Cleanliness and Contamination

Glass Cleaning Validation and Cleaning Verification for Pharmaceutical Glassware and Glass Surfaces

Glass can look clean and still carry a film of cleaning agent, product residue or oil. A water contact angle check on the cleaned surface turns visually clean into a number, so a failed cleaning process is caught before the glassware or glass part goes back into use.

Who this is for: QA, validation and laboratory teams in the pharmaceutical industry, and process engineers cleaning optical, display or architectural glass.

Where it fits: After the cleaning process and drying, before glassware or glass parts are released, and during cleaning validation studies.

What it does not do: It does not identify or quantify residues. Keep your validated analytical method, such as TOC or HPLC, for acceptance limits.

Method

Sessile drop water contact angle on glass

Standard

ASTM C813 (partially compliant)

Setup

About 2 minutes

Book a 15 minute demo Request a quote

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

What regulators and published data say about cleaning verification

1993

the year FDA's cleaning validation inspection guide set the expectation that cleaning procedures be validated, with written procedures, protocols and a final report.

US FDA, Validation of Cleaning Processes (7/93), inspection guide, 1993

Most desirable

how the same FDA guide describes direct sampling of the equipment surface, compared with rinse sampling alone.

US FDA, Validation of Cleaning Processes (7/93), inspection guide, 1993

83.1° to 24.1°

contact angle on titanium before and after plasma cleaning removed surface contaminants, measured with the Dropometer by Harvard researchers.

Ho et al., Biomimetics 10:571 (2025)

Sources: US Food and Drug Administration, Guide to Inspections: Validation of Cleaning Processes (7/93); K. Ho, T. Shiba, C.-Y. Chen, D. M. Kim, Biomimetics 10:571, 2025 (a customer publication that used the Dropometer). Figures and wording come from these sources, not from Droplet Lab measurements.

Quick reference

What this glass cleaning check does and what it does not

A quick reference for QA, validation and process teams checking fit before reading further.

Evidence Box

Problem this solves

Glassware and glass surfaces that pass a visual check but still carry cleaning agent, product residue or oil.

Standard

ASTM C813 uses water contact angle to detect hydrophobic contamination on glass. Our standards page lists the Dropometer as partially compliant.

Peer reviewed method

The Dropometer drop shape method was validated against a KRUSS DSA100E in two peer reviewed papers, in Review of Scientific Instruments and Colloids and Surfaces A.

Customer publication

Harvard researchers used the Dropometer to show contact angle falling as plasma cleaning removed contaminants from titanium (Ho et al., Biomimetics, 2025).

Customer case study

The US R&D site of a global fused silica and quartz maker uses the Dropometer for surface tension and surface energy.

Honest limit

Contact angle flags a surface that is not clean. It does not identify the residue or replace a validated analytical method.

Who this is for

What glass cleaning problem are you trying to solve?

Pick the card closest to your role. Each one points to the part of this page that answers it.

QA or validation manager

Your cleaning validation program relies on swab and rinse results plus a visually clean check. A water contact angle reading adds a fast, reliable surface check between analytical runs and after changes to cleaning.

Contact angle supports, and does not replace, the analytical method in your validation protocol.

Laboratory manager

Manual cleaning of glassware varies between people and shifts, and residues can affect the next test. A contact angle check shows which washer loads or hand washed items are not clean.

Set gates from your own clean glassware, not a value copied from another lab.

Glass process engineer

Optical, display or architectural glass must be clean before coating or bonding. A contact angle check after the cleaning line shows whether the surface is ready, before a coating defect appears.

If surfaces match the baseline, look at the coating or bonding process next.

Fit check

Is a contact angle check right for your cleaning process?

It helps when you need a fast, direct check that a glass surface is clean. It helps less when you need to know what the residue is or how much is there.

Good fit if

You clean glassware or glass lined equipment and rely on a visually clean check.

Manual cleaning methods vary between operators or shifts.

Glassware is shared between products rather than kept as dedicated equipment.

You suspect cleaning agent residue after rinsing.

You are changing the cleaning agent, its concentration or the cleaning cycle.

Glass must be clean before coating, bonding or printing.

Less relevant if

You need to identify or quantify a specific active pharmaceutical ingredient residue. That needs a validated analytical method.

Residues are hydrophilic and do not change how water wets the glass.

Surfaces are too small, curved or wet to place a drop on.

You need microbiological cleanliness. That needs bioburden testing.

Summary

How to verify that glass is clean after the cleaning process

The answer in under a minute.

Clean glass has a high surface energy, so water spreads on it. Cleaning agent, product residue and oils make glass more water repellent. Measure water contact angle on the cleaned, dry surface and compare it with a baseline from glass you know is clean. If the angle is higher, clean again before release.

In a cleaning validation program, your validated analytical method still sets acceptance limits. This check gives a direct, fast reading of the equipment surface for routine cleaning verification. It also gives a quick read on cleaning effectiveness when you validate the cleaning process after a change. ASTM C813 uses the same water contact angle principle, and the Dropometer contact angle kit runs it at the bench. Harvard researchers used it to track contaminant removal; read our analysis of that paper. A fused silica and quartz R&D site also uses the Dropometer.

Expert Quote

Expert perspective on invisible residue

“

Because only the top few ångström decide wetting, an invisible contaminant film, even a monolayer picked up from the air, is enough to change how a liquid behaves on the surface.

The problem

Why visually clean glass is not always clean

In pharmaceutical manufacturing, the purpose of cleaning validation is to show that a cleaning procedure removes product residue, cleaning agent and contaminants from manufacturing equipment surfaces and glassware to an acceptable level, every time. Good manufacturing practice expects written cleaning procedures, a validation protocol and documented results, and FDA's guide describes direct sampling of the equipment surface as the most desirable approach. Analytical methods such as TOC and HPLC quantify residue on swabs and rinses, but they take time and are run on samples, not on every item. Between those tests, many sites rely on a visual check. Glass makes this hard: a thin film of detergent, silicone, oil or product can be invisible, yet it changes how liquids behave on the surface and can carry over into the next batch or test. The same problem affects optical, display and architectural glass, where an invisible film causes coating and bonding defects.

Water beads or breaks into patches on glassware that should be clean.

Swab or rinse results vary between operators or washer loads.

Residue appears after a change of cleaning agent or cleaning cycle.

Coatings or adhesives fail on glass that looked clean.

Results drift in tests run in reused glassware.

Troubleshooting

Root causes behind failed cleaning verification on glass

Why:

Detergents and surfactants left after poor rinsing leave a film that changes how water wets the glass and can interfere with later tests.

How to detect:

Water contact angle above the clean baseline, often uneven across the surface.

Corrective action:

Check rinse volume, water quality and cleaning agent concentration, then recheck. Our guide to sample prep and contamination control covers keeping samples clean while you measure.

Why:

Hydrophobic product residues can remain in corners, threads and rough areas after the cleaning procedure.

How to detect:

A high angle at specific spots, while other areas match the baseline. Confirm with your analytical method.

Corrective action:

Review the effectiveness of the cleaning method for that item and residue, and the worst case locations in your validation protocol.

Why:

Skin oils, silicone grease, lubricants and packaging films are hydrophobic and spread easily across glass.

How to detect:

The angle rises sharply against clean glass. Our contact angle reference values for common surfaces give a sense check.

Corrective action:

Handle cleaned items with clean gloves, remove silicone grease from shared equipment, and store cleaned glass covered.

Why:

Manual cleaning methods depend on the operator. FDA's guide asks how much manual cleaning varies between batches and products.

How to detect:

Readings vary more by operator or shift than by item.

Corrective action:

Tighten the procedure, train operators, compare with automated cleaning where possible, and trend readings by operator to show improvement.

Why:

Clean glass picks up airborne films during drying and storage. Only the outermost molecular layers decide wetting, so even a thin film matters.

How to detect:

The angle rises with storage time after cleaning. The wettability glossary entry explains why a thin film changes the reading.

Corrective action:

Set a clean hold time, cover or wrap cleaned items, and recheck items stored past it.

Why:

Etched, scratched or coated glass wets differently from new glass, whatever its cleanliness.

How to detect:

The angle differs from the baseline even after repeated cleaning. The superhydrophobic glass experiment shows how much surface treatment changes wetting.

Corrective action:

Set a separate baseline for treated or worn glass, or retire damaged items. A cleaning change will not fix this.

Not sure whether the glass is clean?

Bring a clean reference item and one you suspect to a call, and we will compare them on screen.

Book a 15 minute demo

Quality records

What a cleaning verification record contains

Each check produces a record you can file with the cleaning batch. These records can feed into your existing quality records under good manufacturing practice, ISO 9001 or your site's quality system.

Item identity

Glassware or part ID, equipment used, and the product or test it was used for.

Cleaning details

Cleaning procedure, cleaning agent and concentration, manual or automated cleaning, operator and time.

Method settings

Probe liquid, drop volume, spots and the reading time after each drop lands.

Readings

Contact angle at each spot, the median and the spread.

Decision

Release, clean again or send for analytical testing against your specification, with the operator and date.

Drop images

Automatic edge and baseline detection gives the same reading from the same image, so a reviewer can recheck any result.

Measurements

What to measure on cleaned glass

Primary check

Water contact angle on the cleaned, dry glass surface

Why it matters: It responds to hydrophobic films from cleaning agent, product residue and oils.

How to interpret: Compare with a baseline from glass you know is clean. There is no universal pass value; set gates per item and cleaning process from your own correlation study with your analytical method.

When it is not enough: It cannot identify the residue or show residues that do not change wetting.

Uniformity

Spot to spot spread on each item

Why it matters: Residue often stays in corners, necks and rough areas.

How to interpret: A median inside the gate with one high spot still means part of the surface is not clean.

When it is not enough: It cannot reach inside narrow necks or tubing.

Hold time

Contact angle versus storage time after cleaning

Why it matters: It shows how long cleaned glass stays clean in your storage conditions.

How to interpret: Set the maximum clean hold time where the angle starts to rise.

When it is not enough: Packaging and room air change the result; recheck when they change.

Correlation

Contact angle against swab or rinse results

Why it matters: It links the fast check to the analytical method in your validation protocol.

How to interpret: Plot contact angle against residue results from the same items to set a contact angle gate.

When it is not enough: Treat the correlation as part of method validation; it is specific to one residue and cleaning process.

Validation

Validated measurement approach for glass cleanliness

How the Dropometer itself has been validated, and where customers have published with it.

Peer reviewed method

The Dropometer drop shape method is published in Review of Scientific Instruments and Colloids and Surfaces A. Contact angle was validated against a KRUSS DSA100E, with a published accuracy of 0.35°.

See the validation papers

Customer publication on contaminant removal

Harvard researchers used the Dropometer to measure titanium before and after plasma cleaning removed surface contaminants; contact angle fell from 83.1° to 24.1° on machined discs (Ho et al., Biomimetics, 2025). It is titanium, not glass, but the same principle.

Read our analysis of the paper

QC Protocol

How to add contact angle to a cleaning validation protocol

The steps below assume one item type, one residue and one cleaning process. Repeat steps 1 to 3 for each new combination, and when developing cleaning procedures.

1

Lock the method

Fix the probe liquid, drop volume, reading time, spots and drying conditions. Our contact angle measurement guide covers the basics.

2

Baseline clean glass

Measure items you know are clean, for example new glass cleaned and confirmed with your analytical method.

3

Correlate with your analytical method

Measure items with known residue levels from swab or rinse testing, and set release, clean again and test bands from where the angle tracks residue.

4

Check after each cleaning

Check items after the cleaning process and drying, before release, at the worst case spots in your protocol.

5

Release, clean again or test

Release items inside the gates. Clean again outside them, and send repeat failures for analytical testing.

6

Trend and review

Trend readings by item, operator and cleaning agent lot, and use them in the periodic review of cleaning and after any changes to cleaning.

We completed our gage R&R study on the unit and it performed very well.

Brandon Barbee

Corporate Quality Engineer - Zeus Industries - Polymer Manufacturing

Download the glass cleaning verification SOP

A two page SOP you can adapt for your glassware and cleaning process, with the method lock, gate setting steps and a record sheet.

Contact angle measurement with the Dropometer

Example output

What a glass cleaning verification record looks like

Example data only. Your gates and values will differ and must come from your own correlation study.

How to read this record

Example gate: within 5° of the clean baseline with spread up to 3°. Green rows are released, amber rows are cleaned again, red rows are cleaned again and sent for analytical testing.

Sessile drop contact angle measurement: DI Water on Teflon surface, left contact angle 44.9°, right 45.7°

Example glassware cleaning log

Item	Cleaning	Spots	Median (°)	Spread SD (°)	Versus baseline	Result
Clean reference beaker	Validated wash, TOC confirmed	5	8.4	1.1	Baseline	Reference
Beaker B-114	Washer cycle 2	5	10.2	1.6	+1.8	Release
Flask F-031	Manual cleaning	5	17.9	4.2	+9.5	Clean again
Vessel lid L-07	Manual cleaning, silicone grease used	5	48.6	9.7	+40.2	Clean again and test

Example data, not customer data. Water contact angles on borosilicate glass, read at a fixed time after deposition.

Signal order

The order to check signals after cleaning glass

Check these in order. Each one rules out a cause before you look at the next.

1

Contact angle versus clean baseline

Shows whether a hydrophobic film is left on the surface.

2

Spot to spot spread

Shows whether residue is patchy, often at corners and necks.

3

Operator and cleaning method

Links a drift to manual cleaning, cycle or cleaning agent changes.

4

Hold time

Links a drift to storage after cleaning.

5

Analytical confirmation

Swab or rinse testing confirms the residue and the level on items that fail.

FAQ

Common questions about glass cleaning validation and verification

Questions QA and validation teams ask before adding contact angle.

What is the difference between cleaning validation and cleaning verification?

Like process validation, cleaning validation shows, with a validation protocol and documented studies, that a validated cleaning procedure reliably removes residues to an acceptable level. Many programs treat three consecutive successful cleaning runs as a successful validation. Cleaning verification checks that a particular cleaning worked, item by item or batch by batch. A water contact angle reading is useful for routine verification between analytical tests.

Can contact angle replace swab or rinse testing?

No. Swab and rinse samples tested with a validated analytical method identify and quantify residues against your cleaning validation acceptance criteria. Contact angle only shows whether a hydrophobic film is on the surface. Use it to screen items, to compare cleaning methods and to trend routine cleaning, and correlate it with your analytical method first.

What does ASTM C813 cover?

ASTM C813 is a test method that uses water contact angle to detect hydrophobic contamination on glass. Clean glass wets well, so a higher angle points to an organic film. See our ASTM C813 page for how the Dropometer supports the method.

Which residues does contact angle detect?

Mainly hydrophobic residues: oils, silicone, many surfactant films and hydrophobic product residues. Highly water soluble or hydrophilic residues may not change the angle. That is why the check sits alongside, not instead of, your analytical method.

Where does this fit in GMP and regulatory requirements?

FDA's 1993 inspection guide, PIC/S guidance and ICH Q7, the good manufacturing practice guide for active pharmaceutical ingredients, all expect validated cleaning procedures and documented results, and FDA's guide expects written procedures for each piece of equipment. Contact angle records can feed into your existing quality records. Whether you use them in a validation protocol is a decision for your QA and validation team.

Does this work outside the pharmaceutical industry?

Yes. The same check is used on optical, display and architectural glass before coating, bonding or sealing. Our construction guide covers glass and facade work.

What should we look for in an instrument for this check?

It should measure low contact angles on glass reliably, use a published method, give repeatable results across operators and keep drop images with each reading. Our guide to the best contact angle goniometer for industrial QA compares options.

Business impact

What changes when you measure glass cleanliness

Typical changes in how teams work. Benchmarks are given only where a published source exists.

Before and with a contact angle cleaning check

Metric	Before Dropometer	With Dropometer	Indicative Benchmark
Routine cleaning verification	Visual check	Contact angle on each item or batch	FDA calls direct surface sampling the most desirable method (FDA, 1993)
Manual cleaning variation	Seen only in analytical results	Trended by operator and shift	FDA's guide asks how much manual cleaning varies (FDA, 1993)
Clean hold time	Set by habit	Set from measured drift	No published benchmark; track on your own items
Cleaning changes	Analytical testing only	Fast screen first, then analytical testing	No published benchmark; track on your own items

See the check on your own glassware

Bring a clean reference item and one you suspect. We will show the difference on a call.

Book a 15 minute demo

Estimate the cost of cleaning related findings and rejections

Enter your own numbers. The result is an estimate, not a Droplet Lab claim.

Instrument cost ($) Each Dropometer unit is $5,000; the default models 1 unit.

Share of these events prevented (%) Use your own investigation records; start low if unsure.

Cost per inspection finding or CAPA ($) Investigation and remediation cost for one finding.

Inspection findings or CAPAs per year Tied to cleaning or contamination issues specifically.

Cost per customer or batch rejection ($) Value of one rejected batch or shipment.

Rejections per year From contamination-related causes only.

Cost of one recall ($) Your own estimate for one recall event.

Annual recall probability (%) Your own historical rate, not an industry average.

Result

~0

Annual exposure (current)

~0

Annual exposure avoided

~0

Payback period

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

Limits

What a glass cleaning check cannot tell you

Honest limits, so you know when to reach for another test.

It does not identify or quantify residues.

Hydrophilic residues may not change the reading.

Very low angles on clean glass need a consistent drop volume and reading time.

Wet or still drying glass gives wrong readings; measure only dry surfaces.

Narrow necks, tubing and threads cannot be measured directly.

Gates do not transfer between glass types, residues or cleaning processes.

The check flags glass that is not clean. Residue identity, residue level, microbial cleanliness and compliance decisions still need validated analytical methods and your QA process.

Book a 15 minute demo Request a quote

How this page was created

Editorial and technical transparency notes for this page.

Transparency Details 4 checklist items

01

Drafting assistance

Drafted with Claude Opus 5.5 (Anthropic) using web search for sources, then edited by the Droplet Lab team.

02

Technical review

Reviewed and edited for technical accuracy by the Droplet Lab Team.

03

Verification steps

Standard identifiers, units, thresholds and key procedural claims are checked against cited sources before publication.

04

Updates

Reviewed every 12 months or when the underlying method changes.

Report a correction

Spotted an issue in this summary? Send a correction request and our team will review it.

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

Sources

References

1.

US Food and Drug Administration. Guide to Inspections: Validation of Cleaning Processes (7/93), 1993. https://www.fda.gov/validation-cleaning-processes-793

2.

ASTM C813. Standard test method for hydrophobic contamination on glass by contact angle measurement. https://dropletlab.com/industry-standards/astm-c813/

3.

Ho, K., Shiba, T., Chen, C.-Y., Kim, D. M. Plasma treatment to remove titanium surface contaminants and improve implant biocompatibility: an in vitro study. Biomimetics 10:571, 2025. Customer publication that used the Dropometer; the link goes to our analysis of the paper. https://dropletlab.com/validation/citations/analysis/plasma-treatment-to-remove-titanium-surface-contaminants-and-improve-implant-biocompatibility-an-in-vitro-study/

4.

Chen, X. et al. Contact angle measurement with a smartphone. Review of Scientific Instruments 89(3):035117, 2018. https://pubs.aip.org/aip/rsi/article-abstract/89/3/035117/368179/Contact-angle-measurement-with-a-smartphone

5.

Surface tension measurement with a smartphone using a pendant drop. Colloids and Surfaces A: Physicochemical and Engineering Aspects. https://www.sciencedirect.com/science/article/abs/pii/S0927775717307744

6.

Droplet Lab validation papers

7.

Customer publications that cite the Dropometer

8.

ASTM C813 explained

9.

Fused silica and quartz R&D case study

Glass Cleaning Validation and Cleaning Verification for Pharmaceutical Glassware and Glass Surfaces

Featured Studies & Publications

What regulators and published data say about cleaning verification

1993

Most desirable

83.1° to 24.1°

What this glass cleaning check does and what it does not

Evidence Box

What glass cleaning problem are you trying to solve?

QA or validation manager

Laboratory manager

Glass process engineer

Is a contact angle check right for your cleaning process?

Good fit if

Less relevant if

How to verify that glass is clean after the cleaning process

Expert perspective on invisible residue

Why visually clean glass is not always clean

Root causes behind failed cleaning verification on glass

Not sure whether the glass is clean?

What a cleaning verification record contains

Item identity

Cleaning details

Method settings

Readings

Decision

Drop images

What to measure on cleaned glass

Water contact angle on the cleaned, dry glass surface

Spot to spot spread on each item

Contact angle versus storage time after cleaning

Contact angle against swab or rinse results

Validated measurement approach for glass cleanliness

How to add contact angle to a cleaning validation protocol

Lock the method

Baseline clean glass

Correlate with your analytical method

Check after each cleaning

Release, clean again or test

Trend and review

Download the glass cleaning verification SOP

What a glass cleaning verification record looks like

Example glassware cleaning log

The order to check signals after cleaning glass

Contact angle versus clean baseline

Spot to spot spread

Operator and cleaning method

Hold time

Analytical confirmation

Common questions about glass cleaning validation and verification

What changes when you measure glass cleanliness

Before and with a contact angle cleaning check

See the check on your own glassware

Estimate the cost of cleaning related findings and rejections

Estimate exposure from cleaning related findings, rejections and recalls.

What a glass cleaning check cannot tell you

Standards, evidence and related cleanliness workflows

ASTM C813

ASTM F22

Fused silica and quartz R&D

Ho et al. (2025): our analysis

Dropometer

Cleanliness and contamination

Silicone and invisible residue detection

Wafer cleaning and photoresist adhesion

How this page was created

Drafting assistance

Technical review

Verification steps

Updates

Report a correction

References