It makes wonderful kitchen countertops. But putting aside its fabulous looks, soapstone (or soaprock) has many outstanding physical qualities. Especially those focused on coping with high heat levels. Some of the refractory properties soapstone has are even better than those found in heat resistant firebricks. How about well performing baking stones? Then stove or the once popular original fireplace linings.

Two colors of soapstone, dark green and light brown versions. A truly efficient baking oven system needs to meet well two certain thermal characteristics. The faster the heat absorption into the mass is the better (speed of heat soak from a heat source). Thermal Conductivity. That’s the first one. The second is Specific Heat. It’s about material heat storing capacity (how well and how much of heat is stored in the body mass – heat retention.) As it’s designed, soapstone fulfills both criteria and more extremely well – including a very high heat withstanding on reheating over and over again (unaffected like firebricks or better.)

Soapstone volume vs. weight Conversion Calculator

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• This automatic soapstone weight versus volume conversion calculator lets you instantly convert measurements of soapstone between mass kilograms g, ounces oz, pounds lb., kin, jīn, cubic volume in centimeters cm3, inches in3, feet ft3, yards yd3, meters m3, kin per cubic shaku (Japan shaku^3), jīn per cubic chǐ (China chǐ^3), Metric, US, Asian units from/to amounts into scale needed.
• The online tool is based on soapstone mass density 2.956g/cm3
• You may enter whole numbers, decimals or fractions ie: 5, 25, 29.33, 1/4, 17 3/8

Composition and properties of soapstone

By nature’s creation, in mass sense, this rock solid soapstone is composed of 67% of silica (Silicon dioxide – SiO2) and 33% magnesia (Magnesium oxide – MgO). Hence its wonderful thermal properties can be useful either in culinary arts practice or for room heating – ovens – stoves – pits – fireplaces. Or all such segments in one unit. It remains food-grade healthy in heated up or in cold state. Plus it will not degrade by heating nor peel down from reheating over and over again. In the other extreme of its resistance to stress from heat, soapstone copes real well also with freezing conditions. It is truly one amazing not just refractory material.

The high talc content, between 30% to 80% in its body, makes soapstone relatively soft. It definitely is not a hard rock like a granite for instance. When this soap stone gets heated reaching temperatures around 1100°C = 2012°F it hardens marginally (approximately twice as high level to what can be achieved in a semi-opened-source from wood fire heat.) But, at the same time, it does not shrink down in the heat as clay would. E.g. fireclay or pottery clays have a lots higher shrinkage ratio. Metal casting molds can be carved from this stone. It has very high degree of thermal stability. Hence another positive aspect related to business with refractories.

Thermal and mechanical data

Solid soapstone rock

Physical – Mechanical – Thermal – properties of soapstone :
Specific heat capacity 785 J/Kg.°K
Thermal conductivity 12.5 W/m.°K
Melting point temperature > 2912°F = 1600°C (+)
Comprehensive strength 34 MPa (Megapascals)
Transverse strength 12.6 MPa (Megapascals
Permanent linear change
on Reheating to 1022°F = 550°C
nearly none (n/a)
Modulus of rupture 13.2 MPa (Mega-pascals)
Apparent porosity
(this is NOT a softness)
Non-porous
(or perhaps 99.9% non-porous)
Mass density of soapstone :
2.956 g/cm3 grams per cubic centimeter
1.709 oz/in3 (cu in) ounces per cubic inch
2,956.0 kg/m3 kilograms per cubic meter
184.54 lb/ft3 (cu ft) pounds per cubic foot
4,982.5 lb/yd3 (cu yd) pounds per cubic yard
137.05 kin/cu-shaku Japanese kin per cubic shaku
218.3 jīn/cu-chǐ Chinese jīn per cubic chǐ
Naturally occurring properties variations should be expected.

Does soapstone wear off a lot?

Scratching it hard with a pointy-sharp hard metal object in guided motion will cause creation of a groove line (equally so with hard and dense firebrick which as a matter of fact also can be sanded with sandpaper as well.) However, I do not think a bread or pizza paddle is an issue though if the floor or hearth are looked after in a normal manner.

The answer will be hidden within this logical secondary question: Do benchtops, bath & kitchen soapstone counter-tops wear off? Nope … countertops do not wear off like shoes do! We use kitchen countertops made from soapstone in the morning, for making lunch and dinners and in between these hours. This explains it clearly doesn’t it?!

Wear characteristics of soapstone when it comes to usage;
in fireplaces & ovens’ hearths / floors. Down the road there are no problems with soapstone floors and hearths. Just the same as with countertops and various such surfaces. But those individuals who for their own reasons reckon they may use an oven heavily while at the same time perhaps somehow abuse it too much by placing e.g. heavy cast metal objects inside (the same thing then could apply to ceramic tile or firebrick surface), you can always make the floor segmented. The section which might need to be turned up side down or replacement, sometimes in the distant future, can simply be lifted, rotated or taken out and replaced with a new part of the same size. Which leads me to say once again …

… here is one very important aspect to consider! Related to heat expansion and in general long life for various materials exposed to high-heat levels. Soapstone is not excluded from this group. It’s about the temperature differences in material phenomenon. While temperature rises upwards and even more importantly on the other way backwards when the material is cooling down again. It expands and shrinks. It’s much better if fragments instead of thinner tiles or larger to large slabs are used. When a tile or a slab section in a floor heats up, these individual blocks get hot and they slightly expand, equally in theirs centers as well as around their edges (unless the heat source radiates onto a narrow area only, e.g. would be a burner flame.) But when things starts to cool down, edges or sides of some tiles might/always cool down in a faster rate than their centers. Meaning, they, the edges SHRINK down. This is such a common situation … which happens quite often. At this point the center of the slab or tile still keeps higher heat, there the body still remains EXPANDED. Ahh, bummer, a tile is expanded-larger in size in the center while at the same time it’s shrunk-smaller around its sides! Only a small difference is needed and that results in crack/s development/s in the cooled down areas. Potentially spreading across the whole tile surface. THEREFORE it’s better to use, right from the start, smaller segments of the size like bricks have for instance. With such smaller pieces everything breathes in and out again – nicely moves there and back again.

* The 392 degrees Fahrenheit or 200 degrees Celsius mark is when silica shrinks. It’s most often when material cracks. A slab or tiles can be protected from cracks happening – by slowing the cooling-down rate around this temperature level. Just by closing a door and proper outer thermal insulation. Insulate the large slab on which firebricks or soapstone pieces sit – then as it gradually cools down temperature in the whole slab will be even – it’s so simple.

Body of soapstone from certain sources can sometimes contain also alumina (Al2O3) and Calcium oxide (CaO), but these would form only a very minimal quantities. Too minor for consideration in regards to refractory properties.

Additionally on soapstone properties …

… and characteristics. Focusing on mostly from refractory matters perspective.

Due to the denser – heavier Magnesium oxide – MgO part, soapstone has density of 2.956g/cm3 (calculated 2.95556 grams per cubic centimeter.)
Soapstone has thermal conductivity 12.5 W/m.°K (property for heat conduction – temperature dependent heat sink into the stone as the material body. An opposite term would be a thermal resistance.)
Soapstone has the volumetric heat capacity of 785 J/Kg.°K (Specific heat – heat stored as energy – the higher the number the better capacity.)

Thermal conductivity (heat transfer within body)

  1. Silica – SiO2 thermal conductivity at r.t. is 1.38 W/m • °K = 0.9246 per 67%
  2. Magnesia – MgO thermal conductivity at r.t. is 35 W/m • °K = 11.55 per 33%
  3. Soapstone thermal conductivity calculated SiO 67% + MgO 33% = 12.4746 W/m • °K

Thermal conductivity at r.t. = room temperature level.

Specific heat (heat sink capacity)

  1. Silica – SiO2 specific heat at r.t. is 740 J/Kg • °K = 495.8 per 67%
  2. Magnesia – MgO specific heat at r.t. is 877 J/Kg • °K = 289.41 per 33%
  3. Soapstone specific heat calculated SiO2 67% + MgO 33% = 785.21 J/Kg • °K

Specific heat capacity at r.t. = room temperature level –
joules (J) kilogram (Kg [amount, body size]) per kelvin (K) unit.

Technical data

Density of soapstone: 2.956g/cm3

Thermal conductivity of soapstone at room temperature: 12.5 W/m • °K

Specific heat capacity of soapstone at room temperature: 785 J/Kg • °K

Magnesium oxide – MgO thermal conductivity is 35 W/m • °K

Magnesium oxide – MgO specific heat capacity is 877 J/Kg • °K

Magnesium oxide – MgO mass density is 3.58 g/cm³

Silicon dioxide – SiO2 thermal conductivity is 1.38 W/m • °K

Silicon dioxide – SiO2 specific heat capacity is 740 J/Kg • °K

Silicon dioxide – SiO2 mass density is 2.65 g/cm³

Soapstone vs. firebrick comparisons

Here we compare soapstone with refractory properties of firebricks which contain:

33% Aluminum
63% Silica
1.2% Ferric oxide
1.2% Titania
1.6% Accessory oxides

For comparison – 33% alumina + 63% Silica firebrick properties

Thermal Conductivity :

  1. Alumina – Al2O3 thermal conductivity at r.t. is 25.08 W/m • °K = 8.2764 per 33%
  2. Silica – SiO2 thermal conductivity at r.t. is 1.38 W/m • °K = 0.8694 per 63%
  3. 33% Alumina firebrick thermal conductivity – SiO2 63% + Al2O3 33% = 9.1458 W/m • °K

Specific Heat Capacity :

  1. Alumina – Al2O3 specific heat at r.t. is 880 J/Kg • °K = 290.4 per 33%
  2. Silica – SiO2 specific heat at r.t. is 740 J/Kg • °K = 466.2 per 63%
  3. 33% Alumina firebrick specific heat – SiO 67% + Al2O3 33% = 756.6 W/m • °K

These technical data numbers clearly explain why soapstone rock performs better (marginally perhaps) in heat-absorbing speed and also in heat retention.

Soapstone has higher thermal conductivity 12.5 W/m • °K than the 33% alumina content grade firebrick which is at 9.15 W/m • °K in comparison with the two materials. The soapstone has roughly three times (~3x) the bulk density 2.95g/cm3 … these firebrick types have 2g/cm3. Meaning soapstone has also heavier mass.

* Notice that, just like with Graphite, soaprock does not contain the Aluminum (Aluminum oxide – Al2O3) which clay based refractory products have a lot of in them.

Conclusion

The specific heat capacity of the soapstone comes to 785.2 J/Kg.°K where the firebrick is at a lower 757 J/Kg.°K heat storage. All is calculated @ room temperature level. Soapstone wins, it collects slightly more heat and gets hotter as well.

However, on the other side of the chamber – through the wall on the out-side opposite from the heat source, some suitable kind of thermal light-weight insulation has to be applied to prevent much of the heat energy loss. Otherwise the stored energy will run out into the opened air fast.

When it comes to culinary arts practice aspects (and suitability of soapstone for contact with food) – speed of the heat energy being transferred from the heat source through the hot face into the body mass, and also (obviously), how much of the heat energy is retained as in storage, are important considerations. After the bulk is saturated by the heat it performs, cooks or bakes for a long time, from the initial heating up. Soapstone isn’t toxic and it is material very suitable for contact with any foods.

There is only one small negative aspect I can think of. Soapstone does not have porosity. If a material is not porous it will not absorb moisture. A steam for instance. Such particular non-porous characteristic, that in itself is wonderfully positive for baking or for using such structure outdoors in freezing climates. Hence, on the other hand, when fresh pizza base is placed on a heated soapstone surface, and if it’s to be cooked rapidly – a method for producing the most delightful culinary results – the steam from under the fresh base isn’t absorbed by the rock because it behaves as something highly glossy (a glass would do the same.) Much steam gets generated in the early moments of placing the wet base onto the hot floor surface. Therefore there is the chance of not achieving that desired higher crisp. Although do not fret yet. Here are a couple of ways for how to overcome this problem. One is to simply uplift the base 1 to 3 times within the first 10 to 15 seconds. Second is to create the center surface out of several pieces, which creates those little gaps in between the segments for the steam to run out. Low heat and mid range firebricks do have the optimal porosity so no any issues. There is always a way for how to develop, improve, service, fix, repair any subjects needed!

When compared with firebricks, soapstone absorbs the heat faster and it also has bigger/better heat storage capacity. Soapstone gets a little more hotter too at the same time.

Convert a cubic volume of soapstone into …

mass in kilograms Kg, pounds Lbs, Japanese Kin, Chinese Jīn units.

soapstone equivalent measurements
cubic volume
– blocks -
Kilograms
– kg -
Pounds
– lb -
Japanese
– kin -
Chinese
– jīn -
1 cm3 of soapstone 0.003 Kgs 0.007 Lbs 0.005 kin 0.006 jīn
1 in3 of soapstone 0.048 Kgs 0.107 Lbs 0.081 kin 0.097 jīn
1 ft3 of soapstone 83.7 Kgs 184.54 Lbs 139.51 kin 167.41 jīn
1 yd3 of soapstone 2,260.02 Kgs 4,982.5 Lbs 3,766.71 kin 4,520.05 jīn
1 m3 of soapstone 2,956.0 Kgs 6,516.86 Lbs 4,926.67 kin 5,912.0 jīn
1 cubic shaku (Japanese)
of soapstone
82.23 Kgs 181.29 Lbs 137.05 kin 164.46 jīn
1 cubic chǐ (Chinese)
of soapstone
109.15 Kgs 240.64 Lbs 181.92 kin 218.31 jīn

* shaku – Japanese foot, length unit = 303.0 mm – 11.93 inches – 0.9942 feet
  ( cubic volume block: 0.303m * 0.303m * 0.303m = 0.0278 m3 or 0.983 cu-ft )

* kin – Japanese unit of mass, 1 kin = 1.323 Lb or 0.6 Kg

* chǐ – Chinese foot, unit of length = 33.3 cm – 13.11 in – 1.0925 ft
  ( cubic volume block: 0.333m * 0.333m * 0.333m = 0.0369 m3 or 1.304 cu-ft )

* jīn – Chinese mass unit, 1 jīn = 0.596816 kilograms or 1.31575 pounds

Slabs from soapstone of a cubic volume

Block of soapstone of sizes Width 12″ x Length 24″ x Height 4.5″ (makes it 1296 in3 – cubic inches = 0.75 ft3 – cubic foot volume) weights exactly 138.4 lb – pounds = 2 214.44 oz – ounces. Or for any other measure in kilograms and grams of soapstone use the soapstone converter above for the conversion answers.

Soapstone slab of sizes Width 230mm x Depth 460mm” x Height 115mm (in volume 12,167.00 cubic centimeters = 0.012167 m3 – cubic meters) works out exactly 35.97 kg – kilograms in mass. For any other measure in ounces – pounds of soapstone or Asian including cubic volume units, easily calculate the outcome with the soapstone measures converter above.

References for Silicon dioxide and Magnesium oxide information:
1. Air Force Materials Laboratory (U.S.) “Thermophysical Properties of High Temperature Solid Materials”, Vol 4. (Google books)
2. Goodfellow Cambridge Ltd., “Metals, Alloys, Compounds, Ceramics, Polymers, Composites”