Bolivia: vanguardia sindical en “el país más olvidado y dependiente

The recommended kiln for filter production is the Mani kiln. It is a downdraft kiln (Section 6.2) designed specifically for filter production because existing smaller updraft kilns were found to be inefficient and did not have the capacity to fire a large number of filters. This larger

capacity, more fuel-efficient kiln was designed using the following criteria: 1) simple design; 2) inexpensive construction; 3) fuel efficiency; 4) capability of using alternative fuels; and, 5) buildable using local labor and materials. Mani Flat Top or Mani Arch Top kilns are currently being used by 16 factories. Other designs may be used provided they fire evenly—with less than one cone differential in heat distribution (Annex F)—and produce consistent results. Understanding the Mani kiln and its design should aid in successful firings. Listed below are some of the features that have been designed into the Mani kiln which potters and kiln masters should be aware of when firing the Mani kiln. Many of these features are applicable to other kiln designs.

When firing with propane, the Yemen factory recommends using several smaller canisters

which helps maintain pressure.

6.4.1

Flat Top Design

The flat top design, inspired by the Minnesota Flat Top, is actually not flat but rather domed, with the center being at least one inch higher than the sides. The domed “flat-top” requires 23 cm long (9 in) angle iron corner braces with tie rods and can be built in less than half a day. However, because there is a

structural limit to the distance that can be spanned with a domed roof, if a kiln interior larger than 1m3 is required, an arched-top Mani kiln design should be used. Mani arch kilns can be designed to hold up to 200 filters; however, building an arched roof requires an elaborate form with angle iron on all four corners and tie rods to secure the walls from being pushed outward by the weight of the arch.

6.4.2 Kiln Walls

Mani kilns are built out of common construction grade bricks that

have been fired above 1,000°C (1,832°F), which is higher than the 850-900°C (1,560°-

1,650°F) firing temperature of filters. Since kiln walls absorb a lot of heat before it is transferred to the wares, and Mani kilns are primarily used in hot climates, the kiln wall is just one brick 15 cm (6 in) thick. This thickness makes heating the kiln and the wares inside easier and faster.

6.4.3 Firebox

The firebox, which consists of the grate area and the ashpit, is the area where fuel and air are combined for combustion. The grate was incorporated into the Mani kiln after initial firings found that an accumulation of ash and embers in the firebox did not allow sufficient air to enter the firebox for combustion. By using metal rebar to divide the firebox horizontally, wood can be placed on the grate and the space below serves as an ash pit. With this configuration, ash can be removed and the embers spread without disturbing the burning wood. The rebar were designed to protrude 10-15 cm (4-6 in) on either side of each

firebox so they could be removed and replaced easily. Over time, the rebar distorts from the heat and can be difficult to remove if it is not checked regularly and replaced when showing signs of wear. The third generation Mani kiln (3GMK) uses bricks for the grate that can be replaced easily if they break. For this reason, the floor and grate bricks are never mortared in place. The ash pit should be as big, or bigger, than the area above the grate.

6.4.4 Bag Wall

Bag walls divide the combustion space from the chamber where filters are stacked and serve to direct heat upwards and deflect flame from direct contact with the wares. The original Mani kilns did not have bag walls. The 3GMKs have adjustable, perforated, stepped back, pyramid design bag walls in the rear corners of the kiln, in front of both fire ports. They are about one- third the height of the interior of the kiln. This design allows the flame entering the kiln to be dispersed both to the bottom of the kiln and upward for more even heat distribution.

Figure 6-5: Flat Top Mani Kiln

(Hernandez 2006)

In Kenya they found that temperature resistant steel, purchased or found

in a scrap yard, make long lasting grates that do not warp easily under

6.4.5 Temperature Controls

The Mani kiln has three temperature controls. During the preheating and water smoking phase, the

manually operated damper in the chimney should be pulled out to the full open position to allow the moisture to escape. During the firing process, it can be pushed in or pulled out to control the draft and pressure within the kiln.

The loose floor bricks over the firebox and the flue channels that run under the floor of the kiln chamber can be separated slightly to allow more heat through the kiln floor and to reduce the chance of cold spots. The bricks over the firebox, from about the middle of the kiln to the flame port, should be separated by about 0.6-1.25 cm (1/4-1/2 inch). The bricks closer to the flue opening should not be separated, as this will cause the heat to be sucked directly into the flue opening.

In addition, the bricks over the firebox and chimney flue can be put in an alternating rowlock (on their sides) and flat position (Figure 6-6). This allows heat from the kiln to get under and into filters stacked upside down.