Pottery and Ceramics Glaze
There are as many approaches to raw glaze as there are potters and lately much has been written about the subject. This was not the case when I was a student, in fact the information I received was that the method was too difficult, needing special techniques; and due to the requirement that raw glazes have large quantities of clay in them, the glazes were very limited.
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All those beautiful Sung glazes I so admire are in fact once fired and
firing is historically a very recent innovation. Glazing
bisque ware is mainly a European invention used to overcome the difficulties of raw or once-fired tin glazes for majolica ware. Later it was used during the industrial revolution when the pottery process was divided into separate tasks requiring little or no skill.
Why raw glaze?
Raw glazed bowlRaw glazing gives me the spontaneity I was seeking due to the shorter time lapse between the pot's conception and its completion. The pots can be thrown, trimmed and glazed in one day, but I prefer to throw pots in the afternoon and in the morning I turn and finish them, glaze them and then start the cycle again. This cycle breaks the monotony of throwing for eight hours straight and gives my work day more variety. I use brushwork and glaze trailing for decoration, and I wait until a kiln load of pots is dry and then I do all the decorating in one sitting. I find decorating needs a real flow to develop and this is quite difficult in small sessions on a daily basis.
ClaysMost clays which accept slip with few problems will raw glaze at the leather hard stage. Clays that are difficult to raw glaze are difficult to throw and fire, so if you are having problems you should maybe use a new clay body. A base clay with good green strength and a moderate shrinkage rate to bone dry of no more than 8% is a good choice for raw glazing. The addition of a coarser grade of silica aids in the ability of the clay to withstand the shock of rewetting. The clay goes from hard leather hard, to where the clay crystals are in contact with each other, back to the plastic state and this causes swelling of the pot wall. Using 100 mesh silica imparts more than enough tooth to the body and doesn't notably change its firing characteristics. It still rings clearly when struck after firing and displays almost zero porosity. For a long time I added 65 mesh sand which was quite coarse compared to 60 mesh silica. It was also excellent in that it gave a nicely textured clay body but it needed a lot of grinding to make the base and galleries of the pots smooth after firing.
Red Iron Oxide1
My current clay fires gray and is very fine.
Fine sand helps it withstand the shock of rewetting.
A white porcelain body is more difficult to glaze in some shapes.
White stoneware for big pots and hand building.
If the glaze doesn't match the shrinkage of the body from the plastic state one of two things will happen. If there is too much clay in the glaze, the surface will show cracks like the surface of a clay pan and these usually lead to crawling when fired. The solution to this is to calcine a portion of the clay until the glaze fits. More usually the glaze will chip or flake off the pot, particularly at edges of rims and handles. This indicates that the glaze isn't plastic enough and needs more clay added to it, it is surprising how much clay you can add and not have any noticeable change in the glaze quality. Otherwise a more plastic clay or bentonite may be substituted for some of the clay present in the glaze batch.
For my technique the necessary amount of clay is the equivalent of bentonite has approximately six times the shrinkage of clay. 10% ball clay and 3% bentonite equals about 30% of ball clay. Bentonites are volcanic clays which have the ability to absorb huge quantities of water into their structure and hence shrink considerably on drying. Not all bentonites shrink six times as much as clay but most come somewhere near it. Western Australian bentonite, which is now hard to obtain, has only three times the shrinkage and is less useful in low clay glazes but because of the fluxes it has present, it would be an excellent material to use in a glaze with say 20% clay. At present Australia is supplying bentonites from the Hunter Valley of NSW, not from USA as was the case a while back. So far the Hunter Valley ones seem fine to me.
Bentonites are difficult to add to water. There are two approaches. If you are mixing a batch of glaze, mix the bentonite in the dry state with the other glaze materials, dispersing it evenly throughout, then mix it with water in the normal way and pass the mix through a 60 or 80 mesh sieve. If you want to mix it into an existing glaze slop, sprinkle it on top and leave it overnight, then stir the batch with a paint stirrer in an electric drill and run it through the sieve again before use.
With the exception of some special glazes like iron blues, I have been able to obtain most glaze qualities in the form of raw glazes.
Potash Feldspar 15
Ball Clay 28
This glaze is a very unusual glaze with 30% silica and is an excellent raw glaze. With 2% iron it is a good Celadon and with 10% iron a Teadust/Tenmoku. It is very craze resistant and durable. I originally developed it from the Calcium Alumina Silica Eutectic.
Glazes are made up with sufficient water to give a weight of between 2.84 lbs. and 3.06 lbs.
Not many articles talk about this aspect of raw glazing, but it is probably the most important one for a high success rate with the technique. Raw glazing demands consideration of the design and structure of the pot. You must consider how you are going to hold the pot when dipping it into the glaze bucket. Although big pots can be sprayed, it is not economical for production runs of pots. To facilitate dipping most of my pots are either undercut or have a substantial foot ring. A turned foot ring is ideal and one soon learns the type which integrates with your pot designs and gives enough grip to turn the pot upside down and dip into the glaze, remembering that the clay is usually still flexible at this stage and after dipping softens considerably and quickly, so speed is essential here.
In diagram 1 type A is manageable and B is a little difficult to grasp and C gives the best grip of all. Remember that the foot ring must be thick enough to withstand the pressure necessary to hold the pot upside down when dipping.
As a lot of my forms don't have a foot ring, I have found that a bevel as in diagram 2 works well, allowing the pot to be dipped to within 1/8 inch of the bevel. Remember that the diameter to be held must fit comfortably into the hand. Any pot over 4 inches on its base is best held with two hands. This may necessitate a different design especially as these pots are likely to be much heavier and therefore harder to hold. Wide flat forms are held between two hands on their rims and the whole dipped in one operation. The rim is then touched up with a brush when the glaze has dried. From a structural point of view pots for raw glazing need to be thrown a little thicker than normal. It is important to glaze in such a way that as little stress as possible is caused to the pot. This normally means dipping quickly. Dipping one half and then the other is a sure way to split the pot, particularly if it is fairly wide and flat.
Sharp corners, especially on the inside of pots, should be avoided. Casseroles are rounded inside even though from the outside they appear sharply angled. Pots as in diagram 3(a) will almost certainly crack at the change of direction. They should be made as in diagram 3(b).
I can't overemphasize the importance of even cross sections. Nearly all my failures are a result of the cross sections of the pots being uneven. It may be that I turned a base thinner than the walls or a section of the wall was thinner than that above it. If these pots survive the initial rewetting they are sure to crack on drying. Sometimes they can be repaired by brushing some thick glaze into the cracks but this is not always a solution. The best solution is to have pots with even sections
MethodWhen I make mugs, cream jugs and similar pots, I apply my handles after the pots have been turned and while they are still soft leather hard. Then I allow the handles and the pots to equalize in stiffness before glazing. One sure way to make handles collapse is to glaze them before this equalization has occurred. Once the handles and the pots show the barest sign of whiteness on their edges, I glaze them and I usually don't bother waxing feet or lid settings. I pour glaze into the inside and then pour it out quickly while turning the pot to completely cover the interior. Then I immediately dip the pot down to my fingertips, give a little twist of the wrist, pull the pot out, shake downwards sharply, turn it over and place it back on the pot board. If the pot has a large rim, I run my finger around the rim to remove most of the glaze and then leave the pots to first soften and then dry. The next day when the pots are again leather hard, I sponge any runs off and sponge any rims and lids clean. I have used this technique on articles from 18 inch jugs to 1.2 inch tea strainers and I've never lost a handle or had a pot split, providing it complied with the criteria of the design section.
TeapotsThe greatest test of any raw glazer is teapots. After assembling the pots I put them into the damp cupboard to equalize the moisture content of the parts and their joins. Usually with the next day's glazing I pull them out and allow them a couple of hours in the open air and glaze them after everything else. I again pour glaze inside, about a cupful, and pour it out, starting on one side of the spout and turning until the last bit of glaze pours from the spout. I then blow down the spout to clear the strainer holes and immediately dip the outside, giving a twist as I go. When I drain out the pot I usually put one finger on the rim for extra support while I shake off the excess glaze. Again once the pot is upright on the pot board I blow down the spout as hard as I can to ensure the strainer holes are clear. I then wipe the rim with my finger to clear it of glaze and touch up when the pot has returned to the leather hard state.
Bowls, Plates and LidsUsually if a bowl can be held by the foot ring I use the same method as mugs, except that I like to glaze the underneath of a foot ring. To do this I use a slip trailer with a large opening filled with glaze. I squeeze the glaze out into the area contained by the foot ring till it is covered and then simply suck up the excess. The inside and outside are then glazed as above.
When glazing objects that have nothing to hold on to, like lids or plates, I first dip my hands in glaze, then pick up the piece with both hands, immerse the lot in the glaze, letting go completely for a fraction of a second, ensuring that any area the fingers touch is covered in glaze, and catch it. I then remove it, give it a sharp shake to drain off the excess glaze and place it on a pot board for sponging the next day My pot boards are wooden and relatively non porous and raw glaze is easy to remove from them with a sponge.
Any pots I think I will have difficulty with, I wax with either a paraffin wax, kerosene mixture or liquid wax emulsion. These pots usually need touching up, but wait until the glaze is dry and use a mop type brush to carefully build up the glaze. Raw glazes handle and fettle very easily due to the clay imparting excellent strength to the glaze surface. Another advantage of raw glaze is that it allows me to pick up the pot without damaging its surface when I am decorating with brushwork
Firing and Firing CyclesUsually firing raw glazed pots is the same as firing a normal bisque cycle then adding a glaze cycle to the end of it. Some clays demand very gentle treatment during the water of crystallization release phase and quartz inversion periods of the firing. It is my experience that raw glaze will also present few firing problems and lately I've been once firing in my fiber kiln in 13 hours. In the wood kiln the cycle is between 16 and 20 hours. A 22 to 24 hour firing cycle is very conservative, but it offers a starting point for any one starting out. It is accomplished by leaving one burner on overnight to get from 32 degrees F to 694 degrees F. A 16 hour cycle is a good average firing with average temperature rise of about 122 degrees per hour. It should go without saying that before packing the kiln, the pots should be thoroughly dry. It is my practice to pack the kiln with all the pots touching each other as the completion of shrinkage that occurs ensures good gaps between the pots by the time the glazes begin to melt. Large pots and platters may need some powdered alumina under them to allow for the large shrinkage and ensure that they don't get caught on the shelf and crack. If it isn't your normal practice to pull test rings to determine that oxidation is complete before you start reduction, I suggest you do, because you will not have a chance to rectify lack of oxidation in a later firing and bloating may occur.
Glazing a piece of pottery that hasn't been bisque fired first is very tricky, and it will take some practice to get good at it. Believe me, you will have a few oop's incidents, but eventually you will get good at it by following the instructions in this article. The plus side to this is the short period of time it takes to get a finished piece of pottery.
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