In addition to solving technical challenges, it is a constant task of industrial furnace manufacturers to apply all their know-how to limit the health risk of their employees and users of their furnace technology.


The Technical Rules for Hazardous Substances (TRGS) offer the state-of-the-art of occupational medicine and hygiene and other proven knowledge about activities involving hazardous substances, including their classification and labeling. These rules specify requirements of the Hazardous Substances Act within its scope of application.

With the complete substitution of aluminum-silicate wool in its product range, Nabertherm GmbH was successfully able to comply in total with the requirements of TRGS 619, setting new standards in the furnace industry.

Scope of Application

TRGS 619 explains the substitution possibilities of amorphous aluminum-silicate wool products, which are primarily used for thermal insulation in furnace and incinerator construction, heating systems and exhaust systems for motor vehicles, especially for application temperatures above 900˚C.

The substitution follows the goal to eliminate or reduce to a minimum the hazard entailed in activities when dealing with hazardous substances.


Aluminum-silicate wool, also known as ceramic fiber (refractory ceramic fiber = RCF), consists of amorphous fibers produced by melting a combination of Al2O3 and SiO2, usually in a 50:50 weight ratio (see also VDI 3469 Sheet 1 and Sheet 5 and TRGS 558). It can also include ZrO2. RCF products are mainly used at temperatures above 900°C and primarily in equipment that operates intermittently or under intermittent application conditions.

AES wools (alkaline earth silicate wools = high-temperature glass wools) consist of amorphous fibers produced by melting a combination of CaO, MgO and SiO2 and are intended for high-temperature applications. AES wool products are generally used at application temperatures of up to 1200°C (2192°F) and in continuously operating equipment and domestic appliances.

Polycrystalline wools (PCWs) consist of fibers with an Al2O3 content above 63 wt. % and a SiO2 content under 37 wt. %. They are produced from aqueous spinning solutions in the “sol-gel method.” The water-soluble green fibers formed initially as a precursor are then crystallized by means of heat treatment. Polycrystalline wools are generally used at application temperatures above 1300°C (2372°F) and in critical chemical and physical applications.

Determination of Substitution Possibilities

The employer is obliged to always check what hazards can arise during the use of refractory products. The substitution solution must achieve an overall reduction in the dangers posed by hazardous substances at the workplace. At the same time, it should not lead to an increase in other hazards at the workplace or to an increased impairment of other goods to be protected (e.g., fire and explosion hazards, furnace breakouts accompanied by the escape of molten materials).

Hazardous Properties of Fibrous Dusts from High-Temperature Wools

Elongated particles have a carcinogenic effect if they are sufficiently long, thin and biostable. Fibers that meet the TRGS 619 criteria under paragraph 2 are deemed to be sufficiently long and thin (critical fibers).

Potentially carcinogenic fibrous dusts can be released during activities involving RCFs and PCWs. According to current scientific knowledge, a risk of cancer cannot be ruled out in the event of inhalation of these fibrous dusts. The fibrous dusts released are assessed as a category-2 or category-3 carcinogen in accordance with TRGS 905 “List of substances that are carcinogenic, mutagenic or toxic for reproduction.”

Accordingly, fibrous dusts from RCFs are to be assessed as category-2 carcinogens (substances that are to be regarded as carcinogenic for people). There are sufficient indications that exposure to the substance may cause cancer.

Within the meaning of TRGS 905, under the term “all other inorganic fiber dusts” (number 2.3 para. 6 of TRGS 905), fibrous dusts of polycrystalline wools (PCW) are to be assessed as category-3 carcinogens (substances that are of concern because of a possible carcinogenic effect on people, but about which there is insufficient information for a satisfactory evaluation).

Fibrous dusts from AES wools are not classified as carcinogenic.

TRGS 558 “Activities involving high-temperature wool” describes protective measures for activities involving high-temperature wools.

Substitution Principles

Employers are obliged to ensure that any risk posed to the health and safety of employees by a hazardous material at the workplace is eliminated or minimized by the measures defined in the risk assessment. To meet this obligation, the employer should preferably arrange for the substitution of the hazardous material.

In particular, the employer should avoid activities involving hazardous materials or should substitute hazardous materials with substances, mixtures, products or processes that are not hazardous or less hazardous to the health and safety of employees in the respective application conditions. As a matter of priority, the employer must check whether a substitution is technically possible for products made of RCF.

A substitution should be examined within the framework of an overall assessment based on the entire life cycle of the possible products used. Products made of aluminum silicate wool must always be substituted if:

  1. The technical properties (application temperatures, thermal-insulation properties, long-term behavior and service life) are equivalent.
  2. Lower overall health risks exist for employees throughout the entire life cycle.

Additional reasons for considering the use of substitute solutions can include costs, environmental-protection aspects and energy and resource efficiency. It must be emphasized, however, that higher costs incurred for a substitute solution do not automatically result in a do-not-use assessment. In particular, if the substances to be substituted pose a high risk, greater weight must be attributed to the reduction of risk.

The result of the substitute selection must be documented in the risk assessment and disclosed to the competent authorities on request.

Products that do not contain fibers classified as category-1 or -2 carcinogens can be used as fibrous substitutes with a lower health risk while satisfying the requirements of application temperature and other application conditions.

Implementation of the Substitution of Aluminum Silicate Wool Products in Industrial Furnace Design

Due to their good insulation ability, aluminum-silicate wool fibers (RCF) were typically used for operating temperatures ranging from 900-1400°C (1652-2552°F) for many years. By classifying these fiber materials in category 2, it is mandatory to examine the possibilities of substitution and to initiate this where it is technically possible. The argument of higher costs alone is not sufficient to continue to hold on to RCF.

AES-fiber materials, which are classified as non-critical, can often not be used as the only substitute for RCF due to their product characteristics. PCW fiber materials are the technical alternative in many cases. However, the significantly higher procurement costs of these high-temperature fibers complicate the transition.

TRGS 619 in adapted interpretation has been applied in many European countries and outside Europe. In countries like France or the U.S. state of California, huge company groups have not allowed the use of carcinogenic fiber materials for many years. After the revision of TRGS 619 in 2013, manufacturers of insulating materials responded and widened their product portfolio by mixing fibers (e.g., from PCW and AES materials) to close this gap. Until now, however, the international furnace industry has hesitated to make use of these substitution alternatives partially for technical but mainly for cost reasons.

Nabertherm GmbH has, from the beginning, intensively analyzed the possible substitution of RCF by suitable alternative materials.

In 2015, the first kiln series was delivered RCF-free from Nabertherm GmbH. Substitution possibilities were determined for all furnace families by the Nabertherm design team, which has 65 engineers. Almost every furnace model was touched and structurally modified so that RCF fibers are not used in the hot side or as rear insulation.

Nabertherm can now proudly report that its entire furnace range has been changed in close cooperation with leading international fiber suppliers. Today, almost no standard furnace delivered to customers is insulated with RCF-fiber material. Substitution could be almost cost neutral in most cases utilizing alternative design solutions. Costs should not be a reason to not use non-carcinogenic fibers in the future. Classified fibers must only be used for a few applications where no alternative to RCF fibers exists. These are handled very restrictively, however, and will be mutually agreed upon with the customer.

Nabertherm GmbH employs more than 450 employees at its manufacturing plant in Lilienthal/Bremen in northern Germany. Every year, more than 7,500 kilns and furnaces with working temperatures of 30-3000°C (86-5432°F) are produced and delivered for arts and crafts, laboratory, dental, thermal-process technology, advanced materials, glass and foundry.

As a manufacturer of high-quality German-made products, Nabertherm GmbH feels obliged to position itself at the top of the industry and offer leading products with respect to quality and safety, particularly in health-related issues.


For more information: Contact Frank Bartels, vice president sales, Nabertherm GmbH, Bahnhofstr. 20, 28865 Lilienthal, Germany; tel: + 49 (4298) 922 0; e-mail:; web: The German TRGS 619, May 2013 edition was an information source.

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