Joe Molinaro - Eastern Kentucky University
Richard Burkett - San Diego State University

INTRODUCTION

In July of 1994 we were invited to Cuenca by the Paul Rivet Foundation to conduct a seminar which focused on kiln design. We visited more than twelve private studios where pottery is being produced for sale and saw the kilns that are presently being used. These kilns are mostly of a traditional design and fired with wood. Our goal was to view the kilns in use today, discuss problems related to their design and use, and make recommendations for ways in which the present kiln technology might be improved in order to facilitate a more successful way of working.

There are many very large factories in the area which produce ceramics, but the smaller ceramics studios are also currently producing a wide range of both functional and decorative ware of extremely high quality. When one looks at the sophistication of the ware, the use of such low-cost technology is remarkable. The potters, as a rule, are industrious and talented, leading to a small industry that thrives in the midst of large factories which have state-of-the-art technology. Using ÒprimitiveÓ kiln design and firing with wood, the ceramists in these small studios have been able to produce wares that reflect a higher level of thinking and seeing as they relate to the general field of clay work. Their only limitations are money and facilities (i.e., space, materials, etc.), both being areas in which they often have little control. Nonetheless, these artists are enduring and even rising above these limitations as they seek ways to constantly improve and move forward with the resources at hand. This document is meant to first pay tribute to their efforts and, through technical advice, offer support and encouragement. While the answers to many of the problems they face cannot be addressed in this document, it is our hope that we are able to assist them in some small way as they continue to work in clay.

KILNS OF CUENCA

While the traditional kilns used in the ceramic studios throughout the city of Cuenca are generally of a quite practical design that has been in use for centuries both in Ecuador and throughout the world, it is important to see how they function for those currently producing pottery for todayÕs market. Given the materials and resources available to the ceramists working in Cuenca, this document will take a look at the different types of kilns in use and how they might be improved for better, more cost efficient use, as well as improving the safety of such kilns.

To begin, let us look at the types of kilns currently being utilized by the small ceramic studios throughout the city. First, there is the traditional wood-fired kiln made of local bricks. These kilns are either round or square in design. They have a firebox on the underneath side and are usually stoked from one or two firemouths (*see fig. 1). These kilns are loaded from openings on their tops and then closed by placing old, broken pot shards on top of the wares as they sit inside the chamber. The wares are often stacked on top of each other, but a few potters use homemade clay shelves to stack the glazed ware. The kiln is then fired using wood fuel from the onset. The kiln is capable of reaching temperatures that melt the common types of raw lead glazes, although temperatures within this type of kiln are quite uneven. Often there are other types of wares fired in these ovens. These wares have no glaze and are therefore stacked piece-to-piece making shelves unnecessary. The pottery work of the Jatumpamba potters, located just outside of Cuenca, is a clear example of how these kilns are used without glazing the ware. While this effect of no glaze may be aesthetically pleasing, it is often not desired when wanting to produce items that are to be used for the serving and eating of food. Because of the low temperature needed for melting lead glazes and the difficulty of achieving higher temperatures with the traditional kilns, lead remains the main source of flux for the glazes. This presents critical problems when both producing and using pottery. Later in this document we shall examine further the use of lead glazes in the studio environment.

Another type of kiln used in some workshops is the commercial electric kiln (*see fig. 2). These kilns are easy to use and the potter is able to achieve even firings at the higher temperature ranges needed for many of the safer glazes and brighter colors. These kilns range in size and can be purchased through vendors both in Ecuador and abroad. Since both the cost of electricity and the kiln itself are often too high for the individual studio potter, these kilns are often not an option. Many of the studio potters working in Cuenca are family businesses with not much initial capital, therefore it is imperative to keep the operating costs low. Using the technology which is currently available with the traditional kilns and firing with wood and/or gas appears to be the best approach for the potters and is the focus of this document.

Lastly, there is the small gas kiln (bottled gas) which is relatively new to the area for the studio potter. Some of the studios now have gas burners which are homemade (*see fig. 3) and are mostly used for the production of raku ware. The burners are simple in design and easily constructed using local materials. Although these small gas kilns are often built for raku, they could be used for both bisque and/or glaze firing. The kilns are most often made of local brick, but some have begun exploring the use of ceramic fibers. Since fiber can be either difficult to obtain or too costly to purchase, local bricks and clays are often utilized for constructing kilns which use gas as their main source of fuel. Gas, which is more affordable than electricity, is a viable option for any studio potter wishing to reach higher temperatures, fire more evenly and keep costs to a minimum. A combination of better kiln design, different fuel options, and new glazes will make a safer workplace for the potter and his family while assuring the quality and success of the wares produced.

The following are recommendations which can be used by the potters who have traditional wood-fire kilns. These recommendations are meant to serve as a guideline for modifying the existing traditional kilns in order to help them reach higher temperatures and fire more evenly while still using available materials.

1. The kilns in the area are mostly square in shape and vary in size. While this is easier to build with bricks that come in either square or rectangular modules, it is less conducive to even heat distribution. Keeping in mind that flames are curvaceous, a round kiln (*see fig. 4) can facilitate having a more even heat distribution. Kilns which have a ware chamber that is about as tall as it is wide also fire more evenly.
2. Lining the inside walls of kilns built from common brick is important for insulation. This lining must not be so weak that it flakes off and possibly sticks to the glazed ware. A mixture of 50% plastic clay, 40% grog (10% coarse, 15% fine, 15% medium) and 10% Portland cement (sieved) is recommended for most uses. To this mixture add the minimum amount of water necessary to make a very stiff mortar which can be troweled or tamped onto the inside of the kiln.
3. The inside wall mixture (above, #2) should be applied to a damp, textured surface for best adhesion. Using a wet board, this mix can be firmly packed onto the walls.
4. The top of the kiln should be better insulated (*see fig. 5) in order to keep the heat inside the kiln chamber. This will allow for more even heat distribution and higher temperatures since the heat loss through the top will be reduced.
5. A layer of ANY type of surface clay (this clay can be of poor quality) mixed with sawdust and/or straw (50/50 clay/straw by volume) should be applied onto the broken pot shards which cover the ware on the top of the kiln. A layer of this mixture about 4-12 cm thick should be adequate, depending on the wall thickness of the chamber. This will add additional insulation on the top of the kiln and will both quicken the firing and/or assist with reaching higher and more even temperatures.
6. The clay mixture used on the top of the kiln for insulation can be used more than once with a small amount of new clay/sawdust added each time. The larger chunks of old clay/straw insulation may be laid over the shards and the spaces between filled with new and/or recycled insulating mixture. In addition, a layer of straw or newspaper between the broken pot shards covering the ware and the added clay insulation will help keep the layers separate. It will also help to keep the clay covering from crumbling onto the glazed ware below.
7. A more permanent solution to adding insulation to the top of the kiln might be to make a castable form (*see fig. 6-A) for the top of the kiln using a mixture (by volume) of 30% clay, 20% sawdust/wood chips/ground corn cobs/etc., 40% grog, and 10% Portland cement. The grog in this mix should be no larger than 5 or 6 mm in particle size, but should also be a complete range of particle sizes from fine to coarse.
8. The castable form can be made in sections allowing one or more parts to be removed while the remainder is left in place (*see fig. 6-B). Handles made of steel can be cast into each section making removal easier. Cast sections should be at least 12-15 cm thick. Castable can be mixed like concrete using the least amount of water possible to make the mixture stick together. Castable should NOT be mixed using too much water and should not be as liquid as concrete.
9. The castable top should be an arch form and packed well. This form should be allowed to dry for one to two weeks before the first firing. The initial firing should be extremely slow to allow the castable to dry during the firing without exploding. After the first firing, normal firing procedures can be used.
10. If the kiln is covered with clay or has a castable arch top, then an opening in the top of the arch (*see fig. 7) is necessary in order to have the smoke and heat exit the chamber. This will also assist with reaching higher temperatures since the firebox will have a greater air flow for combustion. This opening should be approximately the same diameter of the total area of the firemouth openings and the chimney (if added) works best if it tapers as it rises.
11. The height of the chimney, which can be made from either a clay chimney tile or metal stovepipe, might begin with a height of 45 cm, with the tapering of the chimney being slight. This is a recommendation only as it is advisable to experiment with the chimney and flue dimensions for best results. A higher chimney may assist with more air being pulled from the firemouth opening thus creating a hotter flame. CAUTION: it is not advisable to build a heavy brick chimney on top of a castable arch. Taller chimneys may be made from heavy sheet metal with the bulk of the chimneyÕs weight supported by fastening the metal chimney to another structure. Care should be taken to keep wooden roof structures away from the chimney to avoid the chance of setting buildings on fire.
12. Round (or square) kilns that are fired with wood and have only one firebox for fuel may have additional fireboxes added (*see fig. 8). Having a firebox on opposite sides of the kiln (not uncommon for the traditional kilns), or having three in the case of round kilns, will increase the chances of reaching higher temperatures and more even heat distribution. If more than one firebox is used on a kiln, they can be smaller in size than when only one is used. Experimentation with these dimensions is critical as the size of the kiln and firing chamber will dictate the size and amount of fireboxes needed for the type of temperature desired.
13. The addition of more air at the firebox opening (through the use of a fan or blower) will assist with reaching higher temperatures since the wood will burn hotter and faster. This, along with the addition of a chimney to pull more air, will allow higher temperatures to be easily obtained using the same materials used in kilns today.
14. Gas burners (*see fig. 9) may be introduced as a source of fuel when economically possible. Gas burners will allow for a cleaner atmosphere (less smoke) and possibly reach higher temperatures in less time, depending on the burners and size of the kiln.
15. Burners can use either the venturi system of air flow or have a forced air blower attached to the burner. Orifices for the gas burner must be drilled to match the gas type and pressure used. Normally, an orifice of 1 to 3 mm is used, but experimentation with this is needed depending on the variables listed.
16. Gas burners can be placed in the firebox and used with wood both during the slow preheat stage of firing as well as the end of the firing when higher temperatures are desired. However, it is best to provide separate openings for the burners.
17. Burner ports for gas burners can be effectively built into the kiln in addition to having fireboxes for wood if the kiln is designed for both types of fuels. This would allow both fuels to be used without difficulty and thus help keep costs to a minimum. Otherwise, existing kilns having large firemouths which are being converted to using gas will need to have the large openings bricked in to accommodate the burner (*see fig. 10). A space of approximately 5 Ð 10 cm is necessary around the burner in order to have adequate secondary air flow to complete combustion of the gas.
18. The stacking of the glazed ware inside the firing chamber should be done in a way which does NOT allow pieces to stick together. Kiln shelves are the best support for this.
19. Since kiln shelving can be too costly for some potters, a simple system of using saggars (*see fig. 11) might prove equally effective to achieve the desired results. Saggars can be easily made of local clays. They house the piece during firing, and also serve as a support system for the ware. An alternative to using saggars would be to use unglazed pottery wares (such as planters where the inside is left unglazed) as saggars, filling each one with glazed ware before setting another planter on top. This type of ware is commonly stacked lip to lip, then foot to foot, and so on until the height of the kiln is reached.
20. Lastly, ALL kilns should be built in areas that are properly ventilated and preferably NOT in or near living quarters. While this may be difficult for many studio potters given the restrictions of available space, proper ventilation through the roof of the kiln space is the least one should have to ensure the heat, smoke and gases are adequately removed from the immediate surroundings. An exhaust fan on the top of the roof in the room where the kiln resides will assist greatly with this removal of unwanted smoke and gas. A short tile chimney or sheet metal smokestack which draws the smoke and kiln fumes out above the roof is also recommended.

GLAZE PROBLEMS AFFECTING KILN DESIGN

There are several considerations to take into account when assessing the kiln situation within the ceramic studios of Cuenca. While most of the traditional kilns are of sound design and have been used throughout history with generally good success, one must look carefully at HOW they are used in the studio and WHAT are the desired results. If the potter wishes to move away from using lead glazes and to begin to fire the kilns higher, then the kilns currently being used must be modified unless more expensive kilns (i.e. electric) can be purchased. To begin this section we wish to examine more closely the problems related to the use of lead glazes so the solutions presented are more fully understood.

To begin, the lead for the glazes used in many of the small studios throughout Cuenca often comes from lead plates removed from old car batteries. The use of raw lead glazes (from any source) imposes several hazards on the public, the potter, and his family. Using raw lead oxide in glazes (especially when burned from metallic lead) causes lead to be released into the air during firing. These heavy lead fumes from the firing fall fairly quickly from the air and land on all surfaces near the kiln. Since the traditional kilns used by many of the local potters are located in or near the living space of the potter and his family, this problem becomes more paramount. Small children and the elderly are often the first to be affected by this because the fumes given off during firing are toxic. Young children, because of their low body weight and small size, are affected more by the presence of lead in their environment. One suspected effect of even the lowest levels of lead poisoning in children is lowered intelligence. More severe ingestion of lead, which is extremely toxic, can result in many health problems and even death. This alone becomes reason enough for the potters in Cuenca to both reconsider the use of lead glazes in the studio as well as consider other options for kiln design. The first step toward a more healthy approach to working with glazed ware in these kilns might be to stop using raw lead glazes, and in particular, the practice of stripping car batteries and burning them for lead oxide. If lead MUST be used, then fritted lead may be the best alternative. The expense of obtaining the necessary frits should easily balance when compared to the health risks for the potter, his family, and the users of the ware.

Other problems associated with the use of lead are related to the intended function of the ware and the possibility of lead release and poisoning of the user. It appears that most of the raw lead glazes used are both soft and likely to leach lead when used with food. Some potters noted that when they tried using fritted glazes their work stuck together, but with lead glazes the pieces that touched during the firing were easily separated. This is evidence of a soft, high lead glaze. Also, many studios appear to use copper as a main source of colorant to produce a variety of green glazes. While this color is attractive, the use of copper with ANY lead glaze, even fritted glazes, will make the glaze unsafe with food.

Not only are the health concerns surrounding lead glazes a serious consideration for the potter, his family, and the local market, they should also be taken into account if one is looking to the future where possible sales in a larger market (i.e. international) are to be considered. Most countries now have strict laws against the importation of such lead-releasing wares. Even many of the larger factories in Cuenca are feeling this pressure to move to all lead-free glazes for pieces which are to be used with food. Work produced for purely decorative purpose may not be hazardous for the consumer, but the problems associated with the production of such ware are nonetheless serious. The utilitarian work produced in the smaller studios is often of high quality and good design and could possibly be sold in both local and international markets. Thus the concerns surrounding the use of lead glazes are critical if the producer wishes to pursue these markets into the future. Since lead glazes are more forgiving of uneven firings, they remain easy for the potter to use given the materials and equipment they have available to them. Studios can remove lead glazes from use by having kilns which reach a higher temperature and fire more evenly. Many of the earlier suggestions in this document on kiln design will help the potter with ways to reach higher temperatures using existing kilns and therefore enable them to produce a lead-free product. The following are suggestions to help find ways to move away from using lead glazes in the studio.

1. Stop using ALL lead glazes, in particular on wares which are meant for the serving of food.
2. When lead glazes are used, they MUST be fired in a well ventilated area where people are not generally present. Keep ALL glazes in an area which is not accessible to others, especially small children.
3. Use fritted lead glazes whenever possible, if lead must be used.
4. Seek ways to find alternatives to using lead glazes. Potters could work more closely with the Paul Rivet Foundation as a central center for testing and research, so glazes can be found for the locally available clays without the local potters having to incur the expense and time of testing new glazes.
5. The Paul Rivet Foundation might best serve the needs of the local studio potters by providing a facility for glaze and kiln experimentation. Materials can be housed at the Foundation for use within workshops and seminars.
6. The Paul Rivet Foundation might also serve the needs of the local studio potter as a center for the purchase, analysis, testing and sale of needed materials. Buying in bulk to reduce costs and passing this savings onto the individual potters in Cuenca will facilitate the use of non-lead glazes and provide access to otherwise costly tools and materials. Contacts and cooperation with outside distributors (i.e., ceramic companies in the U.S.) would be a way that the Foundation can serve as a central location for the sale and distribution of such needed items for the potters.