Vince Pitelka, 2009

1. Draw rings should be used in all salt and soda firings to gauge surface development.
2. All wares should be wadded. Pay attention to the aesthetics of wadding, as discussed below.
3. Soda solution is caustic. Wear safety goggles or a face shield while mixing your soda solution and while charging the kiln. Never spray soda solution on the burners or plumbing.
4. Unless you want very heavy soda accumulation on your wares, fan the spray of soda solution across the left wall and floor of the firebox and never directly towards the bagwall. Move the wand constantly to keep from spraying in one area and causing the refractory to pop and scatter bits all over your wares.

For both salt and soda, wadding and shelf wash are made from the same recipe – 40 EPK, 10 OM-4, and 50 Alumina.

Unless the shelf-wash coating on the door bricks and door jamb is still fresh and clean after the last firing, re-coat the contact surfaces with shelf. Both contact surfaces on every kiln post should be coated with shelf wash. If the previous coat is still in good shape you do not need to add more. If the mating surface of a kiln post is rough or irregular, clean it up with a bench grinder or angle grinder. When chipping or grinding posts or shelves always wear appropriate safety goggles and if grinding wear a respirator.

To mix wadding, put all the dry materials in a bucket or bowl and add water a little at a time, mixing with your hand. When it reaches plastic clay consistency, wedge well to ensure thorough blending. Mix a generous amount, and once you are done wadding your pots, keep the leftover in a plastic bag. Even if it dries out, it is very easy to re-hydrate because it contains 50% non-plastic materials and absorbs water easily. Alumina is expensive, so don’t throw away extra wadding, and don’t use it to seal up cracks in the outside of the door. For sealing up the cracks and gaps in the bricked-up door, mix equal parts recycle slurry, sand, and fine sawdust, stiffened with dry crushed clay if necessary to get appropriate plastic clay consistency.

Always use draw rings in salt and soda firing. When making draw rings, use the same clay that is in the wares. Roll out coils about 3/8" in diameter, wrap them in a loop with a 1" hole, overlap the ends, and press down with your finger inside the hole so that the overlapped ends make a wide, stable base and the rings stand up vertically. Always bisque-fire the draw rings. If you are planning to do a lot of salt or soda firing, make a big supply and include them in your next bisque fire. Before you start loading the kiln, coat the bottom surface of your bisque-fired draw rings with kiln wash.

When bricking up the door, remember to leave several 2.5"-square ports, one low down and one fairly high up, and find or fashion an insulation-brick plug for each port. If you wish, you can have the top and bottom ports serve as spyholes for the cone packs, and leave an additional port in the middle to accommodate the draw rings. Whenever possible these ports should be placed just above the level of the nearest kiln shelves. If it is not convenient to do so, you may need to build up a platform with posts and broken pieces of kiln shelf to support the cone packs and/or draw rings.

If there are three ports, place the draw rings in two rows front-to-back so that they are easily accessible inside of the middle port, and have the front-most draw ring at least four or five inches from the door in order to ensure that it receives good soda deposition and thus serves as an accurate indicator. I generally place a total of eight or ten draw rings, with four or five in each row. I have never pulled that many draw rings, but you always want to have more than you need. If you are using the top and bottom ports for both your draw-rings and your cones, place one row of draw rings and one cone pack inside each port. You can also place either your cones or your draw rings inside the spy ports on the back side of the kiln.

Charge the kiln at least twice (see the section on "charging" below) before pulling the first draw ring. When pulling draw rings, wear gloves and protective tinted goggles or glasses, and use a long, thin steel rod with a very small hook bent on the end to snag a single draw ring and withdraw it from the kiln. A length of 1/8" steel gas-welding rod works well. Have a plastic bucket of water handy, and drop the draw ring into the water. Give it a few minutes to cool, and then dry it off completely and check the surface for the amount of soda glaze. Draw rings give no indication of fired clay color or flashing, because they have been cooled instantly. Clay that normally flashes beautifully in soda firing will be a dull, cold gray on a quick-cooled draw ring. Draw rings serve only as an indication of salt or soda deposition.

In salt and soda firing, we use brick pieces as furniture to guarantee stability. We use standard hardbrick soaps as 9" posts, hard brick straights cut into quarters as 4 ½" posts, and hardbrick soaps cut into quarters as 2 ½" posts. Use two parallel thin (3/8"-diameter), short rolls of wadding below and above each kiln post. Do not let them extend beyond the edges of the kiln post or the shelves.

As you wad your wares, remember to think about the aesthetics of wadding, since it leaves distinct, visible marks. Place your wads carefully, and use wads of an appropriate size for the piece being wadded. Use wads about 3/8" to ½" in diameter for all small and medium-sized vessels, and larger wads as needed. Vertical forms like mugs, cups, bottles, vases, covered-jars, pitchers, etc. generally only need three wads. Larger-diameter forms like sculptural pieces, flat slab pieces, plates, casseroles, and bowls must be supported by more wads in order to avoid warpage.

Very small lids (2" or smaller) can usually be supported by three wads, but all other lids need more wads in proportion to their size. Use small rolls (sometimes slightly flattened) rather than round balls when wadding lids. Make sure that the wads really do support the lid and prevent it from touching its seat at all. If a lid has an overhang that extends beyond the body of the pot, it generally does not need to be wadded at all in soda firing unless you are doing a fairly heavy soda effect.

Don’t pack the kiln too full. That’s not an issue at all in a normal oxidation or reduction firing, but in atmospheric vapor-glazing processes you need to allow room for the vapors to circulate. Leave at least ½" between wares. As long as you leave adequate spacing, don’t hesitate to place wares close to the arch, because that will just help to pull the vapors through the set. If you have small items and want heavy soda accumulation, wad and place them right on the bagwall. Do not place ware in the flue.

The only unfired clay in your kiln should be the cone packs, and they should be made properly from as little clay as possible and thoroughly perforated with holes to eliminate any chance of blowing up during the firing. If you are unsure of how to properly make cone packs, consult the instructions in Clay: A Studio Handbook.

Check to make sure that the burner valves are closed, and that the T-handle on the regulator has been unscrewed counter-clockwise until it is loose. Open the main gas shutoff valve on the LPG tank or the supply line. Screw in (clockwise) the regulator T-handle slowly until pressure reads on the gauge and set it at 1 PSI (pounds per square inch). Press and hold the button on one of the Baso valves, light the pilot, and continue to hold the button down for one minute before releasing. Repeat to light the pilots on the other burners. Once the pilots are lit, open the first burner valve slowly until the burner lights. Do the same with the other burners. Keep them on very low (just above the point where they go out) for one hour. During the first stage of the firing all the way up to cone 010, make sure there is no reduction, as per instructions in Clay: A Studio Handbook.

At the end of the first hour, open the burner valves all the way, but keep the regulator set on 1 PSI and leave it like that for one hour. For an overnight preheat, leave the regulator at 1 PSI with the burner valves open all the way and the damper set in far enough that there is significant back pressure at the upper spy hole but no smell of reduction.

This kiln is all hardbrick on the interior, which constitutes a major thermal mass requiring significant BTU input. After one hour at 1 PSI or after the overnight preheat, turn the pressure regulator up to 4 PSI for one hour, and then turn it up to 7 PSI. This kiln should reach body reduction temperature in five or six hours without an overnight preheat, and usually two or three hours after an overnight preheat. After body reduction, you may need to turn the pressure up as high as 10 PSI to maintain reasonable temperature climb, especially in the latter stages of a cone 10 firing.

This kiln reduces easily, and in salt and soda it is not necessary to do a formal body reduction. Just put the kiln in a light reduction when it reaches cone 010, and keep turning up the gas pressure and adjusting the damper to maintain light reduction and reasonable temperature climb through the duration of the firing. After an overnight preheat, this kiln (with a hardbrick interior and with three GACO MR-100 burners firing on high-pressure LPG with adjustable regulator at the kiln), fired as indicated above, will reach cone 6 in six to eight hours, and cone 10 in ten to twelve hours.

On the back end of the MR-100 venturi burner is an adjustable primary air shutter or spinner. Do not open this shutter any more than just over 1/8"and less than 3/16". Running on high pressure propane (anything measured in pounds is considered high pressure in kiln burners), the venturi effect in these burners is very strong, and the escape of gas through the orifice develops considerable vacuum, pulling in plenty of primary air to satisfy the combustion needs. As you turn the burner up the vacuum increases, which pulls in more primary air, so there is no reason to change the adjustment on these shutters. If you open the shutters any further, you are simply allowing the burner to pull in excess primary air, which creates an excessively noisy, oxidizing flame that is actually a cooler flame.

Wait until just before charging to mix your soda solution. Use very hot water, mix it in a plastic bucket using a drill impeller mixer, and screen it when you pour it into the sprayer just to make sure there are not any large undissolved soda crystals or other contaminants that might clog the sprayer nozzle. Use a standard garden sprayer with a three-gallon tank and a metal spray wand. Keep the soda solution hot throughout the entire charging process, and if it starts to cool, put the tank in a sink and run hot water over it for a while. The objective is to have a saturated soda solution, so that you are not charging any more water than necessary in order to inject the needed amount of soda. Soda does not dissolve well in cold water, so we use hot water. If the solution cools significantly in the sprayer, soda will start to crystallize out and the crystals can clog the nozzle. If you are charging in cold weather it is a wise practice to place the sprayer tank in a big tub of hot water next to the kiln, ensuring that the soda ash will stay in solution for the duration of the charging.

I like a fairly light soda effect on my work, and I mix three pounds of soda ash (sodium carbonate) with two gallons of hot water, and I use from a third to half of that. Mix more than you need, and then as you are charging, decide how much to charge based upon the draw rings. Always keep track the amount of soda ash and the amount of water you start with so that you will know exactly how much you use in each firing, and thus will be better able to repeat the results or adjust accordingly in subsequent firings. NOTE: Never fill the sprayer tank completely full. There must always be some air in the tank to compress when you pump it up.

If you specifically want asymmetrical unidirectional soda effects on your ware, open the damper an additional inch or two while charging, and accept the fact that it will take more soda to achieve the desired effect as gauged by the draw rings. If you want a more even effect, keep the damper closed enough to maintain a reduction atmosphere during charging. That holds the soda vapor in the kiln longer and circulates it more evenly. Avoid excessive reduction, however, because that will stall the kiln, and charging soda already stalls the kiln to some extent.

When charging the kiln, poke the nozzle into the charging port quickly and immediately squeeze the valve to start the spray. The nozzle will overheat very quickly if it is not being cooled by the soda solution passing through it. At the same time, try to minimize the amount of soda sprayed on the walls of the burner port or charging port. As soon as you insert the wand and start spraying, fan the sprayer back and forth in order to avoid spraying in just one place, which can cause the refractory to pop and scatter particles on your wares. Fan the soda spray across the firebox floor and up and down the inside wall of the kiln on the side where the burner ports are located. If you spray soda towards the bag wall you risk heavy deposition directly on the wares. Just think about where the soda is going as you spray. Obviously it vaporizes very quickly, but most intensively where the stream of soda solution hits a solid surface. Always avoid spraying any soda on the burners or plumbing.

Standard garden sprayers with metal wands come with a brass nozzle that is soldered to the tip of the sprayer wand. Frequently, the wand overheats, the solder melts, and the original nozzle falls off. When that happens, squeeze (with pliers) or hammer the end of the brass tube flat with a small nail inserted in the opening and then pull the nail out in order to create a spray hole about 1/16" diameter. It does not need to be precise, as long as it produces some sort of spray rather than just dribbling out of the end of the tube. If the original nozzle falls off while you are charging, just beat the very tip flat with anything handy and continue charging, and after you are done you can cut the tip back a bit and re-do it properly.

When you charge, pump the sprayer up enough that it delivers a good spray. Charge for fifteen seconds through the front charging port and then repeat through the rear charging port, and then wait about fifteen minutes before charging again. I usually charge twice before I pull the first draw ring, and again after two more charges. Towards the end I sometimes pull a ring after a single charge.

If you want a heavy accumulation of soda on a particular part of a piece, when loading the kiln position the piece directly inside a spyhole. When charging the kiln, spray soda directly onto the piece, but just do a few quick, short bursts, repeating with each charging as you think necessary. If you choose to spray any soda directly on your ware, be cautious the first few times until you learn the results through a series of firings.

When to charge depends on what kinds of effects you want to achieve. I soda fire to cone 6 and I try for a fairly light soda effect with a soft gloss and plenty of flashing on bare clay areas. As mentioned, I charge between one pound and one and a half pounds of soda. I dissolve three pounds of soda ash in two gallons of hot water and generally use from a third to a half of that. I soda fire to cone 6, so I start charging when cone 5 is down and generally finish as cone 6 is almost down or all the way down. If you want a heavier soda effect, you should start two cones before the firing cone. For a cone 10 firing, if you want a light effect, start charging when cone 9 is down. For a heavier effect, start charging when cone 8 is down. Remember that the kiln will slow down or stall when you are charging, so you may have to turn the burners up a little to keep climbing.

As soon as you are done charging, flush out the sprayer immediately and thoroughly. Unscrew the pump/lid very slowly to bleed off any leftover pressure. Rinse out the tank a few times, fill part-way with hot water, screw on the pump/cap, pump up some pressure, and spray a generous amount of hot water thorough the hose and wand. Unscrew the pump slowly to bleed off any pressure, and then remove the pump, pull the plunger all the way out of the pump housing, and rinse those parts. Rinse the outside of the tank, hose, and nozzle. When in doubt, rinse it some more. Soda is extremely corrosive and you can ruin your sprayer very quickly if you do not wash it thoroughly after each use. Remember that soda solution is very caustic. Wash your hands thoroughly, and if your skin is sensitive, rub your hands with a little vinegar or Epsom salts and water to completely neutralize the soda, and then wash with warm water and soap.

When your firing is done, shut the valve on the supply line to turn off the gas, but make sure you also close the burner valves and unscrew the T-handle on the regulator (counter-clockwise) until it is loose. Close the damper all the way and leave the kiln to cool. If your kiln has an IFB hotface, you may wish to slow down the cooling by placing pieces of kiln shelf or steel plate between the burner tips and the face of the kiln to block off the burner ports. If it has an all-hardbrick hotface, there is no advantage in blocking off the burner ports, because there is no way the kiln could cool too fast with the damper closed and so much thermal mass.

As mentioned, an all-hardbrick soda kiln has a great deal of thermal mass, and it will take at least twelve hours too cool. You can open the damper and start removing bricks from the top of the door as soon as a newspaper twist no longer ignites when inserted in the top spyhole. Take out a half dozen bricks to create an opening into the kiln across the top of the door, come back in an hour, and unbrick the door.

When chipping posts or shelves, always wear a face shield and/or safety goggles. When grinding posts or shelves also wear a respirator. Chip off any wadding from the bottom of the shelves and from both ends of the kiln posts. With a wide metal scraper, scrape all soda-glass residue from the bottom and edges of the shelves. Chip or grind away any glaze-runs and re-coat the upper surface of the shelves with kiln wash, making sure to not get any wash on the edges of the shelf, and let them dry. Chip off any wadding remaining on the floor of the kiln. Use an angle grinder if necessary to ensure a good flat surface in the spots where the posts go.