipe and outline process for a transparent glossy glaze firing at 1050C. A typical glaze recipe includes: Borax Frit China Clay Transparent borax frit               90 parts China clay                                10 parts Water                                       50 parts Glaze milling (particle size reduction) In order to melt a glaze rapidly and produce a high gloss finish it is necessary to reduce the particle size to a powder and ensure that it is well dispersed in water. For this to be achieved the particles of the frit and clay are ground to an average size of 5 microns-less than 1/10th the size of a human hair. Overall a distribution in the range 60-70% less than 15 microns is targeted. Glaze Mill Ball mills are traditionally used to grind the materials to make the glaze. The ball mill is a metal cylinder lined with hard alumina or silica blocks. The mill is filled with graded ceramic balls with a max size of 5cm. to approx 50% apparent volume of the internal space. This allows efficient grinding of the glaze. The glaze materials occupy approx 1/3 the volume. The glaze is sampled after set time periods eg 12 to 20 hours until the required particle size distribution is achieved. Glaze Processing Once the average particle size is achieved the glaze slurry is poured out of the mill into a storage vessel via a coarse sieve. This removes large glaze particles and remnants of the grinding balls from the glaze. Typically a 60 mesh sieve is used. The glaze is then processed further by passing the slurry over thro a fine sieve typically less than 100 mesh and a strong magnet. The magnet can be rare earth magnets or electromagnets. This cleansing process removes more of the coarse residue and contaminants that can give faults after firing. Glaze Quality Controls A few simple tests are used to assess the quality and suitability of a glaze for use and include:- 1. Glaze appearance after firing- by glazing a tile of standard body and firing 2. Particle Size measurement and distribution 3. Solids content of slip = slip density 4. Slip fluidity = Slip viscosity Additional tests may be carried out depending on the end use. For example for hotelware the physical and chemical durability may be assessed. Rheology Control The glaze slip rheology is controlled according to the application method required eg dip, spray or brush etc. This is achieved by control of density, viscosity and thixotropy. A torsion viscometer and density can are often used to measure and control these key properties of the slip. Addition of binders and other chemical additives are usually necessary to achieve the optimum properties for specific application methods. Supply and Packaging; Glaze can be supplied in containers suitable for use from 500ml jars for brush on glaze, 5 to 50 litres for dip glaze and 5 to 1000 Litre tanks for spray glaze. For industrial use tankers containing 10000 litres may even be used. However for overseas or long distance supply the glaze is often supplied as a dried powder. In this case the user needs to mix up the glaze and control the slip characteristics themselves. 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.

What is a glaze?

A mature pottery student recently asked me the question 'What is a ceramic glaze?' I hesitated whilst contemplating how to answer what initially seemed a very simple question. In the end I gave him 2 answers , a non technical one and a technical one. The precise amount of information he required depended much on how he wanted to use the information.

Sometimes too much information only leads to confusion!

For example an evening class student wanting only to paint pottery and later have it glazed and fired by an established potter needs only basic information.

In contrast a student studying pottery making and ultimately wishing to make and fire his own glazes needs a much more detailed chemical approach.

The Answers

These are the answers I gave him:-

1) A pottery glaze is a glassy coating applied to the surface of a ceramic to create a smooth, impervious, and decorative finish.

2) A pottery glaze is a mixture of ceramic materials which may often include minerals such as felspar and silica, a glass like component such as frit, clay as a suspending agent, and pigment as colouration. Many other minerals or oxides may also be used. These materials are finely ground in water to produce a glaze slip. Other organic and inorganic additives may at this stage be added to aid application. The slip is then applied to the ceramic body and fired to a sufficiently high temperature for the components to physically and chemically react to form a smooth glassy coating that bonds strongly to the body. Normally a temperature in excess of 1000C is required.

Do you have any simple questions you want answering? Please let me know.

More information on pottery making can be found at my website The PottersFriend

Pottery Clinic

Glaze blister at high magnification

Post your questions about any pottery or ceramic matter.

All questions are welcome regardless of how simplistic they may appear !

My commitment is to respond in a timely fashion and to provide the best advice that I am able to based on my own perspective and the information provided.

I look forward to answering your questions