Contents
Partially compliant

ASTM C813: standard test method for hydrophobic contamination on glass by contact angle measurement

ASTM C813 finds oily or organic films on glass by reading the water contact angle: a higher angle means more contamination. The Dropometer reads angles from 10° upward with an accuracy of ±0.35° (product specification). Very clean glass can read below 10°, so use a goniometer rated for low angles when you need that value.

What brings you here?

Who this is for
Display, electronics, optics and automotive glass makers, and anyone who cleans glass before coating, bonding or printing.
Compliance tier
Partially compliant: the Dropometer reads water contact angles from 10° to 175°, not the lowest angles on very clean glass.
Measurement
Sessile drop water contact angle, 10° to 175° range, ±0.35° accuracy (Dropometer datasheet).
Setup
About 2 minutes. Battery powered, 1 kg, runs at the cleaning line or in the lab.
Last updated
October 20, 2026
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Free one page checklist for ASTM C813: what to prepare, the settings to lock, how to record readings below 10° and what to note. Made for glass cleaning and QC benches.

Talk to an expert

Book a 15 minute demo on your glass

Tell us your glass, cleaning process and current check. We will measure live and tell you plainly which part of ASTM C813 the Dropometer covers and which it does not.

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View the official standard
Gurdeep-Saini-Photo
Written by
Gurdeep Singh Saini
Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.
COO at Droplet Lab
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Droplet-Lab logo
Technical Review by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
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Verified against ASTM C813
Gurdeep-Saini-Photo
Written By

Gurdeep Singh Saini

COO at Droplet Lab

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

Droplet-Lab logo
Reviewed By

Droplet Lab Team

Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.

Featured Studies & Publications

Research Paper Contact angle measurement with a smartphone
Research Paper Surface tension measurement with a smartphone using a pendant drop

Evidence box

Standard

ASTM C813-20(2024), Standard Test Method for Hydrophobic Contamination on Glass by Contact Angle Measurement, from ASTM Committee C14 on glass. It is the 2020 edition, reapproved in 2024. The earlier C813-90 edition was reapproved several times before the 2020 revision.

Compliance tier

Partially compliant. The Dropometer measures the static water contact angle on glass from 10° to 175°. Very clean glass can read below 10°, outside that range, so the exact value there needs a goniometer rated for low angles.

Instrument performance

Contact angle range 10° to 175°, accuracy ±0.35°, display resolution 0.01°, from the Dropometer product specification.

Peer reviewed method

The drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers. See our validation publications.

Customer evidence

We have no published client paper on glass to ASTM C813 yet. In a 2025 study in Biomimetics, researchers used a Droplet Lab instrument to show the contact angle on titanium falling after plasma cleaning. Read our analysis.

What it does not tell you

A raised angle shows that a hydrophobic film is present. It does not identify the contaminant. Use a chemical method such as FTIR or XPS for failure analysis when you need to know what it is.

Executive summary

What this standard measures, who uses it and how the Dropometer fits

ASTM C813 detects hydrophobic contamination on glass by measuring the water contact angle: clean glass wets well, and an oily or organic film raises the angle. Display, electronics, optics and automotive glass makers use it to check processes for the removal of hydrophobic contaminants and to watch for films picked up from the air. The Dropometer is partially compliant: it reads the water contact angle from 10° to 175° with ±0.35° accuracy, but very clean glass can read below 10°, outside its published range.

ASTM states the method can detect fractions of monomolecular layers of hydrophobic organic contaminants, and that rough or porous glass makes it less sensitive. The contact angle test is nondestructive, so it suits the detection and control of hydrophobic contamination on production glass. You place drops of reagent water, read the angle and compare it with a limit you set. For the terms used, see the wettability glossary entry.

The Dropometer contact angle and surface energy kit runs on battery, so you can check glass at the cleaning line or in the lab. For the full workflow, see glass cleaning validation and verification. A fused silica and quartz R&D site uses the Dropometer for contact angle, surface tension and surface energy. In a published study on plasma removal of surface contaminants, researchers used a Droplet Lab instrument to track the contact angle before and after cleaning titanium.

For background, read the electrical and electronics guide and the contact angle measurement guide.

Further reading: sample preparation and contamination control, water contact angle reference values for common surfaces, a classroom experiment that makes glass superhydrophobic with a flame and choosing a contact angle goniometer for industrial QA.

How the Dropometer fits the contact angle test procedure

The ASTM C813 measurement, step by step, in our own words

From cleaned glass to a recorded angle

These steps outline the procedure and show where the Dropometer reaches its limit. They do not replace the standard. Buy the current edition and follow its drop volume, reading time and specimen rules.

±0.35°

Contact angle accuracy

From the Dropometer product specification. Display resolution is 0.01°.

10° to 175°

Measuring range

Very clean glass can read below 10°. Record those drops as below range.

About 2 min

Setup

Base plate, phone holder, LED and syringe. No PC needed.

1

Take the glass and protect it

Take production parts or clean witness glass from the line. Handle them by the edges with clean gloves or tongs. Note the cleaning step and the time it finished. Prepare specimens as the standard describes.

2

Set up the Dropometer

Place the base plate on a level bench, mount the phone and LED, and fill a clean syringe with reagent water to ASTM D1193. Lay the glass flat on the sessile stage, test side up.

3

Place the drop

Dispense the drop volume the standard specifies onto a fresh area. Keep the volume the same for every drop. Do not touch the glass with the needle, or you may leave a film yourself.

4

Capture and read the angle

Capture the drop at the reading time the standard sets. Automatic edge and baseline detection reads the left and right angles, so two operators get the same reading from the same image. If the drop spreads below 10°, record it as below range.

5

Repeat across the part

Place several drops at set positions, such as the centre and near each edge. Films from handling, racks or drying often sit in one area, so one drop is not enough.

6

Record and compare

Record the mean, spread, positions, cleaning step, time since cleaning and room conditions. Compare the mean with your accept limit. For good handling practice, see sample preparation and contamination control.

7

Watch the full measurement

This video shows a complete Dropometer contact angle measurement, from setup to result. It covers the steps above in one run.

Dropometer contact angle measurement, start to finish.

Validated measurement approach

Independent benchmarking and publication-based validation references.

Peer reviewed method

The Dropometer drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers, one in Review of Scientific Instruments and one in Colloids and Surfaces A. Published contact angle accuracy is ±0.35°.

Read the validation papers

Customer evidence

We have no published client paper on glass cleaning yet. In a 2025 Biomimetics study, researchers used a Droplet Lab instrument to track the contact angle on titanium before and after plasma removal of surface contaminants.

Read our analysis

Set ASTM C813 limits from your own glass

ASTM C813 · glass cleanliness · cleaning process control

The public ASTM summary of C813 gives no pass or fail angle (source: ASTM C813-20(2024) listing). So set your accept limit from your own correlation study. Measure glass that coated, bonded or printed well and glass that failed, then pick the angle that separates them.

Check where your limit sits against the Dropometer range of 10° to 175°. If your limit is above 10°, a drop that spreads below 10° still sits under the limit: record it as below range and accept. If your limit is below 10°, or you must report the exact value on very clean glass, use a goniometer rated for low angles for that part of the work.

Fix the time between cleaning and testing, because clean glass picks up films from the air. Repeat the study when you change the cleaning chemistry, the rinse water, the glass type or the storage and packing.

Example output

Example data for illustration only. These are not measured results. Your own glass, cleaning process and limits decide the numbers.

Example: soda lime glass panels after different cleaning and handling steps

Sample Drops (n) Mean angle (°) Std dev (°) Decision
As received, before cleaning541.32.2Contaminated: clean before use
Detergent wash, reagent water rinse513.41.1Accept
Same wash, then handled without gloves527.84.6Reclean and review handling
Plasma cleaned5Below 10 (below range)n/aAccept: under the 15° limit, value not given
Plasma cleaned, then 24 h in open lab air517.91.4Above limit: reclean or store covered

How to read this table

Each row is the mean of five drops of reagent water. The decision column uses an example accept limit of 15° or lower, set from a bonding trial. The plasma cleaned panel read below 10°, outside the Dropometer range, so its value is not given.

Example output chart

The same example data as the table, plotted against the example accept limit. The plasma cleaned panel is left out because it read below 10°.

Bar chart of example water contact angles on soda lime glass after four cleaning and handling steps, with a 15 degree accept limit and the 10 degree bottom of the Dropometer range marked

Example data, not measured results.

Business impact

What changes when the cleanliness check is a measured angle

Metric Before Dropometer With Dropometer Results
How the check is made Water break test, judged by eye Water contact angle read from the drop image No published benchmark
Result format Pass or fail by eye An angle per drop, 0.01° display, ±0.35° accuracy Dropometer product specification
Where it runs Shared lab goniometer At the cleaning line, on battery for up to 8 h Dropometer product specification
Very clean glass below 10° Lab goniometer rated for low angles Still needs that goniometer for the exact value Dropometer range 10° to 175°, product specification

See the measurement on your own glass

Bring two parts: one that coated or bonded well and one that did not. We will measure both live.

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ROI estimate

Estimate the cost of the coating and bonding failures this test can catch

Enter your own numbers. The defaults are examples, not Droplet Lab results.

Quality cost snapshot

Estimate what wetting related failures cost you.

Example only. Use what you see in a trial on your own samples.
Your own figure.
Your own estimate.

Result

~0
Estimated annual cost today
~0
Estimated annual cost avoided
~0
Payback period

Annual exposure combines audit/CAPA cost, customer rejections, and recall probability; avoided exposure applies the risk reduction percentage.

Settings for contact angle measurements on glass

Lock these before you compare results between shifts or sites

Parameter Recommended Setting Technical Rationale
Test liquid Reagent water to ASTM D1193 ASTM lists D1193 reagent water as a referenced document. Impure water gives false readings.
Drop type Sessile drop, static angle, as the standard specifies The method reads a drop resting on the glass.
Drop volume As the standard specifies, fixed for every drop A changing volume adds spread that is not about contamination.
Reading time As the standard specifies, fixed for every drop Water keeps spreading on clean glass, so the time must not vary.
Measuring range 10° to 175°; record lower readings as below range Dropometer product specification. Very clean glass can spread below 10°.
Fit model Young-Laplace or polynomial, the same for every run Both models are in the Dropometer software. Do not mix them within one study.
Time since cleaning Fixed and recorded Clean glass picks up films from the air, so the angle can rise with storage time.
Environment Record room temperature and humidity The Dropometer runs at 10 to 45 °C and has no temperature or humidity control.

Is ASTM C813 the right test?

Pick the method by the question you need answered

Start: What do you need to know about the glass surface?

Contamination as a measured angle

Signals:

You want a number per part or batch to check cleaning, and your accept limit is above 10°.

Rule-out:

Use ASTM C813, this page. The Dropometer is partially compliant and covers angles from 10° to 175°.

Exact angles on very clean glass

Signals:

Your glass reads below 10° and you must report the value, or your limit is below 10°.

Rule-out:

Use ASTM C813 on a goniometer rated for low angles. The Dropometer can show the drop is below range, not the value.

A quick check over the whole part

Signals:

You want to see at a glance whether water sheets cleanly over a large surface.

Rule-out:

Use the water break test to ASTM F22. Contact angle complements that check with a number.

Why a coating or adhesive fails

Signals:

You need polar and dispersive surface free energy, not only whether a film is present.

Rule-out:

Use a two liquid method such as ASTM D7490.

Decision tree at a glance

Start from the question you need answered. The blue box is the part of this work the Dropometer runs.

Decision tree for choosing between ASTM C813 on the Dropometer, ASTM C813 on a low angle goniometer, the ASTM F22 water break test and a two liquid surface energy method

Is ASTM C813 the right test?

Interpreting contact angles on glass surfaces

What the angle tells you, and what it does not

Higher angle, more hydrophobic film: Clean glass is hydrophilic and wets well. Oils, silicones and other organic films make water bead up, so the glass shows a higher contact angle.
Spread across the part matters: A good mean with one high position points to a handling, rack or drying problem in that area.
Clean glass does not stay clean: Record cleaning time and test time. Glass left in open air can read higher hours later than at the line.
Angle is not identity: A raised angle shows a film is present. It does not say what the film is or where it came from.
Expert perspective

Why water contact angle reveals films on glass

What a liquid responds to is only the outermost few ångström of a surface: the skin, not the bulk. Change that skin and you change the wetting, even if the material underneath is identical.

Dr. Alidad Amirfazli

Department of Mechanical Engineering, York University, Co-founder and Scientific Advisor, Droplet Lab

Common pitfalls and limits

Honest limits of the method and of the Dropometer

Very clean glass can read below 10°, outside the Dropometer range. Record it as below range, or use a goniometer rated for a low contact angle.
ASTM notes that rough or porous surfaces make the test less sensitive. Frosted, etched or textured glass needs its own limits.
Skin oils, powdered gloves, interleaf paper and plastic packing all leave films. Handle glass by the edges with clean tools.
Glass with a deliberate hydrophobic coating reads high by design. This method is for unwanted films, not coatings.
The Dropometer has no temperature or humidity control, and its camera runs at 10 frames per second. Read at a fixed time and record room conditions.
FAQ

Frequently asked questions

What does ASTM C813 measure?

It measures the water contact angle on glass to detect hydrophobic contamination. Clean glass wets well, so a raised angle points to an oily or organic film. ASTM states that the test does not damage the glass and can detect less than a single molecular layer of contamination.

Can the Dropometer run ASTM C813?

In part. The Dropometer reads the static water contact angle on glass from 10° to 175° with ±0.35° accuracy. Very clean glass can read below 10°, outside that range. If your accept limit is above 10°, it covers the check. If you need the exact value below 10°, use a goniometer rated for low angles.

What water contact angle shows clean glass under ASTM C813?

The public ASTM summary gives no pass or fail angle. Set your accept limit from a correlation study: measure glass that coated, bonded or printed well and glass that failed, at the same time after cleaning. See glass cleaning validation.

How is ASTM C813 different from the water break test?

The water break test to ASTM F22 checks by eye whether water sheets over the surface or breaks into patches. ASTM C813 gives a measured angle per drop that you can trend. Many plants use both. See ASTM F22.

Does the test work on rough or coated glass?

With care. ASTM notes that rough or porous surfaces make the test less sensitive, so etched or textured glass needs its own limits. Glass with a deliberate hydrophobic coating reads high by design, so the method does not apply to the coated face.

Can ASTM C813 monitor contamination in a clean area?

Yes. ASTM describes exposing glass that is free of hydrophobic films to the processing ambient, then testing it. A rising angle on these witness pieces shows films settling from the air. Keep the exposure time fixed and record it.

Legal note

This page summarises ASTM C813 in our own words and does not reproduce the standard. Buy the current edition from ASTM before you write a procedure. The Dropometer is partially compliant with this method. Droplet Lab does not certify compliance. Your own method validation does.

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How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

ASTM C813-20(2024) listing; ASTM C813-20 listing (historical); ANSI webstore: ASTM C813-20(2024); BSI Knowledge: ASTM C813-20(2024), current, the Dropometer product specification and Droplet Lab validation papers.

02

Drafting assistance

Drafted with Claude (Anthropic) from the sources listed, then reviewed by Droplet Lab.

03

What we did not do

We did not copy text from the standard. Buy the current edition for the full procedure.

04

Last checked

October 2026

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Correction Request

We work hard to keep this page accurate and up to date. If you spot an error (wrong revision/year, missing requirement, incorrect interpretation, or broken link), tell us and we'll review it.

Contact us to report a correction

References

1. ASTM C813-20(2024). store.astm.org/c0813-20r24.html
2. ASTM C813-20 listing (historical). store.astm.org/c0813-20.html
3. ANSI webstore: ASTM C813-20(2024). webstore.ansi.org/standards/astm/astmc813202024
4. BSI Knowledge: ASTM C813-20(2024), current. knowledge.bsigroup.com/products/standard-test-method-for-hydrophobic-c
5. Droplet Lab. Peer reviewed validation papers in Review of Scientific Instruments and Colloids and Surfaces A. dropletlab.com/validation/publications/
6. Droplet Lab. Dropometer product specification, 2026. dropletlab.com/products/dropometer/