Date: October 6, 2008 

 (Finalized December 12, 2008) 

Subject: Site Visit – Kohler Company, Kohler, Wisconsin 

Clay Ceramics Manufacturing NESHAP 

EPA Contract No. EP-D-06-118; EPA Work Assignment No. 2-09 

RTI Project No. 0210426.002.009 

From: Richard Marinshaw 

To: Jeff Telander 

EPA/SPPD/MMG (D243-02) 

U. S. Environmental Protection Agency 

Research Triangle Park, N.C. 27711 

I. Purpose 

The purpose of this visit was to observe the clay ceramics manufacturing process at the 

Kohler Company (Kohler) facility and to collect information on sources of hazardous air 

pollutant (HAP) emissions and emission controls in support of the redevelopment of national 

emission standards for hazardous air pollutants (NESHAP) for the clay ceramics manufacturing 

industry. 

II. Place and Date 

Kohler Company. 

444 Highland Drive 

Kohler, Wisconsin 53044-1515 

August 11, 2008 

III. Attendees 

Kohler Company 

 

Mike Cassidy, Environment, Health, and Safety Specialist 

Telephone: (920) 457-4441, ext. 77263 

U. S. Environmental Protection Agency 

Jeff Telander 

Telephone: (919) 541-5427 
 

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RTI International 

Richard Marinshaw 

Telephone: (919) 316-3735 

IV. Discussion 

A. Background 

The Kohler Company (Kohler) facility in Kohler, Wisconsin, manufactures ceramic 

sanitaryware, such as toilets and sinks. Kohler also operates sanitaryware manufacturing plants 

in Brownwood, Texas, and Spartanburg, South Carolina. The Wisconsin facility includes six 

plants: the Vitreous Plant, Brass Plant, Foundry Complex, Enamel Shop, Engine Plant, and Die 

Cast Plant. The facility is classified as a major source of HAP emissions due to metal and 

organic HAP emissions from the Foundry Complex. Because the focus of the visit was the 

Vitreous Plant, where the ceramic sanitaryware is manufactured, the other plants are not 

discussed further in this report. 

The Vitreous Plant was constructed in the 1920’s. Whereas the Spartanburg and 

Brownwood facilities manufacture high-volume sanitaryware products, the Wisconsin facility 

produces low-volume specialty sanitaryware products. The plant has a total of five tunnel kilns 

and six shuttle kilns. The tunnel kilns were constructed with the original plant, one of the six 

shuttle kilns was constructed in 1960, while the other five shuttle kilns have been installed since 

2002. Three of the tunnel kilns are currently off-line due to market conditions. Of the two 

operating tunnel kilns, one is used for first fire, and the other is used for refire; refire kilns are 

used for firing products that had already been fired but had minor defects, which were 

subsequently repaired. All six shuttle kilns are in operation. One is used exclusively for refire, 

and the other five are used for first fire, or refire, as needed. Kohler staff noted that it is more 

efficient to use shuttle kilns than tunnel kilns when business is slow. 

B. Process 

The basic process steps used at the Wisconsin facility include raw material storage, mold 

production, glaze preparation, slip preparation, ware forming, drying/finishing, glazing, firing, 

and inspection. The following paragraphs describe the manufacturing process in more detail. 

1. Raw Material Storage 

The primary raw materials used at the plant include ball clay, kaolin, sand, feldspar, and 

flint. Kaolin and ball clay are received in the form of slurries and are stored in tanks until 

needed; the other raw materials are received dry. The Vitreous Plant uses several other materials 

in the process of manufacturing sanitaryware, including plaster of paris for mold making; various 

glaze materials, such as clays and stains; and other materials to facilitate the handling and 

transfer of slurries, such as, binders, anti-foaming agents, fungicides, and algaecides. 
 

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2. Mold Production 

The Vitreous Plant includes a mold-making shop, which produces the plaster production 

molds used to form the sanitaryware products. Mold-making is a multi-step process that begins 

by making a model of the sanitaryware product. Next, a block mold is cast from the model. The 

block mold then is used to make a case mold, which is used to make the plaster production molds 

used to cast sanitaryware. To make the plaster production molds, dry plaster and water are 

processed in a mixer to form a plaster slurry. The slurry is then poured into a case mold. The 

plaster sets up in 15 to 30 minutes. Next, the plaster production mold is removed from the case 

mold and dried in a natural gas-fired dryer for about 24 hours. The dryers operate at about 

120°F. After drying, the plaster of paris molds are ready for the sanitaryware casting step. Each 

case mold can be used to make between 50 and 100 castings of sanitaryware. 

3. Glaze Preparation 

Glaze making is a batch operation, which is specific to the color of the glazed product. 

Glaze materials are metered out into a weigh hopper, then dry mixed. After mixing, the material 

is milled in ball mills, where water is also added. The mixed glaze is then discharged into totes, 

which are transferred by forklift to the glaze spraying area of the plant. 

4. Slip Preparation 

In the slip-making process, clay slurry is pumped from storage tanks to a blunger, where 

dry materials and additives such as deflocculants and fungicides, are added and mixed. From the 

blunger, the slip is pumped into an underground mixer for aging. Waste slip is screened and 

recycled back to the process. 

5. Ware Forming 

Kohler uses two types of casting processes: pressure casting and slip casting. Pressure 

casting can be used for simple shapes, such as pedestals and toilet tanks. For more complex 

shapes, such as toilet bowls, slip casting is used. For either type of casting, the aged slip is 

pumped from the mixer to the casting shop. For slip casting, the slip is deposited into a plaster 

of paris mold. Capillary action draws the liquid out of the slip while building up a layer of solids 

over the inside surface of the mold, forming the body of the ware. After approximately 1 hour, 

excess slip is emptied out to allow the cast form to set up. Then, the mold is disassembled, and 

the ware is removed for air drying. At this point, the ware contains approximately 30 percent 

moisture. Pressure casting is a similar process except that, instead of relying on capillary action, 

hydrostatic pressure is applied to force the liquid out of the slip. 

After casting by either process, the waste slip basins are transported to open storage bins 

in the raw materials mixing area, where the waste slip is screened to remove large pieces and 

contaminants before it is recycled back into the process. 

6. Drying/Finishing 

After casting, the unfired ware, called “greenware”, is placed on racks for drying, or on 

carriers, which are racks attached to an overhead rail/trolley system for transport. Some ware is 

air-dried at room temperature for 3 to 4 days; some ware is air-dried and placed in a tunnel dryer 
 

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for further drying; and some ware is placed directly into tunnel dryers. Dryer operating 

temperatures are approximately 200°F. After drying, the ware, now called “dryware,” is finished 

to remove seams and burrs. Finishing is performed manually using a wet sponge and a palette 

(scraper). 

7. Glazing 

After drying, the green ware is transported to the glaze shop. The Wisconsin facility 

operates a total of 10 glaze spray booths. Two of the spray booths are exhausted to a wet 

scrubber; the other spray booths are all equipped with water curtains and baffles to remove large 

particulate matter (PM) from the over spray. The wet scrubber had been used to control 

emissions from an automated bowl-spraying machine and booth, which was recently converted 

to two spray booths. The water curtains drain into basins below the booths and then to a clarifier 

for pretreatment (solids removal). The baffles are manually scraped to reclaim some of the glaze 

material. Note that there are no defined process lines and a piece of ware can be glazed in any 

booth and fired in any kiln. 

The plant also has four spot spray booths and one respray booth used for touching up or 

re-glazing ware. The spot spray booths are exhausted to fabric filters, and the respray booth has 

a water curtain for controlling emissions. 

After glazing, the ware is loaded onto kiln cars for firing. 

8. Firing 

As noted above, Kohler uses both tunnel kilns and shuttle kilns to fire green sanitaryware. 

For the tunnel kilns, which operate continuously, the ware is loaded onto kiln cars that are 

pushed into the kiln one car at a time according to a set schedule. As the kiln cars are pushed 

through the kiln, they pass through various zones characterized by increasing temperature, up to 

the peak temperature of about 2200° to 2300°F in the firing zone. From that point, the 

temperature decreases as the kiln cars continue to be pushed through, and the ware is gradually 

cooled. 

Shuttle kilns are batch process units, which are used to fire a single kiln load of green 

ware. The kiln car is pushed into the kiln, which is then closed. The burners are ignited, and the 

temperatures are gradually ramped up to a peak firing temperature, then ramped down to 

complete the cycle. The typical kiln cycle for the shuttle kilns at Kohler is 20 to 22 hours from 

the time when the kiln is loaded and the burners are ignited, to the time when the fired ware has 

cooled enough to be removed (cold to cold). 

9. Inspection 

After firing, the product is unloaded from the kilns cars, inspected, finished with a 

grinder, water-tested, and packaged for shipment. If the fired ware has minor defects, the ware is 

machined, reglazed, and refired in a tunnel kiln or shuttle kiln at a temperature of approximately 

2100°F. If the defect cannot be repaired, the ware is discarded. 
 

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C. Emissions and Controls 

The Wisconsin facility uses a combination of wet scrubbers, fabric filters, and water 

curtains to control emissions from the process. Emissions from dry glaze batching operations are 

controlled with fabric filters, which are vented back into the building. A wet scrubber is used to 

control emissions from glaze mixing and milling. As noted above, emissions from glaze 

spraying booths are controlled with either water curtains or wet scrubber. Emissions from the 

ware grinder are controlled with a fabric filter. Emissions from the dryers and kilns are 

uncontrolled.