Asbestos paper machine (Johns-Manville Corp) Rosato (C) InspectApedia Asbestos Heat & Sound Insulation
Asbestos Insulation Products

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Asbestos insulation:

History, manufacture, visual identification: how to recognize asbestos heat & sound insulation in buildings - a visual guide to identifying asbestos in buildings.

This articles series about the manufacture & use of asbestos insulation and related asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials. Page top photo: asbestos paper machine, finishing end. This machine produced rolls of asbestos paper at a rate of 50 tons in 24 hours. The last reel of asbestos paper (shown) weighed 2,500 to 3000 lb.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Asbestos Heat & Sound Insulation Product History & Manufacturing Processes

Asbestos insulation label Johns-Manville Corp (C) Daniel FriedmanThe following text is Adapted from Rosato (1959) p. 94-105 [1] © 2013

Asbestos Insulation Article Series Contents

Asbestos insulation was produced in the form of asbestos papers, corrugated asbestos papers, asbestos-cement millboards, and asbestos-enriched fiber molded products. These developed into a large and valuable industry principally because of their heat and sound insulation combined with their incombustibility.

At left: label from asbestos pipe insulation, a Johns-Manville product, collected by the D. Friedman and photographed in our laboratory.

Chrysotile asbestos is the principal source for the insulation material. The Grade 5 Canadian fibers are generally referred to as asbestos paper products.

Blue Asbestos Sheet & Board Products

Sheets or boards made from blue asbestos are not very suitable for either heat or electrical insulation. These types of fibers are apt to be too stratified and do not provide sufficient strength to resist puncture at the higher temperatures.

Examples of Types of Asbestos Insulation

Examples of types of asbestos insulation are: outer jackets for industrial furnaces, linings for safes or fire-doors, pipe insulation, electric heaters, incinerators, drying ovens, incubators, locomotives, panelling for garages, storage bins for combustibles, automobile hoods, electrical switch rooms, electrical apparatus, packings, gaskets, etc.

Naval ships use large quantities of asbestos insulation. The United States Navy has been one of the prime movers in the development of asbestos heat and electrical insulation. Asbestos has a distinct advantage as heat-insulation material inasmuch as it can be converted to a very loose state.

It will retain this form even at continuous temperatures as high as 700 11 C. The insulating agent in these products is really the trapped air.

Hardcast asbestos on heating boiler © D Friedman at Asbestos is of little use as an insulator at low temperatures when atmospheric moisture condenses on it. Wet asbestos, even in a loose form, ceases to be an insulator.

For low temperature or cold insulation, the air bubbles within the insulator have to be completely sealed off and protected against absorption of water with such water proofing materials as asphalt, silicates, and cements.

When reviewing the subject of insulation, it is important to appreciate both temperature and time conditions. Generally, insulation such as the type used for pipes and ovens concerns long time exposures which can range from one to twenty years.

At left our photo illustrates asbestos insluation on the exterior of a heating boiler.


However, it is important to appreciate that insulation requirements also exist for such extremely short time conditions as those encountered in modern day rockets and missiles.

These types of insulation products generally have no similarity to the longer time conventional type of insulators.

Asbestos-phenolic molded plastic insulators are presently being used in rockets and missiles where temperature conditions ranging from 3,000 to 20,000°F are encountered. Time durations range from seconds to minutes. A review of these types of insulators is given in the chapter on plastics.

Asbestos insulations are very diversified. In most cases, the fibers are combined with other such lightweight materials as magnesium carbonate (85 per cent magnesia).

With wet process insulating materials, small quantities of clay are added as a binder and in some cases silicates are added. Insulation on steam-pipes can be applied wet. They are bound with clay, silicates, or similar binding materials. Drying is effected by the application of steam through the pipes.

Tremolilte asbestos ceiling insulation (C) Daniel FriedmanIn order to reduce the cost and the time for installing continuous sheet asbestos, methods have been devised where asbestos is sprayed on walls and ceilings.


Asbestos applied in this manner provides for thermal insulation, fire protection, sound insulation, resistance to corrosion when metal surfaces are covered, and elimination of wall "sweating." For sound insulation and acoustical purposes, fiber up to 1 /2 in. in length is used.

Photo at left: nearly pure tremolite asbestos insulating blocks glued to the basement ceiling of a commercial building in White Plains, NY © Daniel Friedman.

Silicates have been the primary binders for paper.

Asbestos corrugated paper is made by crimping ordinary asbestos paper and pressing it into permanent corrugated shapes.

Depth of corrugation is approximately one-quarter inch. The sheets of the corrugated paper are covered with flat asbestos cement sheets to form sandwich structures.

The nodes of the corrugated sheets are treated with sodium silicate to provide for adhesion. This type of insulation provides for large volumes of dead air space; it is sometimes referred to as asbestos air cell insulation.

Asbesos miliboard is generally classed with paper inasmuch as it is actually a thick asbestos paper, just as cardboard is considered a thick wrapping paper.

The miliboard is a rigid type of asbestos board insulation that cuts or drills easily and can be nailed or screwed to supporting beams. It can withstand temperatures up to 1,000°F without smoking.


Asbestos Paper & Paper Insulation Manufacture, Properties, Uses

Asbestos fibers from asbestos paper duct wrap - lab  photo (C) Daniel Friedman

Two standard types of paper machines are used to manufacture various asbestos paper products.

These machines are typical of the standard pulp paper making machines; i.e., the Fourdrinier machine and the cylinder machine. Typical of the pulp paper business, modifications have been made on these types of machines in manufacturing asbestos products. The more general type of paper asbestos making machine used is the cylinder.

At left: asbestos fibers collected from a sample of asbestos paper duct wrap, taken through our lab microscope [DF].

The standard method of making paper is essentially the same for all types of machines.

The web of paper is first formed on a traveling wire in the case of the Fourdrinier, or on a cylinder mold of the cylinder machine.

The wet sheet is further dewatered by the press section of the machine, dried out by various types of dryers and finished or smoothed out by calenders.

Fourdrinier Paper Machine for Making Asbestos Paper Products

Fourdrinier asbestos paper machine, Adapted from Rosato (1959) (C) InspectApediaA sketch of the Fourdrinier machine is shown in Figure 5.1 at left [click to enlarge]. The asbestos fiber which goes into the paper machine beaters is very finely ground.

During the processing of the stock, extreme care is taken in order to eliminate impurities and knots. Sometimes the stock is sent through a sand trap and strainers before it passes onto the paper machine.

Water is the medium used to carry the asbestos pulp. This water, generally referred to as white water, is recirculated through the paper machine.

When the stock is initially made, it can contain approximately three per cent fiber. After it has left the stock chest, where it initiates, the high fiber content stock is diluted with white water to a stock of approximately one-half per cent.

The stock chests are cylindrical in shape. They are lined with noncorrosive metal to minimize contamination of stock. Agitators keep the stock moving and also provide for correct consistency.

By gravity, the stock is deposited on a wire screen.

A large percentage of the water drains from the stock through the wire screen and through suction boxes or rolls located next to the stock being carried over the wire screen. Generally, the stock is carried from the wire screen to woolen felts. The felt is an endless blanket similar to the wire screen which is also endless.

These materials continually move and carry the paper pulp. During this operation, the paper stock can be pressed between one or more sets of pressure rolls.

After this operation it is carried around steamed-ash heated dryers where the stock which is approximately 60 per cent dry becomes 95 per cent dry.

Felts are also used to carry the paper around the dryers. The next step is for the paper to go through calenders where it is given a smooth surface. The product is finally wound on a reel which completes its cycle. See Figure 5.2.

Asbestos paper machine (Johns-Manville Corp) Rosato (C) InspectApedia

Figure 5.2. (Courtesy Johns-Manville Corp.)

Asbestos paper machine (finishing end) . This machine produces about 50 tons of paper in 24 hr. The last reel weighs 2,500 to 3,000 lb.

Paper sheets have been made using all asbestos fiber or the various mixes such as asbestos pulp, asbestos sisal, asbestos glass, etc. Inasmuch as asbestos is an extremely fine fiber, it provides for improved blendings of the thicker or coarser fibers.

Experience is required in order to determine the proper combination of fibers and to determine also the proper technique of producing suitable asbestos paper products.

Control of the white water is important in order to reduce fiber waste in the mill. However, it is also important to recognize that the amount of short fiber in the white water will affect the solidity of the finished paper product.

Modern paper machines have reclaiming systems; very little fiber is lost and the recirculated fiber is used to its maximum efficiency.

Cylinder Asbestos Paper Machine

Asbestos paper used as HVAC duct wrap (C) Daniel FriedmanVarious types of machines are available which basically involve rotating cylinders.

Asbestos stock is directed into a large vat which contains a revolving cylinder. The cylinder and vat should be made of noncorrosive material such as brass, copper or plastic.

As the cylinder rotates in the stock, fibers are collected on the cylinder. Various types of pulp beaters are available such as Hollander beaters. The beating or agitating action is generally performed by paddles located on the cylinder.

Photo at left illustrates asbestos paper used as heating duct wrap on the exterior of metal air ducts.

Details are at ASBESTOS PAPER DUCT INSULATION. Click to enlarge.

The paper stock which collects on the drum is passed onto a belt. Generally, the paper is sandwiched between two felts and directed over suction boxes in order to remove some of the water. The paper is directed through roller presses in order to continue to drive out the water.

Drying cylinders or. cans similar to the Fourdrinier machine are also used for removing most of the remaining water. Then, this paper is either rolled on reels or stacked. The thickness of the paper and its directional strength are affected by such variables as rotational speed of drum, type of agitation of the stock in the vat, and type or combination of fibers used.

Details about asbestos in or on HVAC ducts and similar products are at

Asbestos Millboard Manufacturing Process Details

Asbestos millboard (C) Daniel FriedmanRosato detailed the production of asbestos millboard. He described asbestos millboard as a "heat insulation" product, an apellation with which we disagree.

Because of its dense and cementious nature asbestos millboard, while fireproof, can conduct heat towards combustible materials that it might cover, such as wood framing.

Without an adequate air space or other insulation, heat conducted through non-combustible materials can still result in a fire. (Rosato 1959, 100-102)

Asbestos miliboard is made of very short asbestos fiber. The fiber must be uniformly fine and free from all foreign material and impurities. However, the uniformity of fiber is more critical in asbestos paper manufacture than in millboard manufacture. Asbestos fiber is placed in pulp beaters.

From the beater, the pulp moves to a stirring vat which is approximately three times larger than the beater. Buckets are used to remove the pulp or slurry from the vats to strainers. The pulp is subjected to varied screening and agitation in order to produce a homogeneous product.

After the filtering actions have been completed, the pulp is directed through pipelines to the millboard machine; this machine consists of a screen, press, and mold. The screening part consists of large boxes containing revolving screen drums. The watery mixture is kept moving with a paddle; it is filtered through the revolving screens.

The water is drained off through the interior of the boxes.

The next phase of the screening process involves draining the sheet by means of pressure applied by a conveyor belt on the screen. The conveyor belt carries the sheet to the pressing part of the machine. In the pressing operation, the porous conveyor belt moves under a pressing cylinder which squeezes out the water with high pressure.

Millboard machines are also available which contain more than one revolving screen. This type of machine provides for plying sheets at the pressing cylinder in order to produce thicker sheets.

The resulting plied sheet becomes homogeneous after the pressing operation.

The next important operation in the miliboard machine involves conveying the sheet to the molding cylinder. The molding cylinder is placed above the pressing cylinder. Both are connected by endless conveyor belts carrying the pulp layer from the pressing cylinder to the molding cylinder. The conveyor belt is carried along through the action of friction between the two cylinders.

Only the pressure cylinder is power driven. When the pulp comes in contact with the molding cylinder, it is transferred directly from the felt to the cylinder. This transfer occurs inasmuch as the adhesion to the wet surface of the cylinder is greater than the adhesion to the felt.

The molding cylinder determines the size and thickness of the millboard. Convolute wrapping of the pulp is done on the cylinder. Its circumference is generally equal to a single or double width of commercial millboard. Notches are located on the cylinder parallel to the direction of the axis so that when the miliboard has to be removed, a knife cut is made through the notch.

After the cutting operation, the sheets of asbestos pulp still contain a high percentage of water. Most of the water is removed by further pressing the sheets between rollers or in presses. The final operation involves subjecting the sheets to the drying process. Drying ovens are used where the sheets are either passed through on a continuous moving belt or suspended in the ovens. Drying temperatures are in the range of 180 to 200°F.

The pressing operation in the millboard machine is different from that of the paper machine. The thin papers can be pressed and dried much more thoroughly and rapidly than the miliboard.

In the commercial type of millboard machine, in order to produce one pound of finished miliboard, approximately 500 lb of water is required in the mixing vat. Means of conveying, storing, and pumping water are of major importance in this type of operation

Details about asbestos cement sheet products used in roofing, siding, and panels are provided


Asbestos cement board is a hard cementious product easily distinguished by eye from more friable and dangerous soft asbestos panels used as building fireproofing such as these almost


More about this building material can also be read


Asbestos Felt Manufacturing Procedure & Use in Roofing

Asbestos felt and papers impregnated with asphalt are used on the roofs of buildings and homes where resistance to outdoor weathering and insulation is important. The material, used as a flashing on walls, can be applied to concrete, brick, etc. Ordinary roofing felts are made of vegetable fibers which are organic in composition and do not have the permanence of asbestos felts.

Roofing sheetscan be made up of single layers or plied layers saturated, or coated or both saturated and coated with asphalt compositions.


In certain applications, such materials as wire reinforcements are added. The finished sheet materials are generally supplied as flat sheets or wound up in rolls. The application of these types of products to roofs, generally involves one layer of asphalt saturated rag felt, covered with asbestos perforated felt, and then completely coating the felts with asphalt.

The asphalt material suitable for impregnating felts is usually soft in consistency at room temperature, and ranges from semiliquids to semisolids. The types of asphaltic substance include pure native asphalts, residual asphalts, wood-tar pitch, sludge asphalt, and wurtzilite asphalt.

A white asbestos-coated roofing felt is made by hot pressure rolling a layer of asbestos felt onto asphalt-impregnated roofing paper. The layer is made water repellent and more adherent to the asphalt paper by impregnating the asbestos felt with 5 to 10 per cent stearic acid.*

* Clarvoe, G. W., (to Johns-Manville Corp.) U.S. Patent 2,593,010 (April, 1952).

Magnesia-Asbestos Insulation Manufacture & Properties

Magnesia-asbestos insulation is a very well known and popular product used for fire proofing and heat insulating. This type of insulation is generally identified by the name of 85 per cent magnesia.

Eighty-five per cent magnesia derives its name from the fact that it contains not less than 85 per cent of magnesium carbonate. Asbestos fiber is incorporated to act as a binding and reinforcing agent. The fiber principally used is chrysotile; crocidolite and amosite are used also.

The manufacturing process involves thorough mixing of he constituents with sufficient water to permit easy handling in a filter press. The products can be pressed into different shapes. Generally, three days in steam-heated kilns will dry out the product.

Its density is 12 lb per cu ft. In comparison, a solid block of magnesium carbonate weighs 130 lb per cu ft. The 85 per cent magnesia contains approximately only 10 per cent solid material by volume. The remaining 90 per cent is air and it provides for effective insulation.

This product produces exceedingly low thermal conductivity since its structure contains minute dead air cells formed by the interlocking crystals of magnesium carbonate. The product can withstand temperatures as high as 600°F without loss of any insulating value.

When it is subjected to alternate heating and cooling, it does not crack; it is not affected by structural changes which would change its insulating properties. Direct impingement of 2,000°F flame produces only small surface cracks. After 378 hr of intermittent immersion in boiling water no change was observed in its insulating effectiveness.

Average figures for thermal conductivity (K factor) follow. Thermal conductivity, Btu in. per hr per sq ft per °F:

Thermal Conductivity of Magnesia-Asbestos Insulation

Mean Temperature
Thermal Conductivity
100 degF /35
200 degF .38
300 degF .42
400 degF .46

When insulation must operate above 600°F, it is used with diatomaceous silica insulation. Diatomaceous silica occurs naturally; it consists of fossilized plants called diatoms. These diatoms are mixed with asbestos fiber and the resultant insulation can be used in the range of 600°F to 1,000°F.

When the two materials are used together, the application is called combination insulation. The diatomaceous silicaasbestos insulation is applied to the hot surface in sufficient thickness to reduce the temperature at its outer surface to 600°F or lower.

Eighty-five per cent magnesia insulation is used as the outer insulation. This double layer construction effectively utilizes the greater heat resistance of diatomaceous silica as well as the lower thermal conductivity of 85 per cent magnesia.

Diatomaceous silica-asbestos blocks can weigh as much as 40 lb per cu ft. In order to reduce the weight, asbestos fiber, hydrated lime and diatomaceous silica have been used with binders of sodium silicate and calcium chloride. In one application, four parts of asbestos fiber and four parts 42-degree sodium silicate are slurried in fifty parts of water.

One part of calcium chloride dissolved in two parts of water is added and agitation is continued until the reaction is complete. A voluminous precipitate of calcium silicate in the hydrous form results from this reaction. At that point, two parts of diatomaceous silica and two parts of hydrated lime are introduced.

This mixture is run into the mold, the charge dewatered and pressure is applied. Weights as low as 20 lb per cu ft can be obtained with the usual hard diatomaceous silica type insulation.

Asbestos Spray-on vs Asbestos-Foam Laminate Insulation Products

Reader Question: was there asbestos used in spray foam insulation in the 1970's?

I was hoping you might be able to tell me if there was Asbestos used in spray foam for insulation in the 70's

My son in law passed from Mesethlioma last year and in the 70's when he was about 8-10 years old lived on a hippie commune where they built Domes out of fiberglass and steel and then sprayed the foam for insulation. This was in Warren County Virginia. - Anon. [by private email] 27 August 2015

Reply: use of asbestos in asbestos-foam laminate products

Asbestos-foam laminate product - US Patent app.Asbestos fireproofing spray material was used in buildings but as a "foam" product - not that I've come across.


To be clear, when wet, spray-on asbestos coatings looked "foamy" but this is not a plastic foam product such as UFFI, polystyrene, icynene, etc.

See also FOAM SPRAY INSULATION TYPES To recognize those.

There were in fact some asbestos foam and some asbestos-foam-laminate products or asbestos laminated to foam products; often asbestos material was laminated to a foam substrate as you will read below.

This is not the same fireproofing approach as spray-on asbestos coatings cited above.

The illustration of an asbestos-laminate foam product at left is from US4256799 A by Ohashi, published in 1981, filed in 1979 and cited in detail below. This was a multi-layer product incorporating asbestos paper as an upper layer (1), a matted or woven glass fiber as a middle layer, and a metal foil layer, with a foam core. The object was to construct a "fireproof" [sic] foam board type product.

Similarly, Gerek (1970) patented asbestos foam laminates illustrated just below.

Asbestos-foam laminate product - US Patent app.

A patent search for asbestos+foam finds:

Guides for Visual Identification of Asbestos-Containing Products Found On or In Buildings


Continue reading at ASBESTOS ELECTRICAL WIRE INSULATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ASBESTOS BUILDING INSULATION - forms, types, locations of asbestos used as building insulation




Or see INSULATION MATERIAL IDENTIFICATION GUIDE identifying various insulation materials found in buildings.

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