questions and answers about pumice

The use of pumice made how-to history when Vitruvius, a first century BC Roman architect and engineer, specified the use of pozzolana (pumice sands from Puteoli) for producing enduring lime mortar and concrete in his seminal treatise De architectura. Since that millennias-past prescription, the how-to processes that specify the use of pumice have expanded considerably. While still super-charging the performance of modern concretes, mortars, and plasters, pumice is used today in a wide and amazingly diverse variety of industrial processes and products.

1684 illustration of Vitruvius and Caesar Augustus

A 1684 illustration said to depict Vitruvius presenting his ten-volume treatise, De architectura, to his patron, the emperor Caesar Augustus.


question question

What is pumice?


A naturally occurring (created by volcanic events) foamed glass stone, making it 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 particle size.

pumice stones from the Hess deposit

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).


How do volcanoes make pumice?


It begins deep underground, in the fiery heart of an awakening volcano: water mixes with molten rock, pressure builds and builds…until finding 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 frothy stone that flash-cools, hardens, and falls to the earth as pumice. That foamed glass pumice is a hard yet friable stone, amorphous (non-crystalline) in structure, and naturally calcined—a combination of nature-blessed characteristics that make pumice powders, sands, and aggregates incredibly useful to a variety of industries.


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 vesicular[1] form factor: bigger bubbles, cavities, pits, and grooves, and thicker, heavier walls between the voids. Overall, scoria is a rougher, denser stone. Scoria[2] 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.

1Vesicular: (Geology Definition) a small cavity in volcanic rock, produced by gas bubbles in the molten lava.


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 vesicular[1] bubble matrix also factors in.

1Vesicular: (Geology Definition) a small cavity in volcanic rock, produced by gas bubbles in the molten lava.


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. The company that mines and refines the pumice deposit also figures into the “better” equation.

Learn more about Hess Pumice Products company.

hess pumice products logo

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:


photo of HPP grade 3/8 fines

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).

HESS GRADE 3/8 x #8

photo of HPP grade 3/8x#8

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)

Average specimen temperature for both: 75°F. Specimen thicknesses: 2 inches. The primary difference between the two tested grades is that the 3/8x#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.

Website: Lightweight Insulative Concrete. This site looks at the advantages and applications of using foamed-stone aggregate in concrete to build in thermal resistance.


What are the chemical properties of pumice?


Pumice from the Hess deposit is primarily silicon dioxide (SiO2) with a generous splash of aluminum oxide (Al2O3) and small-to-trace amounts of other oxides (see table below). Importantly, no detectable amounts of nasty crystalline-form SiO2.

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

What are the physical properties of pumice?


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

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.

hess pumice mine in idaho usa

The Hess pumice mine at Wright Creek in southeast Idaho, USA


How is pumice “refined?”


Because of the natural purity of the Hess deposit, the refining processes primarily consist of our highly-specialized crushing, screening to grade, and grade blend mixes. 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%) of heavier igneous minerals (like obsidian) found in the deposit. A robust testing process ensures the grades consistently hold to spec.

hess pumice refining plant number one

Hess Pumice Products refining plant #1 in Malad, Idaho, USA


How is pumice packaged for use?


Pumice is packaged in a variety of ways, depending on customer need. It is shipped in bulk containers—rail car or truck, pneumatic or hopper. It is packaged and shipped in palleted production bags from 35 to 50 lbs. (depending on grade). It is packaged in super sacks up to 2000 lbs. or packaged in single-pound quantity in mylar bags. It is packaged in paper barrels, plastic canisters and buckets.

hess pumice production bags on pallet

Stacking bags of pumice on a pallet.


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.


Is pumice available directly to non-industrial customers?


Yes. Hess operates an online Pumice Store (, 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, to people growing bonsai in pumice soil, to woodworkers rubbing up a gleaming finish with pumice dust, to name a few. Through the Pumice Store, Hess supplies both branded pumice products to end users and testing samples to industry R&D projects.

hess pumice store logo and link

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.

Download: Hess Pumice Safety Data Sheet (SDS)


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.

Download: Asbestiform Free Declaration


What does it mean that Hess pumice products are non-crystalline?


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.

Download: Non-Crystalline Silica Declaration


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.

Download: California Prop 65 Declaration


Do Hess pumice products have additives 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).

Download: Organic Status Declaration


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).


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 practices. This statement does not apply to or include any ingredient that may be added to pumice for the purposes of creating a specific product.

Download: Non-Expiration Certificate


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.

Download: Vegan Use and Animal Testing Declaration


Where can one find the library of all Hess Pumice declarations and certifications about its pumice products?


The full library is available for download (PDF files) from the following linked directory at Support Documents.