•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
The Majolica Earthenware Process
Section: Glazes, Subsection: Low Fire
Understanding the advantages and disadvantages of low fire ware and the chemistry of physics of the glazes and bodies used is a key factor to exploiting this type of ceramics
Majolica (or tin glazed earthenware) is a very old pottery process that has long fascinated collectors. The term 'majolica' is Italian (from the port of Majorca), however, the technique was also perfected by the Spanish, French, Dutch, and Slovaks. It is characterized by white-glazed red earthen-ware clay decorated with over-glaze floral and other brush work designs.
Advances in ceramic materials, kilns, and stain technology make it possible or you to produce attractive and exciting majolica-like ware today that is technically superior to any of the classic ware of the past. Although the formidable challenges took hundreds of years to overcome, with a methodical approach now you can have majolica working in weeks! However, some aspects of earthenware are much more difficult to perfect than with stoneware. Glaze manufacturers in the ceramic hobby market have shielded users from this complexity, but at a cost.
Forgive me, however, for not holding true to the traditional materials. In this sense, I use the term 'majolica' loosely. For example, there is no compelling reason to use tin to opacify the glaze and lead flux is no longer needed or even legal. It is reasonable to fire a little higher and longer for better ware strength, and introduce new decoration materials and techniques. In fact, it is not even necessary to use a white glaze with overglaze stains if white and colored slips can be covered with a transparent glaze.
If you have studied or seen majolica ware, you may have dismissed it for a number of reasons. But are they valid? You might say:
- Most museum majolica ware is crazed.
This is true. However, we now have the ability to fine-tune glaze expansion to avoid this problem. In addition, by firing a little higher (i.e. Orton cone 03 instead of cone 06-010), we can develop a tighter and a better bond between glaze and body. These two factors really help prevent expansion crazing associated with water-absorption in earthenware.
- Most museum ware is chipped and cracked and does not appear strong.
True again. However, historical ware was fired at extremely low temperatures. Today, even the most inexpensive hobby kiln can easily fire to cone 03, a temperature high enough to produce much stronger and more functional ware. Most people are quite surprised to hear the tuning-fork like ring of strong well-made majolica ware.
- Higher temperatures must be better.
Maybe. However, consider that many people working with ironware bodies at cone 10 reduction are producing ware that ranges about 4000 psi in tensile strength. Likewise, working at middle-fire produces stoneware of around 5000 psi. Porcelains, on the other hand can be 10,000 psi. However, a big majority of these producers are using glaze that does not fit and that can easily cut the strength of the ware in half without any visible crazing or shivering. This means there are lots of potteries and manufacturers producing ware at 2000-5000 psi firing at stoneware temperatures. However, with a properly fitted glaze, it is possible to achieve 5000 psi at cone 03!
- Lower temperatures produce leachable glazes.
Yes and no. So do higher temperatures. It is possible to have unbalanced glazes, which leach heavy metals at any temperature. However, low fire glazes are normally made from fritted materials that the frit manufacturer has blended to be safe. Boron is the primary flux in these products and it is very safe. Food surfaces in majolica ware are usually just transparent or white zirconium opacified glazes, which present no danger. However, lots of manufacturers and potters are producing ware with large amounts of metallic surface decoration re-fired in the cone 018 range. Which is safer, cone 018 or the much higher 03?
- You are used to using a white firing clay.
True, majolica is made from red burning clay and it might seem a little strange to apply a white glaze over a red clay. However, red burning clays are very abundant and it is impossible to find white materials that mature at low temperatures. Red burning terra cotta clays often vitrify around cone 12. In addition, these clays are normally extremely fine and very plastic. They can be a pleasure to work with.
- It is inexpensive to fire. Firing a kiln to cone 04-03 is vastly easier and less expensive. For example, a typical hobby kiln will reach this range without ever putting the switches on high.
- Ware fired to cone 03 with a good-fitting glaze will ring like a tuning fork.
- Otherwise drab shapes or items take on a whole new life when color is applied. School children can produce amazingly attractive majolica ware.
- Stain-based colors need only be applied in a thin wash for vibrant results and most can be blended to produce intermediates.
- You can show off. If you have painting or brush-working skills, these will shine on majolica. This is because the heavily opacified glaze produces a stiff melt that is very stable, overglaze colors stay put and there is very little feathering on the edges of brush strokes. The fidelity is so good that a finger print on a brush stroke can be visible on the fired surface. Stain powders can be used like water color paints and you can blend colors in a similar manner.
- The majolica process is not fragile. It does not rely on special firing tricks or critical material mixes. It is stable and reproducible and tolerant of variations.
- Majolica clay is robust in the kiln. Majolica clay holds up well during firing making it possible to create very overhung and exaggerated shapes. Mugs do not go oval, handles do not sag, and clay foot-rings do not stick to the kiln shelf.
- Majolica can be bisque fired at cone 03 and glaze fired at a cooler 04. This means that the glaze does not have to pass bubbles from products of decomposition given off from the body. This means a more perfectly glazed surface than otherwise possible with other types of bisquit ware.
- Colors are brighter at lower temperatures and color can often make up for aesthetic deficiencies in other aspects of the ware.
A few things to be aware of:
- Glaze must be applied evenly and much thicker than for porcelain or stoneware. While the white glaze is opaque, the red color of the clay body will darken areas where the glaze is thin. It is also important to apply the glaze in an even thickness over the entire cross section. Do whatever it takes to accomplish this whether it be mixing 10 gallons so you can dip, adding a gum to the glaze, spraying it, waxing and glazing in two stages, heating the ware before glazing, deflocculating the glaze, sanding away the drips, etc.
- All edges must be rounded and thin lips should be avoided. Corners, angles, and sharp edges don't glaze well and the body tends to protrude. Likewise, thinner areas of glaze cover reveal the red color of the underlying clay. Likewise, if walls of ware are too thin, the clay will lack enough absorbency to build an adequate opaque layer of glaze.
- Low fire red clays often contain significant soluble salts that can deposit on the surface during drying, leaving an unsightly whitish scum and affecting the overlying glaze. Make sure your supplier is putting enough barium carbonate in the mix to precipitate these salts. Don't worry about barium leaching, it is under the glaze, it transforms to nontoxic barium sulphate during precipitation, and it participates in glass-building during the firing.
- Crazing and shivering are far more likely at lower temperatures because the less developed interface between clay and glaze is less able to absorb a bad fit. This is actually a blessing in disguise because it forces you to compensate. Crazing is more likely on a body made of clay materials only, shivering is possible on bodies containing talc. Thus an ill-fitted glaze can be improved by selecting the best body or adjusting the glaze recipe. A good approach is to use a base fritted glaze that employs two frits, a high expansion and low expansion one. Any shivering or crazing can be handled by simply increasing the proportion of the frit, which will improve the situation. Alternatively, add 5-15% silica if the ware crazes and 5-15% soda feldspar if it shivers. If possible, get the clay/glaze combination strength-tested to make sure the glaze fits, even if there is no visible problem.
One more thing to remember: Low fire body manufacturers do not normally maintain the thermal expansion of the bodies they make, they do not even measure it. Even if they did, the lack of maturity in these bodies makes it very difficult to identify the factors that vary to raise of lower the expansion. The bottom line: You need an adjustable glaze to adapt to different batches.
- Cover the ware as completely with glaze as possible. The best way to do this is make sure every item has a foot ring. Glaze the entire piece and only remove a small amount of glaze where the foot right contacts the shelf. This will seal the piece so that water is not absorbed.
- Watch out for the temperature at which clay-color transition occurs. Most terra cotta clays change from the classic flower pot red to dark brown over a very narrow range of temperatures. It is not wise to fire near this range since accidental over-firing will result in drastic color differences in the clay. This is not as serious for white glazed ware but it is for transparent glazes. Typical terra cotta clays transition around cone 01, thus cone 02 is not an ideal range. Keep in mind that if you use a transparent glaze the clay will be much darker under the glaze than it is without glaze. This is because the glaze melt matures the surface so it appears several cones darker.
- Manufacturers have limitations on how well they can maintain the consistency of terra cotta bodies. The materials used to make them are much more variable by nature than kaolins, feldspars, silica, etc. used in stonewares and porcelains. Also porosities are high and fired shrinkages low in these bodies for reasons other than maturity so manufacturers do not attempt to maintain these properties to much extent because of the difficulty even understanding the variations let alone doing anything about them. If a batch has 14% porosity one time and 10% the next no one gets upset about it. However they should maintain the drying shrinkage and performance and use formulations that enable this.
- White-dotting in colored areas of glaze is a common problem. It is minimized by bisquing higher than glaze and not firing too quickly.
- If the dried glaze layer is excessively dusty, add some gum to harden it.
- Get a good banding wheel for decorating your ware.
- Use frits if at all possible, not Gerstley Borate (a raw source of boron). The latter is unreliable and its manufacturer cannot stand behind the product's quality.
Getting Started With Your Own Majolica
- Get a terra cotta clay that fires to a nice red at cone 03 with good strength. Try firing it to cone 02, 1, 3, and
5 to verify that it is a true terra cotta. It should be near melting by cone 5 and transitioning to dark brown around cone 1. Also, make sure the material has a fine particle size, a least 100 mesh and finer. This is important to prevent pinholing that occurs in bodies having larger particles that either generate gases that have to bubble up through the glaze or act as vents through which large volumes of other gases are vented at fewer sites on the body surface.
- Get two or three borax frits which have a variety of thermal expansions from low to high. Choose ones that are balanced enough so that you can simply add kaolin to create a working glaze. Choose commonly available frits if possible. I use the following Ferro frits (expansions shown):
Ferro Frit 3124 - Expansion is 7.7
Ferro Frit 3195 - Expansion is 7.1
Ferro Frit 3134 - Expansion is 9.3
- I like these because each works well by itself and they have a range of expansions so I can blend them to fine-tune glaze fit. Another option is to choose one easily available borax frit, which is a complete glaze in itself (like Ferro 3195) and get a high and low 10% amounts. The materials database at https://insight-live.com include information on these.
- Mix a glaze using 85% of the intermediate frit and 15% kaolin (EPK is useful because of its excellent slurry-suspending and gelling properties). Make the slurry viscous enough so that the mass comes to a standstill in about 3 seconds after vigorous stirring is stopped. It should take about 50% water and 50% powder.
- Make large tiles of the clay (10 cm) and glaze-dip them both sides leaving a narrow strip along one edge to stand and fire them on. Cut slots in a piece of insulating firebrick if necessary to stand them on during firing. Apply the glaze quite thick and fire to cone 04, 03, 02 and note the glaze's gloss and tendency to run. If it is running too much for cone 03, add a little kaolin (5%) to raise the melting temperature (if the glaze is crazing add silica instead).
- Perform a boil-ice test by immersing a fired glazed tile in ice water for five minutes, then boiling water for five minutes. Repeat this three or four times and check for crazing or shivering on each. Adjust as follows:
- Glaze shivers: Add 5-10% soda feldspar or nepheline syenite or substitute some of the higher expansion frit
- Glaze crazes: Add 5-10% silica or substitute some of the lower expansion frit
Repeat the above process until the glaze fits well. You now have a reliable transparent glaze.
Add 12% zircon opacifier (i.e. Zircopax or Superpax) and mix the glaze and try it again on a tile. If the color is not white enough, increase the opacifier to 15% or more, but be careful since zircon increases glaze melt viscosity, and thus, raises the likelihood of crawling and pinholing.
When the desired opacity is achieved, stress test, as above, to reveal possible shivering. This needs to be done because zircon will lower the expansion considerably. If the problem appears, replace part of the frit content with a higher expansion frit and redo the test. Repeat the process until the fit is good. You now have a reliable white glaze.
- Obtain a selection of glaze stain powders recommended for cone 04 (in a pinch, you can also use copper carbonate for turquoise, cobalt for blue, and chrome for greens). Get a supply of foam cups, a teaspoon, and bag of the clear glaze developed above (some people use gerstley borate). We will call this the 'melter' (stain powders may not melt and can leave a dry surface on the glaze if they are not mixed with a melter). For each stain, take a foam cup and make an approximate 50:50 mix of the stain with the transparent melter. Add water and stir with your brush until it is a tempera paint consistency. Paint quick test-strokes on glazed tiles. Cross some strokes to see what double thicknesses look like. With a fine brush paint the identification number of each stain using the stain mix itself. Fire the tiles.
- Examine each color to determine if it is adequately melted. If not, increase the melter portion and test again. If any of the stains go on too dark or pasty, reduce the stain portion in favour of melter and retest. If any colors are not correct, it is possible that the chemistry of the melter conflicts with stain color development requirements.
- Paint some of each stain on a piece of bisque ware and apply the transparent glaze over top (it does not need to be thick). Fire to see if the color develops properly. If any colors are not correct, it is possible that the chemistry of the frit conflicts with stain requirements (i.e. chrome-tin stains require the absence of zinc and at least 10% calcia, thus the Ferro frit 3134 above will not develop the pink if used alone).
You now have the over and underglaze colors ready.
- The next step is to develop a slip that will match the body and function properly as an intermediate layer between body and glaze. It is common to employ a simple mixture of ball and china clays for this purpose. However, this can bring out crazing, which does not occur on unslipped areas. This is especially true, if the firing is sufficient to mature the red clay but not the slip. At earthenware temperatures, ball clay and china clay mixtures need the addition of silica or cristobalite to generate thermal expansion properties similar to those of red clay blends. A simple 24:24:24:24 mix of ball clay, china clay, borax frit, and silica with 4% added bentonite is a good starting point for ware fired around Orton cone 03. Although many slips do not employ frit, it is important at low temperatures to develop a glassy phase that will adhere it firmly to the clay surface during firing.
Apply the slip in a thick layer to a leather hard tile. Dry the tile. If the slip shrinks and cracks into flakes with curled up edges, remove the bentonite and retest. If it still does it, add kaolin at the expense of ball clay. If the slip flakes and falls off during drying, add ball clay at the expense of kaolin (10% to start).
- Once you have the slip drying together with the clay, fire a tile with a thick layer. If the slip shrinks more producing a network of cracks, add 4% kaolin and 1% bentonite and remove 5% ball clay. If it flakes off, add 5% ball clay and remove 4% kaolin and 1% bentonite. Repeat if necessary.
You now have a working slip base. The final step is to develop some colored slips.
- Mix 10-15% of a couple stains into the slip (i.e. a powder blue and pastel green) and paint samples of the white and colored stains onto leather-hard tiles. Dry and bisque, then apply the transparent glaze over some areas and leave others bare and fire to cone 03. If the colors are not dark enough or too dark, adjust and fire again.
Now you have everything ready and you know it works. You are in control. I suggest you glue all successful glaze and slip samples on boards and hang them up in plain view as inspiration and direction.
To begin production, I recommend you get a 50 lb. bag of frit (about $100) and mix 5-10 gallons of white and transparent glaze. It is always better to dip the ware completely to get an even layer. If you don't have a pair of dipping tongs, be sure to get some.
Armed with the above working materials, spend the rest of your life investigating the infinite number of combinations, colors, and decorative techniques possible. Enjoy.
Which is stronger: Cone 10R mug or cone 03 mug?
The mug on the left is high temperature Plainsman P700 (Grolleg porcelain). The other is Plainsman Zero3 fired at cone 03. Zero3 has a secret: Added frit which reduces the porosity of the terra cotta base (therefore increasing the density) dramatically. How? The frit melts easily at cone 03 and fills the interparticle space with glass, that glass bonds everything together securely as the piece cools. Although I do not have strength testing equipment right now, I would say that although the P700 mug likely has a harder surface, the Zero3 one is less brittle and more difficult to break.
Low fire nirvana: Use commercial underglazes but make your own clear over glaze
Decorate ware with the underglazes at the leather hard stage, dry and bisque fire it and then dip-glaze in a transparent that you make yourself (and thus control). These mugs are fired at cone 03. All have the same transparent glaze (G2931K), all were decorated with the same underglazes. Notice how bright the colors are compared to middle or high temperature. On the left is a porous talc/stoneware blend (Plainsman L212), rear is a fritted Zero3 stoneware and right is Zero3 fritted porcelain. When mixed properly you can dip ware in this glaze and it covers evenly, does not drip and dries enough to handle in seconds! Follow the Zero3 firing schedule and you will have ware of amazing quality.
G2931K Zero3 transparent glaze on Zero3 Fritware Porcelain
This is an all-fritted version of G2931F Zero3 transparent glaze. I formulated this glaze by calculating what mix of frits must be employed to supply the same chemistry of the G2931F recipe. The mug is made from the Zero3 porcelain body (fired at cone 03) with this glaze. This glaze fits both the porcelain and the Zero3 terra cotta stoneware. The clarity, gloss, fit and durability of this glaze are outstanding.
Three low fire bodies need three different clear glazes. Why?
Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.
Out Bound Links
In Bound Links
G1916M Cone 06-04 Base Glaze
This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.
Pottery fired to a low temperature employing a red-burning terra cotta clay covered with a soft opaque white glaze. Historically, majolica glazes (or tin glazed earthenware) were opacified using Tin Oxide, but now Zirconium silicate is most often used. Most majolica also has colored brushwork design...
By Tony Hansen
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