How to Measure Bentonite Methylene Blue Value: A Lab Guide

What Is Bentonite Methylene Blue Value?

Methylene Blue Value (MBV) is a measure of how much methylene blue a clay sample adsorbs under specified test conditions. Because methylene blue is a cationic dye, its adsorption is strongly influenced by the quantity and surface properties of clay minerals, especially smectite such as montmorillonite. For that reason, MBV is widely used as a quality-control indicator for bentonite and other clay-bearing materials.

The important point is that MBV is not a direct chemical assay for montmorillonite purity. A high result generally indicates a greater capacity for dye adsorption, but the result also depends on mineralogy, particle size, exchangeable cations, organic matter, sample preparation, reagent concentration, and the test method used. MBV should therefore be interpreted alongside other properties such as swelling, moisture, particle size, rheology, or cation exchange capacity when the application requires a complete specification.

The basic test is straightforward. A known mass of prepared clay is dispersed in water, a standardized methylene blue solution is added in measured increments, and the suspension is checked after each addition. The endpoint is identified by a persistent halo around a drop of the suspension on filter paper, or by another endpoint method specified by the applicable standard. The amount of dye consumed is then converted into the reporting unit required by the test method.

This guide explains the chemistry, sample preparation, titration procedure, calculations, endpoint control, common errors, and practical interpretation of bentonite MBV results. It also explains why results from different standards should not be compared as though they were automatically interchangeable.

Why Methylene Blue Testing Matters for Bentonite

Bentonite performance depends heavily on its mineral composition and the behavior of its fine clay fraction. Montmorillonite has a layered structure with negatively charged surfaces and exchangeable cations between the layers. Those characteristics give smectitic clays their ability to adsorb cations, interact with water, and contribute to swelling, rheology, sealing, and binding.

Methylene blue provides a convenient way to assess this adsorption capacity. During the test, positively charged methylene blue ions are attracted to available negatively charged sites associated with the clay. As the dye is added, it is progressively adsorbed. Once the available adsorption capacity under the test conditions has effectively been reached, additional dye remains in the liquid phase. The endpoint test detects that excess dye.

This makes MBV useful in several quality-control settings:

• Bentonite mining and processing: comparing lots, blends, or processed grades.
• Foundry applications: monitoring active clay in green molding sand or evaluating clay materials used as binders.
• Drilling-fluid applications: characterizing clay materials as part of a broader drilling-grade specification.
• Ceramic and mineral processing: assessing the dye adsorption behavior of clay raw materials.
• Aggregate and mineral filler testing: measuring clay-related methylene blue response under the specific standard applicable to the material.

The result is most useful when the laboratory uses one defined method consistently. A number reported as MBV without the test method, sample basis, reagent concentration, and reporting unit can be difficult to interpret.

The Chemistry Behind the Methylene Blue Test

Bentonite is a commercial clay material rather than a single pure mineral. Its principal active component may be montmorillonite, but natural deposits can also contain quartz, feldspar, calcite, mica, gypsum, volcanic minerals, and other phases. Processing can further change particle size, moisture, and dispersion behavior.

Montmorillonite belongs to the smectite group and has a 2:1 layered aluminosilicate structure. A sheet of octahedrally coordinated cations lies between two tetrahedral silica sheets. Isomorphous substitution within the crystal structure creates a permanent negative charge that is balanced by exchangeable cations such as sodium, calcium, magnesium, or potassium.

Methylene blue is a cationic dye. In aqueous suspension, the positively charged dye species can interact with negatively charged clay surfaces and exchange sites. As more dye is added, adsorption continues until the available adsorption capacity under the test conditions is approached.

The simplified exchange process can be represented in words as follows:

Clay exchange site with sodium plus methylene blue gives a clay exchange site associated with methylene blue plus sodium remaining in solution.

The real system is more complicated than this simplified representation. Dye adsorption can involve external surfaces, interlayer regions, and other interactions. That is why the measured value should be treated as a method-specific adsorption index rather than as a universal, absolute measurement of one mineral property.

MBV, CEC, and Surface Area Are Related but Not Identical

One of the most common mistakes in bentonite literature is treating MBV, cation exchange capacity, and specific surface area as interchangeable measurements. They are not.

Cation exchange capacity (CEC) measures the quantity of exchangeable cations a material can retain under a defined analytical procedure. It is commonly reported in milliequivalents per 100 grams or equivalent SI-based units.

Methylene blue value measures dye adsorption under the conditions of a particular methylene blue test. The result can correlate with clay activity and CEC, but the relationship depends on mineralogy and the analytical procedure.

Specific surface area describes surface area per unit mass and is normally determined using a different analytical technique. MBV should not be presented as a direct particle-size or surface-area measurement.

ASTM C837, for example, describes a methylene blue index test for clay and notes that the relationship between methylene blue index and fundamental clay properties depends on the mineral composition. This is one reason a laboratory should avoid converting MBV into a precise montmorillonite percentage unless that conversion has been validated for the material being tested. ASTM Store

Equipment and Reagents for Bentonite MBV Testing

The exact equipment list depends on the standard or internal method selected. A typical visual titration setup includes the following.

Reagents

• Standardized methylene blue solution: Use the concentration and chemical form specified by the applicable method. A commonly encountered concentration is approximately 3.74 g/L for methylene blue trihydrate, but the laboratory should prepare and standardize the reagent according to its adopted procedure rather than assuming that every method uses the same solution.
• Deionized or distilled water: Use water of suitable laboratory quality and document the water specification used by the method.
• Dispersing reagent, where required: Some procedures use a dispersant or other conditioning step. Do not substitute sulfuric acid, sodium pyrophosphate, or another reagent simply because it appears in a different MBV method.
• Hydrogen peroxide, where specifically justified: Organic matter can interfere with some clay tests, but peroxide treatment should only be used when the adopted method permits or requires it. Uncontrolled pretreatment can change the sample and make results difficult to compare.

Laboratory Equipment

• Analytical balance suitable for the required sample mass.
• Class A burette or an equivalent calibrated dispensing system.
• Volumetric flasks for preparing standardized solutions.
• Erlenmeyer flasks or other vessels specified by the method.
• Magnetic stirrer and appropriate stir bar.
• Qualitative filter paper when a visual spot endpoint is specified.
• Clean glass rods or another suitable sampling device.
• Drying oven where the selected method requires moisture determination or dry-basis reporting.
• Desiccator when dried samples need controlled cooling before weighing.
• Stopwatch or laboratory timer.
• Appropriate laboratory PPE and chemical-handling equipment.

Calibration matters. A poorly calibrated balance, burette with a damaged stopcock, contaminated glassware, or incorrectly prepared dye solution can shift the result enough to affect batch decisions.

Preparing the Methylene Blue Solution

The concentration of the methylene blue solution is central to the calculation. If the concentration is wrong, every MBV result calculated from that solution will be wrong in the same direction.

A frequently used laboratory concentration is approximately 0.01 N, corresponding to approximately 0.01 milliequivalents of a monovalent methylene blue species per milliliter. When methylene blue trihydrate is used, a concentration near 3.74 g/L is commonly associated with this level. The exact preparation and standardization procedure must follow the method used by the laboratory.

A Practical Preparation Sequence

1. Check the reagent identity. Confirm the chemical form, assay, lot number, and certificate information for the methylene blue reagent.
2. Condition the reagent if required by the method. Do not automatically oven-dry a hydrated dye simply because moisture affects weighing. Drying conditions can alter hydrated materials. Follow the reagent or test-method instructions instead.
3. Calculate the required mass. Use the molecular weight and target concentration specified by the adopted method.
4. Dissolve the reagent completely. Use a portion of laboratory-quality water and mix until the solution is homogeneous.
5. Transfer quantitatively. Rinse the original vessel and add the rinses to the volumetric flask so that no significant portion of the reagent is lost.
6. Bring to volume. Allow the solution to reach the required temperature before making the final volume adjustment when the method calls for temperature control.
7. Mix thoroughly. Invert or otherwise mix the volumetric flask according to laboratory practice.
8. Standardize or verify the solution. Record the standardization factor or concentration used in all subsequent calculations.
9. Label the container. Include concentration, preparation date, standardization information, expiry or review date, and the analyst's identification according to the laboratory's quality system.

Store the solution under the conditions specified by the method and laboratory SOP. Do not assume that a fixed 30-day replacement period applies to every procedure. Stability depends on the reagent, container, storage conditions, and the laboratory's validated practice.

Step-by-Step Bentonite Methylene Blue Test Procedure

The following is a practical framework for a visual spot-test titration. It should be treated as an illustrative laboratory workflow, not as a replacement for the current edition of a customer-specified or regulatory test standard. ASTM, API, ISO, EN, and foundry procedures can differ in sample preparation, reagent concentration, dispersion, endpoint definition, and reporting.

Step 1: Obtain a Representative Sample

A laboratory result is only as representative as the material placed on the balance.

1. Collect increments from the batch or shipment according to the sampling plan.
2. Combine and reduce the sample using an appropriate laboratory procedure.
3. Protect the sample from additional moisture or contamination.
4. Record the sample identification, source, lot number, date, and condition.

Avoid taking one scoop from the top of a bag and treating it as representative of the entire shipment. Segregation during transport and handling can produce meaningful variation.

Step 2: Determine the Moisture Basis

Decide whether your MBV result will be reported on an as-received basis or a dry basis. Dry-basis reporting is often preferable for comparing clay materials because moisture changes the apparent mass of the sample without increasing the amount of active mineral.

If a dry basis is required:

1. Weigh a representative portion of the sample.
2. Dry it using the temperature and time specified by the adopted method.
3. Cool it appropriately before weighing.
4. Repeat drying and weighing when the method requires a constant-mass determination.
5. Calculate the dry mass or moisture correction.

Do not automatically use 105 degrees Celsius for every clay procedure. Some materials contain components that can change during heating, so the drying condition should come from the selected standard or validated laboratory method.

Step 3: Prepare the Clay for Dispersion

Particle preparation is important because the dye must interact with the accessible clay surface under the test conditions.

1. Reduce the sample to the particle size required by the selected method.
2. Weigh the specified dry-equivalent sample mass.
3. Transfer the sample to the test vessel.
4. Add the specified volume of water.
5. Add the dispersing or conditioning reagent only if the adopted method calls for it.
6. Mix for the specified time and intensity.
7. If heating or boiling is part of the standard method, follow its stated conditions rather than applying a generic five-minute boiling step.
8. Bring the suspension to the required test temperature before titration.

The goal is a reproducible suspension, not simply the fastest possible mixing. Excessive shear, prolonged heating, or an unsuitable dispersant can change the behavior of the sample.

Step 4: Fill and Check the Burette

1. Rinse the burette with a small amount of the standardized methylene blue solution.
2. Fill the burette above the expected working range.
3. Remove any air bubble from the burette tip.
4. Record the initial reading at eye level.
5. Check that the stopcock does not leak.

Read the meniscus consistently and record enough decimal places for the burette being used.

Step 5: Begin the Titration

1. Place the clay suspension on the magnetic stirrer.
2. Establish consistent mixing without splashing.
3. Add the initial quantity of methylene blue specified by the method.
4. Allow the suspension to mix for the required contact time.
5. Perform the endpoint test using the prescribed technique.

Do not change the mixing speed or contact time from one sample to the next. Even when the endpoint is judged visually, consistent operating technique is essential for repeatability.

Step 6: Perform the Spot Test

For a method using a filter-paper endpoint, withdraw a small drop of the well-mixed suspension and place it on clean qualitative filter paper.

At an early stage of the titration, the dye is largely consumed by the clay, so free dye may not produce a persistent blue ring beyond the concentrated clay spot. As the endpoint is approached, excess dye begins to migrate outward with the liquid and creates a blue halo around the central spot.

The exact appearance and persistence requirement depend on the adopted method. A common visual interpretation is:

• No persistent halo: Continue the titration.
• Faint halo that disappears after the specified mixing period: Continue the titration.
• Distinct halo that remains after the specified confirmation period: Treat the endpoint as reached if it meets the method's endpoint criteria.

Avoid relying on a halo's color alone. Filter-paper type, lighting, drop size, sample concentration, operator vision, and dye concentration can all influence the visual judgment.

Step 7: Approach the Endpoint Carefully

Large additions are useful at the beginning of a titration but become risky near the endpoint.

A practical approach is to use larger increments while the sample clearly consumes the dye, then reduce the addition size as the endpoint approaches. After each addition, mix for the specified time before testing again.

When the first faint halo appears, confirm it according to the method. Do not automatically declare the endpoint at the first sign of blue outside the central spot.

Record the final volume only after the endpoint has been confirmed.

Step 8: Record the Complete Test Conditions

A useful laboratory record should include:

1. Sample identification and lot number.
2. Sample mass and moisture basis.
3. Sieve or particle-size preparation, if applicable.
4. Reagent identity and concentration.
5. Reagent standardization factor.
6. Dispersion conditions.
7. Test temperature, where specified.
8. Initial and final burette readings.
9. Total methylene blue volume consumed.
10. Endpoint observation.
11. Analyst identification and test date.
12. Test method and revision or laboratory SOP number.

This information makes the result auditable and much easier to troubleshoot when a batch falls outside its normal range.

Methylene Blue Value Formula

The calculation depends on how the laboratory expresses the dye concentration and how the method defines its reporting unit. For a simplified titration using a methylene blue solution with a known normality, the result can be calculated as:

MBV (meq/100 g) = V x N x 100 / Wdry

Where:

• V = volume of standardized methylene blue solution consumed, in mL.
• N = methylene blue solution concentration expressed in equivalents per liter, numerically equivalent to milliequivalents per mL for this calculation.
• Wdry = dry sample mass, in grams.

For a 0.01 N solution and a 0.500 g dry sample:

MBV (meq/100 g) = V x 2

This simplified relationship is valid only when the reagent concentration and sample mass actually match those assumptions.

Example Calculation

Suppose a laboratory uses:

• Dry-equivalent bentonite sample = 0.500 g.
• Standardized methylene blue solution = 0.010 meq/mL.
• Confirmed endpoint volume = 44.5 mL.

The calculation is:

MBV = 44.5 x 0.010 x 100 / 0.500

MBV = 89.0 meq/100 g

That is the calculated methylene blue adsorption value on the stated sample basis and under the stated test conditions.

The result should not automatically be converted into 93.7 percent montmorillonite. That conversion assumes a reference CEC and a direct proportional relationship that may not hold for the particular bentonite. If a supplier or laboratory reports an estimated active-clay percentage from MBV, the reference value and validation basis should be stated clearly.

Reporting MBV in Grams of Dye per 100 Grams

If the laboratory method reports dye consumption as grams of methylene blue per 100 grams of dry sample, the calculation can be expressed as:

MBV (g/100 g) = V x Cdye x 100 / (1000 x Wdry)

Where:

• V = dye solution volume in mL.
• Cdye = dye concentration in g/L.
• Wdry = dry sample mass in g.

Always check whether the concentration refers to the specific hydrated form of methylene blue used in the method. Mixing anhydrous and hydrated molecular weights in the same calculation is a common source of error.

Should MBV Be Converted to Montmorillonite Content?

Only with caution.

A high MBV often indicates a greater proportion or activity of smectitic clay, but MBV is not a mineralogical assay. Natural bentonite can contain several clay and non-clay minerals, and different montmorillonite compositions can have different exchange capacities.

A simple calculation such as:

Estimated active clay percentage = measured MBV / selected reference value x 100

can be useful as an internal index if the laboratory has established and validated the reference value for its material. It should not be presented as a universal measurement of montmorillonite purity.

For definitive mineral identification or phase quantification, techniques such as X-ray diffraction are more appropriate. CEC testing can provide additional information about exchange capacity, while swelling, rheology, and particle-size tests describe other aspects of performance.

The strongest quality-control program uses MBV as one measurement within a broader specification rather than treating it as the only indicator of bentonite quality.

How to Interpret MBV Results Across Bentonite Grades

There is no single MBV number that makes a bentonite suitable for every application. The required result depends on the mineralogy, formulation, test method, and end use.

A supplier should therefore avoid promising that a particular MBV automatically guarantees performance in a finished product. The application specification should determine which supporting tests are necessary.

MBV Testing Standards: Why the Method Matters

Different methylene blue tests use different procedures and reporting conventions. Results from one method should not be compared directly with results from another unless equivalence has been demonstrated.

ASTM C837

ASTM C837 is the Standard Test Method for Methylene Blue Index of Clay. The current ASTM listing identifies ASTM C837-09(2024) as the active version. The method measures methylene blue adsorption by clay and explains that the relationship between the index and other clay properties depends on the composition of the clay. ASTM Store

This matters because ASTM C837 should not be described simply as a universal bentonite CEC test. It is a methylene blue index method for clay, and its interpretation depends on the material being tested.

API Specification 13A and ISO 13500

API Specification 13A and ISO 13500 cover drilling-fluid materials and associated physical properties and test procedures. ISO 13500:2008 remains the current ISO edition according to ISO's current listing, and it covers materials including bentonite, nontreated bentonite, and OCMA-grade bentonite. ISO +1

For drilling-grade bentonite, the laboratory should follow the current customer-required edition of the applicable API or ISO specification rather than copying a procedure from a general clay test.

Foundry Testing

Foundries often use methylene blue or related active-clay procedures to monitor circulating green sand systems. The exact test procedure should come from the foundry's process specification, equipment supplier, or applicable standard.

The important distinction is that a test designed for a bentonite raw material is not automatically equivalent to a test designed to measure active clay in a complex, recycled green-sand system. Recycled sand may contain carbonaceous additives, thermal degradation products, accumulated fines, and other materials that influence the result.

Do Not Mix Numerical Results From Different Methods

A reported value such as 80 meq/100 g has little meaning without its method and sample basis. Even when two laboratories both call their result MBV, they may use different reagent concentrations, sample masses, pretreatments, endpoint definitions, or reporting units.

For purchasing and supplier qualification, put the method directly on the specification and COA. That small detail prevents many unnecessary disputes.

Common MBV Testing Errors and How to Prevent Them

1. Using an Unstandardized Dye Solution

A prepared solution can change in concentration because of weighing errors, reagent composition, evaporation, contamination, or storage conditions.

Fix: Standardize or verify the reagent according to the adopted method and record the factor with every test series.

2. Treating the First Blue Halo as the Endpoint

A faint ring can appear before the true endpoint. The operator must follow the confirmation rule defined by the test method.

Fix: Use a consistent mixing time and endpoint confirmation procedure. If the method requires the halo to persist after a specified period, apply that condition to every sample.

3. Using the Wrong Sample Basis

A wet sample weighs more than the same quantity of dry clay. If one laboratory reports on a dry basis and another reports on an as-received basis, their values may appear inconsistent even when the material is identical.

Fix: State the sample basis clearly and apply the moisture correction consistently.

4. Over-Grinding the Sample

Grinding can change the physical state of a clay sample and may alter its dispersion behavior. It can also introduce contamination from the grinding equipment.

Fix: Prepare the sample only to the particle-size condition required by the selected method.

5. Incomplete Dispersion

Clay aggregates can hide surfaces from the reagent. Poor dispersion can produce a result that does not represent the adsorption capacity expected from a properly prepared sample.

Fix: Use the specified dispersant, mixing time, temperature, and preparation procedure. Do not improvise a more aggressive treatment without validating its effect.

6. Using a Different Filter Paper

A visual halo endpoint depends partly on how the liquid migrates through the paper. Different grades can produce different spot shapes and contrast.

Fix: Use the filter paper specified by the method, or validate an approved equivalent before introducing it into routine QC work.

7. Ignoring Organic Additives

Organic polymers, lignite, carbonaceous materials, and other additives can interact with methylene blue. This is particularly relevant when testing complex materials such as used drilling fluids or recycled foundry sand rather than clean bentonite.

Fix: Identify potential interferences before testing. Use a validated pretreatment only when the applicable method permits it, and report the pretreatment because it affects comparability.

8. Comparing MBV Without Comparing the Method

This is one of the most damaging reporting mistakes. Two numbers can look directly comparable while representing different analytical procedures.

Fix: Put the method, reagent concentration, reporting unit, and moisture basis next to the result.

How to Improve Repeatability in a Bentonite QC Laboratory

A reliable MBV program is less about making one perfect titration and more about controlling every variable that affects the result.

Use a Reference Material

Run a suitable laboratory reference or control material at a defined frequency. Track its results over time rather than reacting to one isolated measurement.

A control chart can reveal gradual changes in dye concentration, analyst technique, equipment condition, or sample preparation before they become obvious in production data.

Run Duplicate Samples

For important qualification work, test duplicate preparations rather than repeating only the final spot test from the same suspension. Independent preparations provide more information about sample preparation repeatability.

If duplicates disagree beyond the laboratory's established acceptance limit, investigate before releasing the result.

Standardize Analyst Technique

Visual endpoint tests are sensitive to operator technique. Train analysts to use the same drop size, filter paper, lighting conditions, mixing time, and endpoint criteria.

For laboratories with high sample volumes, an instrumental endpoint method may reduce some operator-to-operator variation when an appropriate validated method is available.

Keep a Complete Test Record

A result without traceability is difficult to defend. Maintain records of reagent preparation, calibration, sample preparation, environmental conditions where relevant, instrument checks, analyst identity, and calculation details.

This becomes especially important when MBV is included on a customer-facing Certificate of Analysis.

MBV vs. Swelling Index: What Is the Difference?

MBV and swelling index answer different questions.

MBV measures methylene blue adsorption under defined analytical conditions. It provides information related to clay activity and adsorption capacity.

Swelling index measures how much the material expands when hydrated under a specified procedure. It is a physical performance test rather than a dye-adsorption measurement.

A bentonite can have a relatively high MBV but show different swelling behavior from another bentonite because mineralogy, exchangeable cations, particle size, processing, and test conditions differ.

That distinction matters in product selection. A drilling-fluid buyer may care about rheology and fluid loss. A foundry may prioritize active clay, compactability, strength, and thermal behavior. A civil engineering application may focus on swelling, filtration, gel strength, and sealing performance.

MBV is useful, but it doesn't replace application-specific testing.

A Practical MBV Quality-Control Checklist

Before accepting a result, confirm the following:

1. The sample represents the lot being tested.
2. The sample preparation follows the selected test method.
3. The moisture basis is known.
4. The balance and volumetric equipment are calibrated or verified.
5. The methylene blue reagent identity and concentration are documented.
6. The reagent has been standardized or otherwise verified as required.
7. The suspension has been dispersed consistently.
8. Mixing time and speed are controlled.
9. The endpoint is confirmed using the method's criteria.
10. The final burette reading is recorded correctly.
11. The calculation uses the correct sample basis and concentration.
12. The result is reported with the test method and units.
13. Duplicate or control results meet the laboratory's acceptance criteria.
14. Any unusual sample characteristics or pretreatment are documented.

This checklist is simple, but it prevents many of the errors that cause unexplained batch-to-batch variation.

How Buyers Should Specify Bentonite MBV

For industrial procurement, asking for "high MBV bentonite" is not specific enough. A better purchase specification identifies the measurement method and the supporting properties that matter to the application.

For example, a technical specification can define:

• Required MBV or methylene blue index range.
• Applicable ASTM, API, ISO, EN, or internal method.
• Dry or as-received reporting basis.
• Moisture limit.
• Particle-size requirement.
• Swelling or rheology requirements where applicable.
• Fluid-loss requirement for drilling applications where applicable.
• Foundry strength or active-clay requirements where applicable.
• Sampling and COA requirements.
• Acceptance criteria for retesting and disputes.

This approach gives both the buyer and supplier a clear basis for comparison.

A Certificate of Analysis should ideally show the actual result, not simply a statement that the material is "premium grade." The method and units should also be visible so the buyer can compare batches correctly.

What a Good MBV Test Report Should Contain

A professional laboratory report can be concise while still being technically useful. At minimum, it should identify:

A report structured this way gives the customer enough information to understand what the number represents.

Final Takeaways

Bentonite methylene blue testing is a useful analytical tool for evaluating clay adsorption behavior, but its value depends on disciplined laboratory practice and correct interpretation.

The most important points are straightforward:

• MBV measures methylene blue adsorption under defined test conditions.
• It can provide useful information about active clay and adsorption capacity, but it is not a direct assay of montmorillonite purity.
• CEC, specific surface area, swelling index, and MBV are related properties, not interchangeable measurements.
• Sample preparation and reagent concentration have a major influence on repeatability.
• The endpoint must be confirmed according to the selected test method.
• Results from different standards should not be compared without checking their procedures and reporting units.
• For industrial purchasing, MBV should be considered alongside application-specific performance tests.

For laboratories that test bentonite routinely, consistency is the real objective. Use one validated procedure, control the reagent, document the sample basis, train analysts on the endpoint, and trend reference-material results over time. Those practices make MBV data much more useful for production control, supplier qualification, and customer specifications.

If a bentonite supplier provides MBV data on a Certificate of Analysis, ask for the test method and reporting basis as well. A number without its analytical context is only part of the information you need to judge the material.