Shot content analysis is a fundamental quality control test for thermal insulation wool products such as rock wool, glass wool, and ceramic fiber, quantifying the non-fibrous spherical particles that degrade thermal and mechanical performance. This article explains the technical significance of shot (slag ball) content, the applicable international standards such as GB/T 5480, ASTM C492, and JIS A 9504, laboratory testing procedures, typical acceptance limits, and practical applications of the test results.
Technical Overview
Thermal insulation wool—including rock (stone) wool, slag wool, glass wool, and aluminum silicate (ceramic) fiber—is manufactured by melting mineral raw materials and converting the molten stream into fine fibers through centrifugal fiberization, spinning wheels, or flame-blowing processes. During fiberization, a portion of the melt is not successfully drawn into fibers and instead solidifies into non-fibrous, predominantly spherical or rounded glassy particles known as shot, or slag balls.
Shot content is defined as the mass fraction of these non-fibrous particles—conventionally those retained on a 0.25 mm test sieve after fiber separation—expressed as a percentage of the dry specimen mass. Because shot particles conduct heat far more readily than the entrapped-air fiber matrix, add dead weight, and reduce resilience, an elevated shot content directly increases thermal conductivity and density while lowering tensile strength, flexibility, and acoustic absorption performance.
Shot content is therefore a routine and often contractual quality indicator in product standards such as GB/T 11835, GB/T 13350, and JIS A 9504. Dedicated test method standards, principally GB/T 5480 and ASTM C492, describe the gravimetric water-separation procedure used by accredited third-party laboratories and in-house quality control departments worldwide.
Testing Samples
Rock Wool (Stone Wool) Products
Boards, blankets, felts, pipe sections, and wired-mesh blankets produced from basalt, diabase, or blast-furnace slag. Rock wool typically carries the most demanding shot requirements, with common specification limits of no more than 10% to 12% by mass for particles larger than 0.25 mm, because centrifugal fiberization of basaltic melts tends to generate more shot than glass fiberization systems.
Glass Wool Products
Centrifugally fiberized borosilicate glass wool in the form of boards, blankets, duct wraps, and pipe coverings. Glass wool normally exhibits very low shot levels, with typical specification limits of no more than 0.3% by mass.
Ceramic Fiber (Aluminum Silicate Fiber) Products
Needled blankets, modules, boards, papers, and vacuum-formed shapes used in high-temperature furnaces and kilns. Shot content for these materials is commonly specified in the range of approximately 12% to 25% depending on temperature classification and product grade, and it strongly influences resilience, compression recovery, and hot-face thermal performance.
Loose Fiber, Floc, and Raw Materials
Bulk fiber and granulated wool used as feedstock for wet-laid papers, vacuum-formed components, and refractory composites, where the shot level controls fiber yield, slurry processing behavior, and final part uniformity.
Faced and Composite Products
Specimens with facings, wire mesh, glass tissue, or coatings can be tested, provided that all non-wool layers are carefully removed before specimen preparation so that only the fiber mass is evaluated.
Testing Items
- Shot content (mass fraction, %) for particles equal to or larger than 0.25 mm — the core determinand
- Shot size distribution by multi-sieve fractionation, for example 0.25–0.5 mm, 0.5–1.0 mm, and above 1.0 mm
- Fiber-to-shot mass ratio and effective fiber content
- Supporting characterization on request: dry density, moisture content, and correlation assessment with thermal conductivity
Testing Methods
Reference Standards
- GB/T 5480 — Mineral wool and its products: determination of shot content by the water dispersion–sieving method
- ASTM C492 — Standard Test Method for Shot Content of Preformed Mineral Fiber Thermal Insulation
- JIS A 9504 — Rock wool thermal insulation materials, including shot content requirements and the associated test procedure
- Product standards GB/T 11835 (rock wool products) and GB/T 13350 (glass wool products), which set acceptance limits referenced to the above test methods
Apparatus
- Forced-convection drying oven capable of maintaining 105 ± 5 °C
- Analytical balance with a readability of 0.001 g
- Stainless steel standard test sieve with a 0.25 mm aperture, fitted with lid and receiver
- Shot separation apparatus: agitated beaker, flushing funnel, or elutriation vessel providing a controlled water flow that carries the floating fiber fraction away while the denser shot settles
- Desiccator, wash bottle, filter paper, and handling tools
Test Procedure
Specimen preparation: representative specimens, typically 3 g to 5 g each and taken from several positions across the product, are dried at 105 ± 5 °C to constant mass, cooled in a desiccator, and accurately weighed (m1).
Fiber dispersion: each specimen is immersed in water and agitated so that the fibers separate and become suspended while the denser shot settles. Repetitive water exchange, gentle boiling, or elutriation is used until the fiber and shot fractions are fully disengaged.
Sieving: the settled residue is transferred onto the 0.25 mm sieve and washed thoroughly; particles retained on the mesh constitute the defined shot fraction, while suspended fibers and fines passing the mesh are discarded.
Drying and weighing: the collected shot is dried at 105 ± 5 °C to constant mass, cooled, and weighed (m2).
Calculation and Reporting
Shot content S (%) = m2 / m1 × 100, where m1 is the dry mass of the original specimen and m2 is the dry mass of the retained shot. At least two, and commonly three, parallel determinations are performed; the mean value is reported to the nearest 0.1%, together with the governing standard, sieve aperture, drying conditions, and specimen dry basis.
Precision and Influencing Factors
- Incomplete fiber dispersion can trap shot inside fiber clumps and bias the result low
- Fine fiber fragments contaminating the retained fraction can bias the result high if washing is inadequate
- Water-soluble binder content affects the mass basis; a binder-free or dry-mass basis should be clearly defined
- Facings, mesh, and coatings must be removed before testing to avoid extraneous residue
- Repeatability within a laboratory is generally good for trained operators, while inter-laboratory reproducibility depends on the separation apparatus and washing endpoint
Applications
- Fiberization process control: correlating shot level with spinning-disc speed, melt temperature and viscosity, and line throughput to optimize fiber yield
- Product certification and compliance testing against the acceptance limits of GB/T 11835, GB/T 13350, and JIS A 9504
- Supplier qualification, incoming inspection, and batch release testing for insulation fabricators and construction projects
- Research and development of low-shot, premium-grade insulation with superior thermal and acoustic performance
- Failure analysis of underperforming insulation systems where high shot content, abnormal density, or thermal shortcuts are suspected
- High-temperature industry support: qualification of ceramic fiber blankets, modules, and boards for furnace and kiln linings
FAQ
What is shot in insulation wool?
Shot, also called slag ball, refers to the non-fibrous, predominantly spherical glassy particles that form when part of the molten raw material fails to be drawn into fibers during the fiberization process. Shot is denser than fiber and conducts heat more effectively, so it reduces insulation efficiency.
Why is shot content limited in product standards?
Higher shot content increases thermal conductivity and product density, reduces resilience and tensile strength, and diminishes sound absorption. Limiting shot content ensures the installed insulation delivers the specified thermal, mechanical, and acoustic performance.
Which sieve aperture defines the shot fraction?
The widely accepted convention, used in GB/T 5480 and referenced by the associated product standards, defines shot as non-fibrous particles that do not pass a 0.25 mm test sieve. ASTM C492 and JIS A 9504 describe equivalent separation-based procedures for their respective market contexts.
What are typical acceptance limits?
Typical limits are no more than about 10% to 12% shot for rock wool products, no more than about 0.3% for glass wool products, and roughly 12% to 25% for ceramic fiber products depending on grade. The governing product standard or purchase specification should always be confirmed, as individual requirements vary.
How is shot separated from the fibers?
The specimen is dispersed in water so that the fibers become suspended while the denser shot settles. The fiber-laden water is decanted or elutriated away, and the settled residue is washed through a 0.25 mm sieve; the retained particles are dried and weighed gravimetrically.
How much sample is needed, and what is the turnaround time?
Only a few grams of fiber are required for each determination, but laboratories usually request a representative section of the product, for example a 100 mm × 100 mm piece, to allow multiple parallel specimens. Testing is typically completed within a few working days.
Can faced or bonded products be tested?
Yes. Facings, wire mesh, tissue, and coatings are removed before specimen preparation, and the test is performed on the recovered fiber mass. If a significant binder content is present, the reporting basis should be agreed in advance with the accredited third-party laboratory.