Glass-Like Outer Layer in Pottery and Ceramics
Having covered how to hand-build pottery and ceramics using pinching, coil, and slab handbuilding techniques, slip casting and plaster mouldings, and potter’s wheel and wheel-throwing methods, its now time to look at how to decorate pottery and ceramic figures using glazes. A glaze is that glass-like outer layer in pottery and ceramics that has melted to a glassy state in kiln fire but has not melted enough to runoff the wares to which it is applied on. When pottery is built by hand or by wheel, it is left to dry to leather stage. At leather stage, it can be decorated with drawings using a knife and a colored clay slip, and then is left to dry to bone dry stage. The bone dry pieces are then fired to bisque fire stage of about 850˚C to 950 degree Celsius. The bisque pieces are then left to cool and removed from kiln, and a glaze is applied to them before firing for a second time to melt the glaze to glassy state at temperatures between 1060˚C and 1310 degrees Celsius.
Why Apply Glaze To Pottery And Ceramics?
A glaze is applied on pottery and ceramics for the following reasons:
1. To color and decorate the ware for a higher aesthetic value
2. To strengthen the ware
3. To make the ware waterproof – unglazed earthenware will absorb water and bacteria can flourish in them.
Bridge between Glazes and Clays
A glaze and a clay body slip may just look the same but they are different. The bridge between a glaze and clays is the feldspars. A clay body with a lot of feldspar may become a glaze at certain temperatures.
Mix Your Own Glazes
You can buy ready-made glazes from your pottery supplies in your area. Ready-made glazes are specially made for a particular clay body and are to be fired to a particular temperature – you just choose the glazes based on the colors you want for a particular clay body to be fired at a particular temperature. But you can mix your own glazes – it is not difficult and it’s far much cheaper than buying ready-made glazes. This way you will have more control in your pottery and ceramic. The whole process is a continuous testing every time you do a firing as there is always that extra space in your kiln for test pieces. In as much as mixing your own glazes may look attractive, it is however important to note that some materials used for making glazes are toxic and poisonous and as such precaution should be taken especially on ventilation in your studio, and wearing of gloves and dust masks, when you mix the metal oxides to make glazes.
Three Components of a Glaze
A graze need three components, namely:
1. A binder – this is the alumina from clay and it’s the one that help graze hold on the clay during firing – use no less than 10%
2. Silica – this is the glass forming component – use no less than 10%
3. Fluxes – these are feldspar, and are necessary in lowering the melting temperatures of alumina and clay in the graze – use about 70% to 80%
4. Metal oxides for color – use about 1% to 10%. These may be as good as fluxes
Keep Good Records of What You Do
Metal oxides are mixed with glazes to give colors upon firing. The amount of oxide used, the temperature fired, the atmosphere of firing if reduction or oxidation, thickness of glaze, and presence of other metal oxides in the glaze all determine the final colors obtained. It’s all a question of trying, and to keep on testing for any good surprises in final results/colors. Keep good records of what you do and once you get a good color that is interesting to you, confirm it again by following your log of records - and there you have your own glaze recipe to use for many more firings to come.
Metal Oxides to Use in Glazes
1. Chrome Oxide for forest green color and pink-brown if zinc is present
2. Cobalt oxide or cobalt carbonate. Use 1% for medium blue, midnight blue
3. Red iron oxide and black iron oxide. Use 2 - 5% for amber, rusty red-brown and dark brown in oxidation atmosphere and celadon green and olive green in reduction atmosphere
4. Nickel oxide. Use up to 2% for blue gray and gray-green
5. Rutile or titanium dioxide. Use 5% for stark whites, use impure titanium dioxide for yellow orange and tan orange. Use rutiles for matt surfaces and elongated crystals
6. Vanadium Pent-Oxide Stains. Use 10% for yellow to mustard colors and grays in reduction
7. Zinc Oxide. Use 10% for white colors and for opacity.
8. Tin oxide. Use 5% for creamy whites and pink tinge. Use tin to opacify
9. Praseodymium Oxide. Use 10% for yellows and oranges
10. Manganese dioxide and manganese carbonate, use 2 - 10% for gold metallic colors and specks
11. Ilemenite, use 1 - 6 % for tans and browns
12. Copper Oxide. Use 2% to 6% for red metallic in reduction atmosphere, green-black, grass green in lead based glaze, and turquoise in alkaline glaze
13. Cadmium Sulfide and Cadmium Selenide. Use 10% for red, orange and yellow.
Example of Earthenware Glaze
Matt Stone White Glaze
Feldspar - 22.5%
Whiting - 9.0%
Lead Bisilicate - 45%
Kaolin - 14%
Titanium dioxide - 2.5%
Tin oxide - 7%
(Fire 1100˚C to 1140˚C)
Example of Porcelain Glaze
Transparent Matt Glaze
Feldspar - 44%
Barium Carbonate - 18%
Kaolin - 16%
Whiting - 8%
(Fire 1280˚C to 1305˚C)
Example of Stoneware Glaze
Khaki Reduction Glaze
Feldspar - 45%
Silica - 18%
Kaolin - 8%
Whiting - 11%
Talc - 5.5%
Iron oxide - 11%
Titanium dioxide - 1.5%
(Fire 1280˚C to 1305˚C)
The above are only three examples of glaze recipes. There are thousands of such recipes which you will find the enjoyment to find out by reading widely in ceramic and pottery books and magazines. You can start by going to this website for comprehensive list of glaze recipes which you can test and use.
Commercial Prepared Glazes
Start by using commercial prepared glazes from ceramic suppliers. Follow the instructions provided by the manufacturers by testing them before going on large scale firing. Making your own glaze may require you to start by using recipes by other people and testing by replacing a few ingredients and hopefully one day you will get one combination that works very well. Do not let it go as this will become one of your secret glaze recipes. Always make good notes and logs of what you do – ingredients, firing temperatures, firing atmospheres, clay bodies, times of firing, weather, etc.
Raw lead is poisonous. It is not a good idea to eat and drink from ceramics containers glazed with lead glazes. Lead is one of the most active fluxes and influence colors than any other known material such that it may be irreplaceable in decorating pottery and ceramic vessels. Fritting lead involve heating lead above 1093˚C and quenching quickly the molten lead in cold water and then grounding it to fine powder that is absolutely not poisonous. This therefore means that any lead salts that you use must be fritted. Avoid working with raw lead. If raw lead is used in pottery, as it happens in Mexico, the ware must be fired past 1093˚C. It’s very unlikely any ceramic supplier will agree to sell raw lead to you but always ask them.
You can work a glaze recipe from scratch without having to start by using other people’s recipes and using trial and error methods. There is a formula to do this but it’s a complicated and sophisticated method that you may regret ever having tried to learn it. Its advanced chemistry that will involve learning the chemical composition of raw materials and metal oxides, and their characteristics at each temperature – do not bother with such things which they can do at the university.
Faults in Glazes
1. Crazing – this is when glaze expands more than the clay body and does not ‘fit'. You will see tiny and large craze marks on the surface of your pottery items. Crazing can be intentional as a decoration and in such a case crazing is called ‘crackle’ Silica increases expansion in clay body, and silica decreases expansion in glazes. Control crazing by testing with addition of silica in clay body or in glaze until the problem is corrected.
2. Shivering – the opposite of crazing. The glaze has compressed more than the clay body resulting with glaze popping off and cracking. Too much silica in glaze or too little silica in clay body will cause shivering. Another cause is starting reduction too early, or doing too much reduction, in firing as carbon will displace iron and the glaze will move of the surface of the clay body. Reduction is simply the denial of oxygen to your ware during the firing.
3. Dunting – is a cooling crack through the glaze and clay which is ‘Y’ shaped most of the times. This is due to fast cooling or when your clay body is not able to withstand cooling and may happen regardless of how slow you cool (8 hours) your ware. Try to add some grog to your clay body. Dunting may have nothing to do with silica inversion points at 538˚C and 260˚C.
4. Crawling and Running Glaze – can be due to dirt, dust, fingerprints or hand lotions on bisque figures before glazing. It can also be due to application of too much glaze on the bisque or too high firing temperature. Crawling and Running Glaze can be a form of decoration which is done by adding fluffy chemicals such as magnesium carbonate to the glaze. Dirt and oil on bisque can be avoided by you, or can be cleaned using alcohol or moist sponge.
5. Pinhole and Blisters – blisters on glaze after firing due to: (a) air bubbles because there was dust on bisque before glaze application (b) bisque firing was done at low temperature (c) the glaze has too high quantity of certain glaze ingredient like boron that boils during firing. You control pinhole and blisters by reducing the quantity of the ingredient that boils or try adding zinc oxide. Its however important to note that some pinhole and blisters are intentional as decorative features in pottery and ceramics.
Edges Are Never Glazed
When you glaze your ware, remember that the edges where the wares sit on the shelves during firing are never glazed because the wares will stick on the shelves. Use wax on the outer bottom edge to prevent glaze from sticking there. Good luck in glazing your pottery and ceramic.
The next article is on How to Choose a Kiln
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Huntgoddess from Midwest U.S.A. on August 08, 2015:
I love this !!
I've never made my own glazes. I have just used glazes that my instructors made up, but I'm looking forward to making my own someday.
My main concern is food-safety, though.
ngureco (author) on November 13, 2012:
Homemade brick kilns are the best since you can maneuver them to achieve reduction and oxidation colors of the glazes. Homemade kilns can also be economical as you can build large ones that will contain hundreds of wares at firing unit costs that are low. To achieve a temperature of 1315 degree Celsius for stone-wares, you may need to line the interior of the kiln with ceramic fibre so that you may conserve energy. It is however important to note that the process of building an efficient kiln is difficult and will require lots of your time and persistence.
Good luck, Kariuki, in your kiln building.
Emmanuel Kariuki from Nairobi, Kenya on November 12, 2012:
This is valuable information. I plan to build a kiln for firing but wonder if a homemade brick kiln can have sufficient temperatures to glaze.
Uma07 on September 08, 2010:
I loved the chemistry part in the whole hub and the way you presented the art - full of passion.Thanks for the info. :)
Hello, hello, from London, UK on September 08, 2010:
Thank you for your well written hub. It was so interesting to read. It is a fascinating art.