Rev. 2002-12-25, 2003-10-30, 2004-12-06, 2005-09-06

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Refractories are materials that stand high temperatures. Most of them are clay or ceramic materials. Sources are given below, but are also found under Refractories in the Yellow Pages. You will normally be dealing with industrial sources when buying - refractories are used whenever the temperature gets above 1000F and often lower than that (boilers, etc.). That is about the point that many glasses start to sag.  Most refractories are sold for lining furnaces and ovens not for glass working.  Other materials requiring refractory temps include making cement, firing bricks, melting steel, heat treating metals.

Refractories may be hard fire brick, insulating fire brick, rammable paste that hardens to fire brick, bags of castable hard or insulating material, various forms of ceramic fiber - blanket, board, or sheet and vermiculite.
Generally, the hard materials will withstand higher temperatures and greater chemical attack but will have much lower insulating qualities so they are usually backed with insulating materials.  Ceramic fiber has much higher insulating values but is vulnerable to attack by molten glass.  (What are Conduction Values?)

Fire Brick
Very hard, very heavy, low insulating value brick, usually yellow grey in color with sharp corners.  Discussed both by temperature rating and materials used which resist chemical attack.  Usually available by each (instead of box) and may cost anywhere from $2-3 each to $27-40 each depending on material.
Insulating Fire Brick
Very light, porous brick. Weak, but amazing in that it can be white hot on one side and warm only on the other.  Different temperature ratings have different insulation values, lower insulation value with higher temp rating.
Rammable refractory
Sold as a wet paste in buckets it is intended to be hammered into place with pneumatic hammers and is used for repairing firebrick structures that have become worn or damaged, like the lips of steel crucibles.  Used by glassblowers sometimes as a building material, it requires careful handling and very careful firing up to temperature the first time.  And it has a shelf life- it turns hard after 6-12 months in the bucket.
Castable refractory
A dry powder that is mixed like cement with water and casts into a material like fire brick.  Very strong and tough. Very sensitive to water volume-goes from a stiff mix with dry powder in areas to slop with a few ounces of water.
Insulating castable refractory
A dry mix that is mixed like cement with a lot of fluffy stuff that produces result that is like insulating fire brick. Also sensitive to water volume.  Should not be rodded, etc., because light parts will float and separate.  Does not achieve a good insulation value until fired - using it instead of brick to support heating elements works poorly - and is not very strong - when used freestanding usually cracks or breaks, much better to put under compression with a strap or sleeve.
Ceramic Fiber Blanket and Board
A modern material out of the space program, it works by actually reflecting heat at higher temperatures. It is normally white.  It is available in blanket form in various thicknesses and widths, normally sold as 100 board feet (so 1"x 2'x 50', or 2"x 2' x 25'), and as board 2'x 4' in various thicknesses.  When heated to higher temps, converts partially to hazardous substances that damage lungs if breathed, so current practice is to install behind castable or other surface treatment.  More Below.
Vermiculite Board and Pourable
Vermiculite is a processed result of volcanic action that puffs into lightweight chunks when reheated.  It is used loose for horticulture because it holds water and lightens soil.  It can withstand high heat and has been used as an admixture to cement to make an insulating castable.  It can be used as poured insulation, but it flows and blows and thus needs a tight shell around the body of a furnace to contain it.  It is also available as compacted board from Skamol, discussed below.
DOWN BELOW - Insulating values of various products and more discussion of Ceramic Blanket and Vermiculite Board.

National Refractories & Minerals A fair site, but lacking specific information such as density - which can be requested by e-mail.

NIST Property Data Summaries for Sintered Alumina ( Al2O3 ) Density 3.9 gm/cm3 or about 243 #/cu.ft.

Subject: Crucible Cracks
Date: Sun, 26 Nov 2000 15:23:00 -0600
From: Mike Firth
"Repair Cracks in Invested Pot"
Nothing can be done. Anything used to repair cracks will dissolve in the glass, making it less desirable. One of the disadvantages of an invested pot is that when (not if) the pot cracks, the glass goes into the investment and as the glass level rises and falls, crud sucked back into the glass.
On the other hand, one of the claimed advantages of investment is that you can continue to use the pot after it cracks.
Tank furnaces are designed so that as the glass goes between the bricks, it finds colder temps and "freezes" without damaging the bricks which are selected for the purpose. Investment insulation prevents the freezing until the glass is in and dissolving the investment and one of the negative arguments about investment is that you can not add massive insulation (more efficiency) without exposing more investment to dissolution.
Free standing pot furnaces have drains and removable fronts for replacing the pot when (not if) it cracks and are commonly made with 12" or more of insulation around them.

Thermal Conductivity Data (W/mK)

Mean Temp
























Colour Blueish-White
Classification Temperature 1260C
Melting Point >1760C
Fiber Diameter 2-3 microns
Specific Heat at 1100C 1130 J/kgC
Specific Gravity 2.63
Permanent Linear Shrinkage, 24 hour soak



Posted By: Graham Stone <>
Date: 3/6/2001 - 10:04 p.m.

I was disturbed to learn from Henry that fiber is a known carcinogen. Henry, please tell me you meant "suspected" carcinogen, as was the case some time ago. If things have changed and a link has been established between ceramic fiber and cancer it has massive ramifications for all of us. My (perhaps out of date) information is that no link has been made and that the reason the Europeans were shifting to the so called 2nd generation fibers was that they were taking a pro-active approach to cover their arses IN CASE the stuff is carcinogenic or damaging to the pulmonary system in some other way.

Ceramic Fibre (RCF) is available in blanket, board and paper forms. Kaowool brand from Thermal Ceramics (Morgan) and Fibrefrax brand from Unifrax (Carborundum) are the 2 biggest brands and essentially the same material. Unifrax also make a rigid paper form called 110 paper. If these products are known to be carcinogenic, it raises serious ethical, medical and legal issues for manufacturers and governments (not for the first time, huh?)

My understanding was that any danger from ceramic fiber was regarded as being on a par with conventional fiber glass (bad enough). As I understand it, the crystalline structure of the fibers and the ease with which they become airborne, coupled with the fact that the fibers are small enough to enter the lungs is what has given cause for concern.

The 2nd generation fibers are non-crystalline (technically a glass but different again to fiber glass). These are man-made alkaline earth silicate fibers that are rated low "biopersistence". What this means is that the fibers do not get embedded in the lung tissue. Essentially, they are coughed up.

Thermal Ceramics (Morgan) call theirs Superwool. Superwool 607 is rated to 1100C/2000F, Superwool 612 is rated to 1250C/2280F. Unifrax call theirs Carbowool (at least in this country) for the 1100C/2000F rating and Isofrax for a 1260C/2300F rating. These products are available in the same forms as RCF (with the exception of 110 paper) and usually cost about 20% more than their RCF counterparts. However, some people report that they are not as durable as their ceramic equivalents.

Wetfelt and Moistpack are fibre supplied with colloidal silica in wet solution for molding. Colloidal silica=rigidizer or hardener. It's much cheaper to buy your own rigidizer and spray it onto the fiber than to buy the wet product and usually you don't need complete saturation. Nevertheless, it's my belief that, worthwhile though this practice is, it really works by prolonging the inevitable, or more accurately, simply slowing down the rate of dispersion into the atmosphere.

"QF 180" is a high temperature fiber adhesive for cementing bits of fiber together, but not for covering.

A host of other products are available for "sealing in" fiber insulation. These include Top Coat M, Zirconia Wash Coat, Alpha Maritex (learned this one from Henry) and Dip Lag (learned this one from Cynthia). But what I regard as the best and most long lived is Refrasil, a woven cloth made from larger particled silica threads. Unfortunately, it's also the most expensive but it's the old story. You get what you pay for. -Graham 169

Tom Ash on CraftWeb - Fiberfrax LDS moldable consists of ceramic fibers dispersed in a sticky water based refractory binder. It's kind of like a high temp painters caulking/putty and is even available in a caulk tube that fits standard caulk guns, as well as one and five gallon buckets. The caulk tubes work great for doing the element grooves. It sticks like crazy and fires hard. Manufactured by the Carborundum company. Denver Glass (303-781-0980) sells it as a repair/patching material for their compressed fiber glory hole liners. Not cheap, but great stuff.


Following a table of insulation values for pourable vermiculite is one on Skamol pressed board
Nominal Thermal Resistance Values
Thermal Resistance (a) 0F .h.ft2/Btu (K.m2/W)

Mean Temp.
0F (0C)

16 mm

8 mm

4 mm

2 mm

4-Super Fine
1 mm

-199 (-84)





3.4 (0.59)

-13 (-25)





2.7 (0.48)

75 (24)

2.3 (0.40)

2.3 (0.40)

2.3 (0.40)

2.3 (0.40)

2.3 (0.40)

212 (100)





1.8 (0.32)

302 (150)





1.6 (0.28)

662 (350)





0.94 (0.17)

850 (454)





0.73 (0.13)

(a)The nominal thermal resistances in this table are for 1.0 inch (25.4 mm) of thickness

h represents the heat flow per unit area per unit temperature difference. The larger h is, the larger the heat transfer Q.
The R-value is used to describe the effectiveness of insulations, since as the inverse of h, it represents the resistance to heat flow. The larger the R, the less the heat flow .
R  1F-ft2-h/Btu (therm.) = 0.176228 K-m2/W kelvin meter squared per watt

Skamol Vermiculite Board
a discussion that interlocked across several newsgroups points at this Heta UK Skamolex Vermiculite Panels with wondering about being sold in USA.  It turns out that Bullseye and Seattle Pottery carry it.
I asked the company for samples to see how it crumbled, etc., and got 3x4x1" blocks and a nice notebook.  The hard board seems suitable for walls of kilns, especially well below its fairly high temperature ratings (typ.1100C, 2012F) and is supposed to take grooving for elements.  Since it is created by pressure without significant binders, it should not burn out.
 Three of the samples sent are various densities of V-1100 which is designated for back-up or exposed. The densities are 375, 475 and 600 kg/m3 (23.4 to 37.5 lbs/ft3 compare to water at 1000 kg/m3 or 62 #/ft3)  The higher densities have a higher compressive and rupture strength (1:3 approx) and lower porosity (86:76).  Conductivity increases by about 25% (0.16-0.19 W/(m K)@600C) while specific heat (0.94KJ/(kg K) or 0.224 BTU/(lb F) varies not at all.
Analysis (in part) shows 47% silica, 21% magnesium oxide, 11% potassium oxide, 7% alumina, 4% ferric oxide, balance 3 other oxides and 7% loss on ignition.  Shrinkage is 1%.
 Other densities can be ordered if needed; a data sheet for 410 is included.  Special shapes can be molded. 
 Skamol makes other products including samples sent for VIP-12 (1200 kg/m3) intended for backing cryolite (aluminum ore) cells because it reacts with and blocks leaks) and calcium silicate backup insulation that is very light (0.10 conductivity, 225 kg/m3) and smooth.  Mortars for assembly are also sold.
 "SKAMOLEX BENEFITS: Vermiculite is inorganic, No emission of toxic gasses, No emission of smoke or odor, Non-combustible, No known health hazards,
The types of vermiculite used by Skamolex are examined at the National Institute of Occupational Health, Denmark (Arbejdsmilj Instituttet) on a regular basis to assure no content of asbestos-fibres and quartz.
SKAMOLEX panels are made from exfoliated vermiculite having excellent insulating properties.
Vermiculite is the geological name given to a group of hydrated laminar minerals, which are aluminum-iron magnesium silicates."

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  "Correll has developed a simple way of comparing furnace insulation by calculating equivalent inches of firebrick (E.I.F.). Firebrick is a commonly used construction material for furnaces.  The value of firebrick shall be defined as 1; all other materials insulation properties will be defined as equivalent inches of firebrick.   The table to the right shows the equivalent inches of firebrick for common furnace building materials. "
Used no other place on the Internet, per Google Search. EIF has dozens of meanings.  Exterior Insulation and Finish is closest. Note that fiber insulation is not included in listing. 2009-07-22


Equivalent Inches of Firebrick for Common Furnace Building Materials

The Conduction Value (given below for various products) is (Btuin/hrft2F) the flow of energy in Btu (British thermal units) through each inch of thickness, per hour, per square foot, per degree Fahrenheit difference in temperature across the insulation.  Thus, thicker insulation will cut down the flow, greater temps will increase the flow as will more area.  Also, as the tables make clear, most insulations decrease in quality (have a higher conduction value) at higher temperatures.  This is not just the greater flow with a difference in temperature - if Insboard (the first in the the list) has an inside temperature of 800F and outside of 750F the conduction will be 0.6 (about) while if the temperatures are 1200F and 1150F, the value is 0.8 and one third more heat will be conducted by the 50F difference at the higher temperature.  Almost all the products below show this kind of change; the few that don't have rather high conduction rates to start with. 2004-12-06

Arranged from two posts to CraftWeb, with the group of generally lower conductivity products first, higher after, with some over lap.

Posted by Edward Dluzen
Additionally there is from Zicar
MICROSIL Microporous Insulation
Max Use Temp 1742F
    68F     .132
  392F     .145
  752F     .173
1112F     .208
1472F     .263

Alumina Mat
Max Use Temp 3002F
  315F       .50
1000F       .77
1400F     1.03
1796F     1.33
2192F     1.65
2597F     2.15

Alumina Blanket Type AB & MB
Max Use Temp 2912F
  599F       .50
1000F       .70
1400F       .90
1796F     1.25
2192F     1.60
2597F     2.15

Alumina-Silica Blanket Type ASB-2300 & ASB-2600
Max Use Temp 2300F and 2600F
Bulk Density of 4 pcf
  500F       .56
1000F     1.13
1500F     1.96

Bulk Density of 6 pcf
  500F       .45
1000F       .94
1500F     1.70
2000F     2.80

Bulk Density of 8 pcf
  500F        .39
1000F       .78
1500F     1.31
2000F     2.02

Bulk Density of 10 pcf
1000F       .73
1500F     1.17
2000F     1.72

Alumina Insulation Type ZAL-15 & ZAL-15AA board
Max Continuous use 2822F & 2732F
482F     .40
977F     .60
1472F     .90
1967F    1.30
2282F    1.70
2462F    1.80

Alumina-Silica Insulation Type AXL Board
Max Continuous Use 2300F
752F     .80
1472F     1.20
2012F     1.50

Alumina-Silica Insulation Type AXHTM Board
Max Continuous Use 2600F
752F     .60
1472F     1.00
2012F     1.70
In regard to Zircar products, they are usually quite pricey.
The Thermal Ceramics BTU-Block Board works out to $13.90 per sq ft for one inch thickness via website:   search for Thermal Ceramics. 
The Zircar Microsil Microporous Insulation works out to over $25.00 per sq ft in one inch thickness from the Zircar website:  search for microporous

Posted on Craft Web by David Paterson 2004-10-22
edited this post to add the figures for one of the Microporous Insulation products from Thermal Ceramics, BTU Block Board.
Here Are The Insulation Values Of Different Materials

Insboard 2600 HD Ceramic Fiber Board
26 lbs/cu.ft.


Skamol SUPER-1100 E Calcium Silicate Insulating Board
15.3 lbs/cu.ft. (backing insulation)


Thermal Ceramics Microporous Insulation
BTU-Block Board
18 lbs/cu.ft.


Inswool HP Blanket

6 lbs/cu.ft

8 lbs/cu.ft.

G-26 LI Insulating Firebrick
48 lbs/cu.ft.


Kastolite 30 LI
90 lbs/cu.ft.


Mizzou Castable
139 lbs/cu.ft.

Greencast 94
163 lbs/cu.ft.

Last edited by David Paterson on 10-17-2004 at 11:27 PM

List to left in strict conductivity order of one rating highest or fairly high.
MICROSIL Microporous Insulation
1472F     .263
Thermal Ceramics Microporous Insulation
BTU-Block Board  18 lbs/cu.ft.
1500F  0.30
Skamol Calcium Silicate Insulating Board
15.3 lbs/cu.ft. (backing insulation)
1112F   0.69
Insboard 2600 HD Ceramic Fiber Board  26 lbs/cu.ft. 
2000F   1.2
Alumina Blanket Type AB & MB
1796F     1.25
Alumina Mat
1796F     1.33
Blanket 8 lbs/cu.ft.
1600F   1.4
Alumina-Silica Type AXL Board
2012F     1.50
Type AXHTM Board
2012F     1.70
Alumina Type ZAL-15 board
2282F    1.70
Blanket Bulk Density of 10 pcf
2000F     1.72
Inswool HP Blanket  2300F 6 lbs/cu.ft
1600F   1.85
Alumina-Silica Blanket Type ASB-2300 Bulk Density of 4 pcf
1500F     1.96
Blanket Bulk Density of 8 pcf
2000F     2.02
G-26 LI Insulating Firebrick  48 lbs/cu.ft.
2000F    2.6
Blanket Bulk Density of 6 pcf
2000F     2.80
Kastolite 30 LI   90 lbs/cu.ft.
2000F    4.4
Mizzou Castable  139 lbs/cu.ft.
2000F    7.4
Greencast 94  163 lbs/cu.ft.
2000F   13.6
Graph showing relative conductivity of numbers at left in same order as at left.

Graph of relative insulation values of refractories


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