What Are Ceramic Cones Used For?

Ceramic cones are small, engineered ceramic devices used chiefly in two separate fields. One type, called pyrometric cones, measures the combined effect of kiln temperature and heating time so potters and ceramic technologists know when wares have reached the correct maturity. The other type, commonly referred to in industry as ceramic foam filters or ceramic filter cones, is used in metal casting to remove nonmetallic inclusions and control metal flow, improving the quality and reliability of castings. Both rely on carefully selected ceramic compositions and controlled manufacturing to perform under demanding thermal conditions.

Pyrometric cones: purpose, operation and practical tips

What is a pyrometric cone?

A pyrometric cone is a small, tapered, ceramic device manufactured to deform at a defined amount of heatwork. Heatwork is the cumulative effect of temperature and time during a kiln firing. When the cone softens and bends at its rated heatwork the user has a simple and reliable indication that the kiln atmosphere has reached the intended firing maturity for the ware or glaze. Pyrometric cones are widely used by potters, ceramic engineers and laboratories.

How pyrometric cones work

Cones are formulated so that at a given combination of temperature and heating rate the ceramic material will begin to soften and then bend. Faster heating generally requires a slightly higher peak temperature to achieve the same heatwork; slower heating requires a lower peak. That is why cones are a measure of heatwork rather than temperature alone. Cones are often used as witness devices in the kiln or in the kiln sitter mechanism for automatic shutoff.

Common cone ranges and meaning

Pyrometric cones are indexed in systems such as the Orton cone scale with numbers like 04, 06, 6, 10 and so on. Lower numbers indicate lower heatwork and lower firing “temperature equivalents” but remember they are heatwork indicators. Typical practical uses:

• low-fire glazes and bodies: cones 06 to 04

• mid-fire: cones 5 to 6

• high-fire stoneware and porcelain: cones 8 to 12 or higher.

Placement, reading and good practice

• Put cones on each shelf or a witness shelf toward the kiln center.

• Use a set of three cones when possible: guide cone, firing cone and guard cone to detect under or over firing.

• Read cones at a consistent position and lighting condition. Visual reading remains the most common method, though thermocouples and digital controllers have their place.

• Keep cones in original packaging until use and avoid contaminating them with glaze splashes.

Limitations and common mistakes

• Cones tell you about heatwork, not an exact furnace air temperature at any instant.

• Misplaced or poorly shielded cones give misleading results.

• Relying solely on cone appearance without understanding firing schedules can lead to inconsistent results.

Ceramic cones used as foundry filtration (ceramic foam filters)

What is a ceramic foam filter?

In foundry practice the phrase ceramic cone or ceramic filter most commonly refers to a porous ceramic element used to remove inclusions from molten metal as it flows into a mold. These filters are usually reticulated porous ceramics produced in defined pore sizes (often expressed as pores per inch, PPI) and tailored by chemistry to withstand the melt temperature and chemical environment. They are essential for high-quality aluminum, magnesium, copper and even steel castings.

Why foundries use ceramic filters

Ceramic filters remove nonmetallic inclusions, reduce turbulence, and improve metal flow profile. Benefits include:

• fewer internal defects and reduced hot tearing

• improved surface finish and dimensional accuracy

• better mechanical properties due to fewer oxide inclusions

• lower scrap rates and less costly rework.

Typical materials and their application envelopes

• Alumina (Al2O3): common for aluminum alloys; good chemical resistance up to roughly 1200°C depending on formulation.

• Silicon carbide (SiC): tougher thermal shock resistance; used where higher thermal and erosion resistance is needed.

• Zirconia (ZrO2): for very high temperature or reactive melts such as some steels and nickel alloys; highest cost, highest performance.

Pore sizes and flow characteristics

Filters are produced in PPI values commonly ranging from 10 PPI to 40 PPI for casting applications. Lower PPI (coarser pores) yields higher flow but less particle capture. Higher PPI (finer pores) captures smaller inclusions but increases flow resistance. Selecting the right PPI is a balance between filtration performance and acceptable pressure drop.

How filters are used in practice

• Inserted in filter boxes or filter sleeves in the gating system.

• Often sealed with gaskets or refractory mortars to prevent metal bypass.

• Can be used singly or in staged filtration banks for high purity needs.

Materials, manufacturing and specifications

Pyrometric cones — materials and manufacturing notes

Cones are manufactured from clay bodies specifically blended and heat-treated to produce consistent melting behavior. Strict quality control in formulation and firing ensures reproducible softening points. Cones can be made for general studio use or with tighter tolerances for laboratory control. Recommended handling and storage practices are simple: keep dry, do not chip the tips and store by cone number so you use the right cone for the intended firing.

Foundry ceramic filters — production and quality markers

Ceramic foam filters are usually produced by coating a sacrificial polymer foam with a ceramic slurry, drying, and firing. After firing the polymer burns away leaving an open-cell ceramic skeleton. Key quality parameters:

• chemical composition and phase purity

• pore size uniformity and connectivity

• mechanical strength and resistance to thermal shock

• consistent dimensional tolerance for filter housings.

Selection matrix: which cone or filter to use

Quick decision table

When selecting a foundry filter match material chemistry and maximum operating temperature. For pyrometric cones select a cone matching the body and glaze maturity and confirm with your firing schedule and record-keeping.

Installation, handling and best practices

For pyrometric cones

• Position cones so they see the same atmosphere and heatwork as your wares.

• If using a kiln sitter, confirm the cone grade recommended by the kiln manufacturer.

• Use witness cones on each shelf for consistent data logging.

For foundry filters

• Seal filters properly to prevent metal bypass.

• Use appropriate filter box design to avoid premature breakage.

• Avoid excessive turbulence upstream which can reduce filtration efficiency.

• Preheating filters or using preheated gating can reduce thermal shock.

Performance metrics and monitoring

Pyrometric cones

Performance is qualitative and visual. Track cone behavior alongside thermocouple logs to build an accurate process history. For lab environments use cones with tighter tolerances or couple cone readings with thermocouple validation.

Foundry filters

Key metrics:

• inclusion capture rate (measured via metallography)

• pressure drop and flow rate under pour conditions

• filter integrity after exposure to molten metal

• reduction in scrap and rework rates.
  Routine sampling and quality audits are recommended to confirm filter performance in production.

Comparison: pyrometric cones versus foundry ceramic filters

Typical specification table for foundry filters

Note these are general guidelines. Exact temperature limits depend on formulation and local conditions. Always check manufacturer datasheets for design values.

Environmental, safety and cost considerations

• Ceramic filters reduce scrap and downstream waste but add per-pour consumables cost. The total cost of ownership is typically favorable when filter use reduces rework and increases yield.

• Handling of used filters and furnace residues must follow local environmental regulations.

• For pyrometric cones, disposal is low risk; for foundry filters there can be adhered dross and oxide remnants needing proper industrial disposal.

Tables summary (quick reference)

Table A. When to use which ceramic cone

Table B. Common PPI to application mapping

FAQs (frequently asked questions)

1. What is the difference between a pyrometric cone and a kiln thermocouple?
   A thermocouple measures instantaneous temperature at a point. A pyrometric cone registers heatwork, which is the combined effect of time and temperature. Use both together for the best control.

2. Can I reuse a ceramic foundry filter?
   No. Filters are sacrificial. After exposure to molten metal they are saturated with oxides and have lost structural integrity. Replace filters after each pour.

3. Which cone should I use for stoneware?
   Stoneware commonly fires between cone 5 and cone 11 depending on the body and glaze. Check your recipe and firing schedule and use witness cones on each shelf.

4. How do I choose filter PPI for aluminum casting?
   Balance required cleanliness and acceptable flow. For general aluminum work 20PPI filters are common; precision castings may use 30PPI or higher. Confirm via trial pours and metallography.

5. Are ceramic foam filters compatible with all alloys?
   Most are formulated for specific alloy families. Alumina works well for aluminum; zirconia and SiC are chosen for higher temperature or more reactive melts. Check manufacturer compatibility tables.

6. Do pyrometric cones need calibration?
   Cones are manufactured to a standard but store them carefully and use cones from reliable brands. For critical labs cross-check cone results with calibrated thermocouples.

7. What are signs a foundry filter failed during a pour?
   Uneven flow, metal bypassing filter seals, filter fracture, or persistent inclusions in the casting are signs. Inspect gating and filter seating after any anomaly.

8. Can I put cones directly on glazed ware?
   Do not contaminate work with cone materials. Put cones on separate kiln posts or use witness tiles to avoid slips or glaze runs damaging cones or wares.

9. How do I dispose of used ceramic filters safely?
   Follow local industrial waste regulations. Filters with heavy metal residues may be hazardous; consult your environmental officer or supplier for disposal recommendations.

10. What records should a foundry keep about filter use?
    Track filter type, PPI, lot number, pour conditions, inclusion rates from QA sampling, and scrap rates. This enables continuous improvement and cost justification.

Sources and selected references

Key authoritative sources referenced in this article:

• Pyrometric cone overview and history.

• Practical pyrometric cone usage and charts.

• Ceramic foam filters for metal casting, materials and PPI guidance.

• Foundry product pages and application notes including manufacturer guidance.