Spraying and Firing of Cast-Iron Enamel

Release time:

2026-10-22 10:23

Source:

China Enamel Industry Association

The enameling process for cast iron involves two primary application methods: dry powder spraying and wet (slurry) enamel spraying.

Dry powder application includes both electrostatic powder spraying and the traditional powder-sieving (dusting) technique. In the sieving process, a ground coat (base coat) is first applied via wet spraying and fired; the workpiece is then preheated to a temperature between 750°C and 830°C, after which the enamel powder is evenly dusted onto the surface before the piece is placed into a furnace for final firing. Dry powder application allows for the workpiece to be introduced into the furnace at high temperatures. The ground coat used is typically a "semi-raw" (partially fritted) formulation, which ensures the fired base layer remains gas-permeable. For colored enamel products, a white ground coat is most commonly used.

Wet (slurry) enamel application involves grinding enamel frits together with various additives in a ball mill. Proportions of glaze frit and additives:

Clay: 4–7%

Borax: 0.2–1%

Sodium nitrite: 0.3–1.2%

Sodium carbonate (soda ash): 0.3–0.9%

Bentonite: 0.5–1%

Quartz powder: 5–10%

Sodium aluminate: 0.3–1%

Sodium thiosulfate ("hypo"): 1–1.5%

Clay, bentonite, and sodium aluminate act as suspending agents in the glaze slip, preventing it from settling quickly and facilitating the spraying process. Sodium nitrite, sodium carbonate, and borax help maintain an alkaline environment in the glaze slip, which prevents the formation of rust spots that could otherwise compromise the quality of the fired product. Quartz powder expands the firing range and improves gas permeability. Sodium thiosulfate serves to adjust the electrolyte balance and maintain the alkalinity of the glaze slip. 

The firing temperature is generally kept below 790°C, with a firing duration of 10–15 minutes.

After spraying, the workpieces should be dried in a drying oven. The oven temperature should be set to 200°C, ensuring the workpiece temperature remains below approximately 160°C. It is important not to let the temperature get too high, as excessive heat can cause cracks to form on the enameled surface. Cover coat enamels are generally prepared by milling colored base frits with added pigments. The composition (relative to the frit) is as follows:

Clay: 2–4%

Sodium aluminate: 0.1–0.3%

Pigments: 2–4%

Bentonite: 0.5–1%

Cast-iron enameling is typically performed using a spray gun. Because cast-iron components are generally heavier than sheet-steel parts—and because the internal and external coatings often differ—the dipping method is not suitable.

Spray gun nozzles typically have a diameter of 1.2–1.5 mm, and the capacity of the attached paint cup (for handheld guns) is generally 500–750 ml.

Enamel slip specifications (specific gravity and fineness): The fineness of ball-milled enamel slip is measured using a 60–80 mesh sieve. When using a 60-mesh sieve, the residue should be approximately 0.5%; when using an 80-mesh sieve, the residue should be 1.5–2%.

Specific gravity of the enamel slip: 1.62–1.66 for ground coats and 1.65–1.73 for cover coats.

Previous Page

None

Chromium-based and Cadmium-based Pigments

Next Page

Previous Page

None

Next Page

Chromium-based and Cadmium-based Pigments