ISO/TC 28 - Petroleum and related products, fuels and lubricants from natural or synthetic sources

This document specifies methods for the calibration of tanks above eight metres in diameter with cylindrical courses that are vertical. It provides two methods for determining the volumetric quantity of the liquid contained within a tank at gauged liquid levels. NOTE For optical-reference-line method, the optical (offset) measurements required to determine the circumferences can be taken internally or externally, provided that insulation is removed if tank is insulated. The methods are suitable for tilted tanks with up to 3 % deviation from the vertical provided that a correction is applied for the measurement tilt, as described in ISO 7507-1. These methods are alternatives to other methods such as strapping (ISO 7507-1) and the optical-triangulation method (ISO 7507-3).

This document specifies the process and methodology for the construction, operation, and maintenance of statistical control charts to assess if a laboratory's execution of a standard test method is in-statistical-control and how to establish and validate the 'in-statistical-control' status. It specifies control charts that are most appropriate for ISO/TC 28 test methods where the dominant common cause variation is associated with the long term, multiple operator conditions. The control charts specified for determination of in-statistical-control are: individual (I), moving range of 2 (MR2), and either the exponentially weighted moving average (EWMA) or zone-based run rules [similar to Western Electric (WE) run rules[3]] as sensitivity enhancement strategy to support the I-chart. The procedures in this document have been primarily designed for numerical results obtained from testing of control samples prepared from a homogenous source of petroleum and related products in a manner that preserves the homogeneity of properties of interest between control samples. If the test method permits, a certified reference material (CRM) sample is used as a control sample provided the sample composition is representative of the material being tested and is not a pure compound; if this is done then the laboratory best establishes its own mean for the CRM sample. This document is applicable to properties of interest that are (known to be) stable over time, and for data sets with sufficient resolution to support validation of the assumption that the data distribution can be approximately represented by the normal (Gaussian) model. Mitigating strategies are suggested for situations where the assumption cannot be validated.

This document specifies a method for evaluating the resistance of a lubricating grease to washout by water from a bearing, when tested at 38 °C or 79 °C under specified laboratory test conditions. This test method estimates the resistance of greases to washout from ball bearings under specified conditions. No formal correlation with field service has been established so far. This document is used for development and specification purposes. NOTE For the purposes of this document, the term “% (m/m)” is used to represent the mass fraction.

This document specifies the minimum requirements for mineral or synthetic based lubricants, as delivered, and intended for use in flooded rotary air compressors (vane and screw). This document can be read in conjunction with ISO 6743-3. NOTE For the purposes of this document, the term “% (m/m)” is used to represent the mass fraction of a material.

This document establishes a classification and specifies the minimum requirements for mineral or synthetic base stock-based greases for the categories CKG and CKL according to ISO 6743-6 intended for the lubrication of enclosed and open gear systems. This document does not cover the extreme cases of use in terms of temperature and extreme charges conditions. For use in exceptional conditions, suppliers and purchasers of lubricants are intended to mutually agree on the testing methods and the acceptability criteria of the products, not covered by this document. NOTE 1 This document can be read in conjunction with ISO 6743-6, ISO 6743-9[1], ISO 12924[3] and ISO 6743-99[2]. NOTE 2 For the purpose of this document, the terms “% (m/m)” and “% (V/V)” are used to represent respectively the mass fraction and the volume fraction of a material.

This document specifies the gas chromatographic (GC) method for the determination of saturated, olefinic and aromatic hydrocarbons in automotive motor gasoline and ethanol (E85) automotive fuel. Additionally, the benzene and toluene content, oxygenated compounds and the total oxygen content can be determined. NOTE 1 For the purposes of this document, the terms % (m/m) and % (V/V) are used to represent respectively the mass fraction, w, and the volume fraction, φ. This document defines two procedures, A and B. Procedure A is applicable to automotive motor gasoline with total aromatics of 19,32 % (V/V) up to 46,29 % (V/V); total olefins from 0,40 % (V/V) up to 26,85 % (V/V); oxygenates from 0,61 % (V/V) up to 9,85 % (V/V); oxygen content from 1,50 % (m/m) to 12,32 % (m/m); benzene content from 0,38 % (V/V) up to 1,98 % (V/V) and toluene content from 5,85 % (V/V) up to 31,65 % (V/V). The method has also been tested for individual oxygenates. A precision has been determined for a total volume of methanol from 1,05 % (V/V) up to 16,96 % (V/V); a total volume of ethanol from 0,50 % (V/V) up to 17,86 % (V/V); a total volume of MTBE from 0,99 % (V/V) up to 15,70 % (V/V), a total volume of ETBE from 0,99 % (V/V) up to 15,49 % (V/V), a total volume of TAME from 0,99 % (V/V) up to 5,92 % (V/V), and a total volume of TAEE from 0,98 % (V/V) up to 15,59 % (V/V). Although this test method can be used to determine higher-olefin contents of up to 50 % (V/V), the precision for olefins was tested only in the range from 0,40 % (V/V) to 26,85 % (V/V). Although specifically developed for the analysis of automotive motor gasoline that contains oxygenates, this test method can also be applied to other hydrocarbon streams having similar boiling ranges, such as naphthas and reformates. NOTE 2 For Procedure A, applicability of this document has also been verified for the determination of n-propanol, acetone, and di-isopropyl ether (DIPE). However, no precision data have been determined for these compounds. Procedure B describes the analysis of oxygenated groups (ethanol, methanol, ethers, C3 – C5 alcohols) in ethanol (E85) automotive fuel containing ethanol between 50 % (V/V) and 85 % (V/V). The gasoline is diluted with an oxygenate-free component to lower the ethanol content to a value below 20 % (V/V) before the analysis by GC. The sample can be fully analysed including hydrocarbons. Precision data for the diluted sample are only available for the oxygenated groups. NOTE 3 For Procedure B, the precision can be used for an ethanol fraction from about 50 % up to 85 % (V/V). For the ether fraction, the precision as specified in Table 6 can be used for samples containing at least 11 % (V/V) of ethers. For the higher alcohol fraction, too few data were obtained to derive a full precision statement and the data presented in Table 6 are therefore only indicative. NOTE 4 An overlap between C9 and C10 aromatics can occur. However, the total is accurate. Isopropyl benzene is resolved from the C8 aromatics and is included with the other C9 aromatics.

This document specifies a method for the determination of the rust prevention characteristics of lubricating grease under dynamic wet conditions. This test method is used to assess the ability of a grease to prevent corrosion in rolling bearings operated in presence of water, synthetic sea water or any industrial aqueous pollutant. NOTE For the purposes of this document, the term “% (m/m)” is used to represent the mass fraction.

This document specifies a method for the determination of the rust prevention characteristics of lubricating grease in the presence of a flow of an aqueous test fluid. This test method is used to assess the ability of a grease to prevent corrosion in rolling bearings operated in presence of water, synthetic sea water or any industrial aqueous pollutant, under wash out conditions. NOTE For the purposes of this document, the term “% (m/m)” is used to represent the mass fraction.

This document specifies a procedure, at temperatures up to 100 °C, to determine whether a liquid product, that would be classified as “flammable” by virtue of its flash point, sustains combustion at the temperature(s) specified e.g. in regulations. NOTE Many national and international regulations classify liquids as presenting a flammable hazard based on their flash point, as determined by a recognized method. Some of these regulations allow a derogation if the substance cannot “sustain combustion” at some specified temperature(s). The procedure is applicable to paints (including water-borne paints), varnishes, paint binders, solvents, petroleum or related products and adhesives, that have a flash point. It is not applicable to painted surfaces in respect of assessing their potential fire hazards. This test method is applicable, in addition to test methods for flash point, for assessing the fire hazard of a product.

This document specifies a procedure for rating the tendencies of gas turbine fuels to deposit decomposition products within the fuel system. It is applicable to middle distillate and wide-cut fuels and is particularly specified for the performance of aviation gas turbine fuels. The test results are indicative of fuel thermal oxidation stability during gas turbine operation and can be used to assess the level of deposits that form when liquid fuel contacts a heated surface at a specified temperature. This method is also applicable to aviation turbine fuel that consists of conventional and synthetic blending components as defined in the scope of for instance ASTM D7566[1] and Def Stan 91-091[2]. NOTE For the benefit of those using older instruments, non-SI-units and recalculated numbers are given in between brackets where they are more suitable.

This document specifies a method for the determination of the manual and automated closed cup flash point of combustible liquids having flash points between –30,0 °C to 75,0 °C. However, the precision given for this method is only valid for flash points in the range −8,5 °C to 75,0 °C. This document is not applicable to water-borne paints. NOTE 1 Water borne paints can be tested using ISO 3679[1]. NOTE 2 See 9.1 for the importance of this test in avoiding loss of volatile materials. NOTE 3 Liquids containing halogenated compounds can give anomalous results. NOTE 4 The thermometer specified for the manual apparatus limits the upper test temperature to 70,0 °C. NOTE 5 See 13.1 for more specific information related to precision.

This document specifies procedures and requirements for the transfer of bunkers to vessels by bunker tankers using the Coriolis mass flow meter (MFM) system. It encompasses the process leading to the approval of the MFM system as installed on bunker tankers and post-approval bunkering operation. It covers terminology, specifications, requirements and procedures on metrology, system integrity, metering system selection and installation, MFM system verification, bunker delivery and dispute handling. NOTE Local and international regulations, such as the International Convention for the Prevention of Pollution from Ships (MARPOL) can apply.

This document establishes a common basis for, and assistance in, the classification of applications and multiphase meters, as well as guidance and recommendations for the implementation and use of such meters. The so-called in-line multiphase flow meters (MPFMs) that directly measure the oil, water and gas flow rates, as well as the partial- and full-separation MPFMs are the main focus of this document. Conventional two- or three-phase separators are not included in this document. Only limited reference is made to wet-gas meters, since although wet-gas flow is a subset of multiphase flow, wet-gas measurement is covered by ISO/TR 11583 and ISO/TR 12748.

This document specifies the quality requirements for Liquefied Natural Gas (LNG) used as a fuel for marine applications. It defines the relevant parameters to measure as well as the required values and the test reference methods for all those parameters. This document applies to LNG from any source, e.g. gas from conventional reservoirs, shale gas, coalbed methane, biomethane, synthetic methane. LNG described in this document can come from synthesis process out of fossil fuels or renewable sources. This document identifies the required specifications for fuels delivered at the time and place of custody transfer (at the delivery point).

This document establishes an extended classification which includes sub-categories of environmentally acceptable lubricants and specifies the minimum requirements for mineral or synthetic based lubricants for the categories CKH, CKJ and CKM, according to ISO 6743-6, and their sub-categories of environmentally acceptable lubricants, intended for the lubrication of open and semi-enclosed gears. This document covers the lubricants applied in the open and semi-enclosed gear systems most currently encountered in the industry. It does not cover the extreme cases of use with regards to temperature and extreme charges conditions. NOTE 1 For use in exceptional conditions, suppliers and purchasers of lubricants can mutually agree on additional testing methods and acceptability criteria of the products. NOTE 2 This document can be read in conjunction with ISO 6743-6.

This document specifies Procedure A, using manual glass viscometers, and Procedure B, using glass capillary viscometers in an automated assembly, for the determination of the kinematic viscosity, ν, of liquid petroleum products, both transparent and opaque, by measuring the time for a volume of liquid to flow under gravity through a calibrated glass capillary viscometer. The dynamic viscosity, η, is obtained by multiplying the measured kinematic viscosity by the density, ρ, of the liquid. The range of kinematic viscosities covered in this test method is from 0,2 mm2/s to 300 000 mm2/s over the temperature range ?20 °C to +150 °C. NOTE The result obtained from this document is dependent upon the behaviour of the sample and is intended for application to liquids for which primarily the shear stress and shear rates are proportional (Newtonian flow behaviour). If, however, the viscosity varies significantly with the rate of shear, different results can be obtained from viscometers of different capillary diameters. The procedure and precision values for residual fuel oils, which under some conditions exhibit non-Newtonian behaviour, have been included.

This document specifies a procedure for the determination of kinematic viscosity (ν) at 40 °C in the range from 2 mm2/s to 6 mm2/s by calculation from dynamic viscosity (η) and density (ρ) of middle distillate fuels, fatty acid methyl ester fuels (FAME) and mixtures of these using the Stabinger type viscometer. The result obtained using the procedure described in this document depends on the rheological behaviour of the sample. This document is predominantly applicable to liquids whose shear stress and shear rate are proportional (Newtonian flow behaviour). However, if the viscosity changes significantly with the shear rate, comparison with other measuring methods is only permissible at similar shear rates.

This document establishes the rating of diesel fuel oil in terms of an arbitrary scale of cetane numbers (CNs) using a standard single cylinder, four-stroke cycle, variable compression ratio, indirect injected diesel engine. The CN provides a measure of the ignition characteristics of diesel fuel oil in compression ignition engines. The CN is determined at constant speed in a pre-combustion chamber-type compression ignition test engine. However, the relationship of test engine performance to full scale, variable speed and variable load engines is not completely understood. This document is applicable for the entire scale range from 0 CN to 100 CN but typical testing is in the range of 30 CN to 65 CN. An interlaboratory study executed by CEN in 2013 (10 samples in the range 52,4 CN to 73,8 CN)[3] confirmed that paraffinic diesel from synthesis or hydrotreatment, containing up to a volume fraction of 7 % fatty acid methyl ester (FAME), can be tested by this test method and that the precision is comparable to conventional fuels. This test can be used for unconventional fuels such as synthetics or vegetable oils. However, the precision for those fuels has not been established and the relationship to the performance of such materials in full-scale engines is not completely understood. Samples with fluid properties that interfere with the gravity flow of fuel to the fuel pump or delivery through the injector nozzle are not suitable for rating by this method. NOTE This document specifies operating conditions in SI units but engine measurements are specified in inch-pound units or Fahrenheit because these are the historical units used in the manufacture of the equipment, and thus some references in this document include these and other non-SI units in parenthesis.

This document establishes an overview of test methods in the field to determine flash point and combustibility of petroleum and related products. It presents advice on application and specification development. This document is not intended to be a comprehensive manual on flash point and combustibility tests, and the interpretation of test results, however it covers the key aspects on these subjects.

This document is applicable for the determination of VOC and SVOC with an expected VOC and/or SVOC content greater than 0,01 % by mass up to 100 % by mass. The method given in ISO 11890-1 is used when the VOC is greater than 15 % by mass. This document (method ISO 11890-2) applies when the system contains VOC and SVOC as the VOC result of ISO 11890-1 can be influenced by the SVOC. For VOC content smaller than 0,1 %, the head space method described in ISO 17895 is used as an alternative. ISO 11890-1 and ISO 17895 cannot be used for the determination of the SVOC content. NOTE 1 Some ingredients of coating materials and their raw materials can decompose during analysis and cause artificial VOC and/or SVOC signals. When determining VOC and/or SVOC for coating materials and their raw materials, these signals are artefacts of the method and are not taken into account (examples are given in Annex B). This method assumes that the volatile matter is either water or organic. However, other volatile inorganic compounds can be present and might need to be quantified by another suitable method and allowed for in the calculations. The method defined in this document is not applicable for determination of water content. NOTE 2 If organic acids or bases and their corresponding salts are present in the coating material or its raw materials, the amount that is quantified by this method might not be accurate due to a change in the acid or base equilibrium.

This document specifies several methods for the empirical estimation of the consistency of lubricating greases and petrolatum by measuring the penetration of a standardized cone.

This document specifies a method for the assessment of the persistence of a flame applied to the edge of a wick of non-flammable material immersed in fire-resistant fluid. This test does not determine the behaviour of a spray of fire-resistant fluid. NOTE Such test methods are specified in ISO 15029-1 and ISO 15029-2. This document specifies one of four basic tests for determining flammability. This document does not apply to certain liquids such as HFAE and HFAS liquids.

This document describes the design, use and calibration of volumetric measures (capacity measures) which are intended for use in fixed locations in a laboratory or in the field. This document gives guidance on both standard and non-standard measures. It also covers portable and mobile measures. This document is applicable to the petroleum industry; however, it may be applied more widely to other applications. This document excludes measures for cryogenic liquids and pressurized measures as used for liquid petroleum gas (LPG) and liquefied natural gas (LNG). Volumetric measures are classified as test measures or prover tanks depending on capacity and design. Measures described in this document are primarily designed, calibrated and used to measure volumes from a measure which is wetted and drained for a specified time before use and designated to deliver. Many of the provisions, however, apply equally to measures which are used to measure a volume using a clean and dry measure and designated to contain. Guidance is given regarding commonly expected uncertainties and calibration specifications. The document also provides, in Annex A, reference formulae describing the properties of water and other fluids and materials used in volumetric measurement more generally.

This document specifies methods for obtaining samples of lubricating grease from production lots or shipments and gives instructions for inspecting greases in sales packages.

This document specifies procedures and requirements for the transfer of bunkers to vessels involving bunker tankers, road tankers and shore pipelines. It is applicable to pre-delivery, delivery and post-delivery checks and documentation.

This document specifies the minimum requirements of unused fire-resistant and less flammable hydraulic fluids for hydrostatic and hydrodynamic systems in general industrial applications. It is not intended for use in aerospace or power-generation applications, where different requirements apply. It provides guidance for suppliers and end users of these less hazardous fluids and to the manufacturers of hydraulic equipment in which they are used. Of the categories covered by ISO 6743-4, which classifies the different types of fluids used in hydraulic applications, only the following are detailed in this document: HFAE, HFAS, HFB, HFC, HFDR and HFDU. Types HFAE, HFAS, HFB, HFC and HFDR are "fire-resistant" fluids as defined by ISO 5598. Most HFDU fluids, while displaying an improvement in combustion behaviour over mineral oil, fall outside this definition and are more appropriately considered as "less flammable" fluids.

This document specifies the metrological and technical requirements for flowmeters intended to be used for the dynamic measurement of liquefied natural gas (LNG) and other refrigerated hydrocarbon fluids. For LNG static volume measurement used in custody transfer, see ISO 10976. This document sets the best practice for the proper selection and installation of flowmeters in cryogenic applications and identifies the specific issues that can affect the performance of the flowmeter in use. Moreover, it offers a calibration guideline for laboratory and on-site conditions (mass or volume) by either using LNG or other reference fluids. The choice of calibration fluid will depend on the capabilities of the available flow calibration facilities and the ability to achieve the required overall measurement uncertainty demanded by the intended application. This document is applicable, but is not limited, to the use of Coriolis and ultrasonic flowmeters for dynamic measurements of LNG. In principle, LNG and other refrigerated liquid hydrocarbons are considered in this document. Recommendations in this document are based on the available test results with LNG. These results are probably applicable to other cryogenic fluids.

This document specifies the methodology for the regular monitoring of the test method precision achieved versus the precision published in the standard test method using data from proficiency testing schemes (PTSs) supported by the regular users of standard test methods. The procedures in this document are designed specifically for proficiency testing (PT) conducted on standard test methods, having a published reproducibility, for petroleum and petroleum-related products, which are presumed to be homogeneous, and where the data distribution is approximately normal. In addition, it is applicable to properties of interest that are (known to be) stable over time and transport. This document specifies the methodology for the statistical comparison of standard deviation under reproducibility conditions achieved in PT programmes versus that published. The purpose of this comparison is to find out if the published reproducibility precision is representative of that achievable by the regular participants in the PT programmes. This document also provides guidance on how to use a PT z-score to monitor an individual participant's performance over time (see Annex B).

This document specifies a test method for inductively coupled plasma optical emission spectrometry (ICP-OES) for the detection of the sulfur content from 2 mg/kg to 21 mg/kg in fatty acid methyl esters (FAME). NOTE 1 For the purposes of this document, the term "% (m/m)" is used to represent the mass fraction (µ) of the material. NOTE 2 The method can also be used for the determination of concentrations outside the given limits. The precision statement, however, is only valid for the concentration range given in the scope. NOTE 3 The method described in the document was tested with FAME derived from soybean oil and beef tallow. FAME derived from other feedstock, in particular aged oils, may behave different due to the different nature of sulfur compounds.

This document specifies the minimum requirements for mineral or synthetic based lubricants intended for use in all types of refrigerating compressors. NOTE This document can be read in conjunction with ISO 6743-3.

This document addresses quality considerations that apply to marine fuels in view of the implementation of maximum 0,50 mass % S in 2020 and the range of marine fuels that will be placed on the market in response to the international statutory requirements to reduce exhaust gas emissions. It defines general requirements that apply to all 0,50 mass % sulfur (S) fuels and confirms the applicability of ISO 8217 for those fuels. It gives technical considerations which might apply to particular fuels for the following characteristics: — kinematic viscosity; — cold flow properties; — stability; — ignition characteristics; — catalyst fines. Additionally, it provides considerations on the compatibility between fuels and additional information on ISO 8217:2017, Annex B (see Annexes B and D). NOTE 1 For the purposes of this document, 0,50 mass % S fuels refers to distillate and residual fuels with a sulfur content up to 0,50 mass %. NOTE 2 For the purposes of this document, "mass %" and "volume %" are used to represent the mass and volume fractions respectively. NOTE 3 This document can also be used in conjunction with earlier editions of ISO 8217 in the event an earlier edition is referenced in the commercial agreement between parties.

This document specifies a wavelength-dispersive X-ray fluorescence (WDXRF) test method for the determination of the sulfur content of liquid, homogeneous automotive fuels from 5 mg/kg to 500 mg/kg, which have a maximum oxygen content of 3,7 % (m/m). This product range covers: — diesel fuels containing up to about 30 % (V/V) fatty acid methyl esters (FAME), — motor gasolines containing up to about 10 % (V/V) ethanol, — synthetic fuels such as hydrotreated vegetable oil (HVO) and gas to liquid (GTL) having sulfur contents in the range of 5 mg/kg to 45 mg/kg. Products with higher oxygen content show significant matrix effects, e.g. pure FAME used as biodiesel, nevertheless, pure FAME can be analysed when the corresponding procedures are followed (see 5.3 and 8.1). Other products can be analysed with this test method, though precision data for products other than those mentioned have not been established for this document. NOTE 1 Sulfur contents higher than 500 mg/kg can be determined after sample dilution, however, the precision was not established for diluted samples. NOTE 2 For the purposes of this document, "% (m/m)" and "% (V/V)" are used to represent the mass fraction, w, and the volume fraction, φ, of a material respectively.

This document specifies an ultraviolet (UV) fluorescence test method for the determination of the sulfur content of the following products: — having sulfur contents in the range 3 mg/kg to 500 mg/kg, — motor gasolines containing up to 3,7 % (m/m) oxygen [including those blended with ethanol up to about 10 % (V/V)], — diesel fuels, including those containing up to about 30 % (V/V) fatty acid methyl ester (FAME), — having sulfur contents in the range of 3 mg/kg to 45 mg/kg, — synthetic fuels, such as hydrotreated vegetable oil (HVO) and gas to liquid (GTL). Other products can be analysed and other sulfur contents can be determined according to this test method, however, no precision data for products other than automotive fuels and for results outside the specified range have been established for this document. Halogens interfere with this detection technique at concentrations above approximately 3 500 mg/kg. NOTE 1 Some process catalysts used in petroleum and chemical refining can be poisoned when trace amounts of sulfur-bearing materials are contained in the feedstocks. NOTE 2 This test method can be used to determine sulfur in process feeds and can also be used to control sulfur in effluents. NOTE 3 For the purposes of this document, "% (m/m)" and "% (V/V)" are used to represent the mass fraction, w, and the volume fraction, φ, of a material respectively. NOTE 4 Sulfate species in ethanol do not have the same conversion factor of organic sulfur in ethanol. Nevertheless, sulfates have a conversion factor close to that of organic sulfur. NOTE 5 Nitrogen interference can occur, see 6.5 for further guidance.

This document specifies a method to determine the turbidity point of petroleum products based on distillates from crude oil. This document also specifies how to convert the turbidity point to an aniline point equivalent. This document describes a procedure using automated or automatic apparatus suitable for transparent samples with an initial boiling point above ambient temperature.

This document specifies a method for the determination of the boiling range distribution of petroleum products. The method is applicable to petroleum products and fractions with a final boiling point of 538 °C or lower at atmospheric pressure as determined by this document. This document does not apply to gasoline samples or gasoline components. The method is limited to products having a boiling range greater than 55 °C and having a vapour pressure sufficiently low to permit sampling at ambient temperature. The document describes two procedures. a) Procedure A allows a larger selection of columns and analysis conditions, such as packed and capillary columns as well as a thermal conductivity detector in addition to the flame ionization detector. Analysis times range from 14 min to 60 min. b) Procedure B is restricted to only three capillary columns and requires no sample dilution. The analysis time is reduced to about 8 min. Both procedures have been successfully applied to samples containing fatty acid methyl esters (FAME) up to 20 % (volume fraction). NOTE For the purposes of this document, the terms "% (mass fraction)" and "% (volume fraction)" are used to represent the mass fraction (µ), the volume fraction (φ) of a material.