Explores (in three sections) the characteristics, industrial and consumer product applications, and processes of supplying pumice to market in a Q and A format.
The Characteristics of Pumice
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What is pumice?
Pumice is a naturally occurring (created by volcanic events) foamed glass stone. That rigid frothy character makes pumice lightweight and amorphous (non-crystalline) in form factor and widely versatile in application. Pumice is classified as an extrusive igneous rock that began life as silica-rich magma before its explosive birth. Another descriptive term for pumice: a rhyolitic froth. As pumice is a “froth” of thousands of tiny, thin-walled, interlaced vapor cavities, its absorptive, insulative, filtering, and lightweight properties are proportionally greater than its particle size.
One-inch pumice stones from the Hess deposit in southeast Idaho, USA
Pumice is typically pale in color, though some can be quite dark, even black. Pumice from the Hess Pumice deposit is white to off-white (scoring an 84 on the GE Brightness Scale) and exceptionally pure. Also, pumice is not the same as scoria (see below ).
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How do volcanoes make pumice?
It begins deep underground, in the fiery heart of an awakening volcano: incredible pressure forces water to mix with molten rock. The pressure continues to builds…until overlying rock fractures and the water-infused magma finds a violent, spectacular release. Once ejected into the atmosphere, the trapped water in the viscous, super-heated liquid rock flashes to steam, blasting the magma into a froth that flash-cools, hardens, and falls to earth as pumice—a hard (Mohs scale 6) yet friable volcanic glass stone, amorphous (non-crystalline) in structure, naturally calcined.
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Is pumice the same as scoria?
No. Scoria is a related type of porous volcanic rock, but differs from pumice in that it has a larger and thicker vesicular01 form factor: bigger bubbles, cavities, pits, and grooves, and thicker, heavier walls between the voids. Overall, scoria is a rougher, denser stone. Scoria02 is the red/brown/black common lava rock sold for landscaping ground covers or used in decorative fire pits. Pumice is much more finely structured (microvesicular) and more amenable to fine-tuned refinement. To get even more into the geek weeds about it: Pumice is a felsic volcanic glass; scoria is a mafic volcanic glass.
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What determines pumice density?
Pumice density is, primarily, determined by the wall thickness between the tubular bubbles and cavities within the pumice stone. The quantity and size of the vesicular03 bubble matrix also factors in.
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Is some pumice better than other pumice?
Being a naturally occurring mineral, the chemical makeup, purity, hardness, denseness and other application-critical factors vary from pumice deposit to pumice deposit. These natural variances mean, depending on use, application suitableness and performance effectiveness also vary. Also, the company that mines and refines the pumice deposit figures into the “better” equation.
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Does pumice provide thermal resistivity?
It does. Thermal resistivity (R-Value) will vary slightly depending on natural density and grade size/blend. For example, a pair of Hess Pumice grades tested for thermal resistance showed the following:
LEFT: Hess Grade 3/8 Fines (Mine Grade). Apparent Thermal Conductivity of 1.03 (Btu•in/ft2 •hr•°F). Thermal Resistivity (R-per-inch) of 0.971 (ft2 •hr•°F/Btu•in). Thermal Resistance of Specimen at 2.0 (ft2 •hr•°F/Btu). Specimen density: 55.01 (lb/ft3 ). | RIGHT: Hess Grade 3/8 x #8 (Processed Grade). Apparent Thermal Conductivity of 0.955 (Btu•in/ft2 •hr•°F). Thermal Resistivity (R-per-inch) of 1.047 (ft2 •hr•°F/Btu•in) Thermal Resistance of Specimen at 2.2 (ft2•hr•°F/Btu). Specimen density: 40.57 (lb/ft3 ). (lb/ft3 ).
Average specimen temperature for both grades pictured above: 75°F. Specimen thicknesses: 2 inches. The primary difference between the two tested grades is that the 3/8 x #8 grade contains no particle fines, leaving larger air pocket space between the little pumice stones. Tests using pumice aggregate in concrete show a 4X improvement in the R-value of the cured concrete itself.
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What are the chemical properties of pumice?
Pumice from the southeast Idaho Hess deposit is primarily silicon dioxide (SiO2 ) with a generous splash of aluminum oxide (Al2 O3 ) and small-to-trace amounts of other oxides (see table below). Importantly, no detectable amounts of nasty crystalline-form SiO2 .
CHEMICAL PROPERTIES (HESS PUMICE DEPOSIT)
Silicon Dioxide:
76.2%
Aluminum Oxide:
13.5%
Ferric Oxide:
1.1%
Ferrous Oxide:
0.1%
Sodium Oxide:
1.6%
Potassium Oxide:
1.8%
Calcium Oxide:
0.8%
Titanium Oxide:
0.2%
Magnesium Oxide:
0.05%
Water:
<1.0%
Crystalline Si02 :
None Detected
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What are the physical properties of pumice?
PHYSICAL PROPERTIES (HESS PUMICE DEPOSIT)
Chemical Name:
Amorphous Aluminum Silicate
Hardness (MOHS):
6
pH:
7.2
Radioactivity:
None
Loss on Ignition (LOI):
5%
Softening Point:
900 Degrees C
Water Soluble Substances:
0.15%
Acid Soluble Substances:
2.9%
Reactivity:
Inert (except in the presence of calcium hydroxide or hydrofluoric acid)
Appearance:
White Powder
Whiteness
GE Brightness of 84
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How is pumice mined?
Pumice mining is an environmentally benign process, as the material is removed from surface deposits. Overburden (including top soils) is removed and stockpiled for replacement when a section of the deposit is exhausted and returned to its native state. Pumice is scraped from the deposit using heavy equipment, crushed and screened into several base mine-class grades. Mine grades are sold directly to customers. Select mine grades are transported to a refining facility for additional processing.
The Hess pumice mine at Wright Creek in southeast Idaho, USA
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How is pumice “refined?”
Because of the natural purity of the Hess deposit, the refining processes primarily consist of highly-specialized crushing, screening and separating to grade, grade blending (select grades), and packaging. Hess supplies industry with grades that range from three-quarter-inch aggregate stones to pumice powders crushed to the consistency of smoke. For a handful of ultra-pure grades, Hess uses specialized impurity extraction processes to remove the trace (2%) non-pumice igneous minerals (like obsidian) found in the deposit. A robust testing process ensures the grades consistently hold to spec.
Hess Pumice Products refining plant #1 in Malad, Idaho, USA
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In what ways is pumice packaged for use?
Pumice is packaged in a variety of ways, depending on customer need for end use. Pumice is shipped in bulk containers—rail car or truck, pneumatic or hopper-type. Pumice is packaged and shipped in palleted production bags weighing 35 to 50 lbs, depending on grade. Pumice is packaged in palleted super sacks up to 2000 lbs. or packaged in single-pound quantities in mylar bags. Pumice is also packaged in paper barrels, plastic canisters, and buckets.
Stacking bags of pumice on a pallet.
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What are the recommended storage practices for pumice?
Hess Pumice recommends that packaged pumice be stored in a dry area, protected from the elements/weather. Recommended storage temperatures range from -20° to 120°F, although no minimum or maximum storage temperatures have been established.
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Is pumice available directly to non-industrial customers? Small business retailers? Consumers?
Yes. Hess operates an online Pumice Store (pumicestore.com ), backed by an in-house fulfillment center that quickly puts pumice in the hands of end users for a wide variety of needs—lightweight pumice aggregate to university engineering students building concrete canoes, enthusiasts growing bonsai in pumice soil, woodworkers rubbing up a gleaming finish with pumice dust—to name a few.
Pumice Store , Hess supplies both branded pumice products to end users and testing samples to industry R&D projects.
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Is breathing pumice dust a health risk?
It is not. Pumice is classified as a nuisance dust. It is fully amorphous (non-crystalline in structure), and thus (the Hess pumice deposit in particular) has no detectable amounts of crystalline silica or asbestiform minerals.
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Does pumice contain asbestos?
Pumice from the Hess deposit is entirely free from all asbestiform minerals, including tremolite asbestos. Nor does Hess’ pumice contain respirable crystalline silica.
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What does it mean that pumice and pumice products are non-crystalline silicas (NCS)?
Breathing the dust from silicas that are wholly or partially crystalline in structure is known to cause respiratory harm. Pumice is amorphous in structure (non-crystalline) and thus poses no respiratory risk. Independent laboratory testing consistently confirms that the crystalline silica content of pumice products from the Hess deposit is below detectable limits. OSHA classifies pumice powders from the Hess deposit as no more than a nuisance dust.
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Does pumice meet the California Prop65 requirements?
The pumice products (raw or processed) from the Hess deposit contain no contaminants or byproducts known to the State of California to cause cancer or reproductive harm (toxicity) as listed under Proposition 65 State Drinking Water and Toxic Enforcement Act. Additionally, the crystalline silica content of Hess’ pumice products have been determined to be below detectable limits.
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Do pumice products from the Hess deposit have additives or contaminations of any kind?
All Hess pumice grades are 100% natural mineral and contain no chemical additions or alterations, no pesticide residue, no heavy metals. Furthermore, in this natural state, Hess pumice products are not carcinogenic, mutagenic, or toxic for reproduction, are non-crystalline in form, and thus safe to use in any process or product, including cosmetic products. Furthermore (per ISO 16128-2L2017), Hess Pumice products have a Natural Origin Index of one (1) and are thus considered a natural ingredient.
The Hess pumice grades are also deemed allowable in organic processes as approved by the Organic Materials Review Institute (OMRI) for use as soil amendments, crop fertilizer carriers, seed coatings, and other horticulture or crop management and production aid processes. See NOP rules 205.105, 205.203(d), and 205.206(d)(2).
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Is pumice regulated by the U. S. Department of Transportation?
Pumice is a non-hazardous substance and is not regulated by the U.S. DOT (freight classification 55).
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Does pumice expire or become less effective over time?
Pumice is an inorganic natural mineral with an indefinite shelf life and is not subject to an expiration date, but there are recommended storage practices01 . This statement does not apply to or include any ingredient that may be added to pumice for the purposes of creating a specific product.
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Are pumice products suitable for vegan use?
Pumice products mined, processed, and distributed by Hess Pumice Products are free from any animal-derived ingredients, do not contain dairy or animal products of any kind, and are thus suitable for use by vegetarians and vegans. In addition, no animal testing is associated with Hess pumice products, raw or processed.
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Where can one find the library of all Hess Pumice declarations and certifications about its pumice products?
