# Why a clean-looking aerospace part can still fail a residue test

By MarketScale Newsroom · Published 2026-10-12 · Engineering & Construction on MarketScale
Canonical: https://www.marketscale.com/industries/engineering-and-construction/why-a-clean-looking-aerospace-part-can-still-fail-a-residue-test-1247892

> Element Materials Technology explains how NVR and gravimetric tests weigh the residue and particles a cleaned aerospace part still carries.

## Key points

- Nonvolatile residue testing weighs what a solvent rinse removes from a part, so a film the eye cannot see becomes a pass or fail figure against the drawing.
- IEST-STD-CC1246E, the document that replaced MIL-STD-1246 in 2002, sets residue limits from 10 nanograms to 25 milligrams per 0.1 square meter, and its particle levels count down to 5 microns, far below the 100 micron width of a hair.

One of Element Materials Technology's customers has flown a private jet carrying a single component to the lab, waited a few hours while it was precision cleaned, and flown the part straight back out to be installed. Katie Legowski, business operations manager for Element Enfield and Hartford, told the story on Coast to Coast: Precision Cleaning Techniques with Element, a webinar archived as a MarketScale Studio segment and facilitated by a host from Element. Cleaning is the last operation a part sees, she says, and by then the schedule is usually gone.

> We have some customers that will fly in a private jet with one component on it for us to precision clean components in a matter of hours and then take that private jet right back out to the end use to install that component. — Katie Legowski, Business Operations Manager, Element Enfield and Hartford (AS&D)

Speed is the visible pressure. Joe Moore, engineering specialist for precision cleaning at Element Los Angeles, built the webinar around a harder one: a component can be machined perfectly to print and still carry oil, fibers, abrasive media or chemical residue that no inspector can see, and that film or grit fails a test the eye would pass. For the quality engineer signing off aerospace, defense or medical device hardware, the question has moved from whether a part looks clean to whether a number proves it is.

## Cleanliness is a line on the drawing now

Moore says cleanliness requirements are increasingly written directly on part drawings and in specifications, which makes cleaning an engineered requirement with a limit attached. Left behind, contamination can restrict flow in a small passage, change heat transfer, interfere with a seal or coating, or seed corrosion and wear, he says. In the worst case, he adds, that means equipment failure, injury or loss of life.

> At the end of the day, looking clean and being clean are two very different things. — Joe Moore, Engineering Specialist, Precision Cleaning, Element Los Angeles (AS&D)

The document most of those call-outs trace back to is older than the trend. Astro Pak, a precision cleaning company, says in an explainer on its own site, updated in April 2026, that the US military wrote MIL-STD-1246 in the 1960s to set cleanliness levels, that the Institute of Environmental Sciences and Technology's IEST-STD-CC1246 replaced it in 2002, and that the current revision E dates to 2013. What has moved is how many parts get held to it and how closely: Moore says original equipment manufacturers and outside auditors now review a lab's procedures, training records, equipment, test methods and documentation, and the controls have to change as the requirements do.

The IEST describes the standard as a set of mutually agreed limits for surface and liquid cleanliness, covering particles and molecular residue, written for procurement and design contracts where contamination control decides reliability and not meant for every product. That is the language a drawing borrows when it puts a cleanliness level next to a dimension.

## How a solvent rinse becomes a weight

Nonvolatile residue testing, or NVR, measures what remains after a volatile solvent evaporates: oil, grease and processing fluid that is invisible on the part. Element's team describes it on the recording as a two weighing process. First comes a blank analysis, a control run with the same solvent and equipment but no part, so any residue contributed by the test itself is known.

Then the component is rinsed with a controlled volume of solvent, the rinse goes into a container that has already been precisely weighed, and the solvent is evaporated away. The container is weighed again, the blank is subtracted, and the difference is the mass of residue the part was carrying.

> NVR testing allows us to take contamination that may be too small to see and turn it into an actual measurable number. — Element Materials Technology precision cleaning team, Element AS&D

Precision Cleaning Group, a cleaning services company, says on its own site that ASTM F331 is the test method behind this, and that the extract is stripped of insoluble particles before the residue is weighed, so the particle count and the residue weight can come from one rinse. The same page explains why the blank matters. Rinsing is an indirect method, it says: detaching contamination from the surface lets the lab examine an entire component, including complex geometries a microscope cannot see into, but particles can be lost during extraction and the work needs very clean conditions so the test adds no contamination of its own.

The limits the result is judged against sit in a table. Astro Pak's reproduction of IEST-STD-CC1246E section 5.2.2 runs the NVR levels from R1E-5, which allows 10 nanograms, up to R25 at 25 milligrams, each per 0.1 square meter of surface or 0.1 liter of solvent, with the standard treating 0.1 liter of rinse as standing in for 0.1 square meter, about one square foot. On a drawing the code reads particle level first and residue level second. Astro Pak's worked example is 50R1: particle level 50 plus no more than 1 milligram of residue per 0.1 square meter sampled.

## Counting what the filter patch catches

Gravimetric testing, meaning measurement by weight, goes after solid particles rather than film. Element rinses or flushes the part with a controlled amount of test fluid, passes the fluid through a very fine filter patch, then dries and weighs the patch to get the total mass of particulate that came off. An automatic particle counter then reads the patch to report how many particles there are and how large each one is, so the result can be matched to an allowable size distribution instead of a single weight.

Precision Cleaning Group says ASTM F312 governs that count and size distribution, which an analyst can also do by scanning the full filter area under a microscope and tallying each size range by hand. Infinita Lab, a testing laboratory network, writes on its own site, in a page updated September 27, 2026, that the automated version is a light optical microscope that images the membrane and sorts each particle into a size class, and that labs add scanning electron microscopy with elemental analysis, which reads the chemistry of one particle, when they need to trace it to a machining chip, abrasive grain or rust. A part that misses the limit goes back for another cleaning cycle and another test, Element's team says, until the level is met.

> To put the cleanliness that we can achieve on-site in Hartford and LAX into perspective, we verify down to a 100 micron particulate level, which is essentially the width of a human hair. — Element Materials Technology precision cleaning team, Element AS&D

The table behind that claim counts far smaller things. In Astro Pak's reproduction of IEST-STD-CC1246E, level 50 permits 141 particles between 5 and 15 microns, 17 between 15 and 25, six between 25 and 50, one between 50 and 100, and none larger; the level number marks the size above which only one particle may appear. One level up, level 100 allows nine particles between 50 and 100 microns and a single one between 100 and 250. A micron is a thousandth of a millimeter, so a 100 micron detection limit sits at the boundary between those two levels, and the bins below it are where the table does most of its counting.

Element also sells the test without the cleaning. A manufacturer that cleans in-house can send the part or just a sample of the fluid it used to rinse or flush it, including PAO oil, a test oil that Moore says is commonly used to collect contamination as it moves through a component or system. The report lists particle count and size, whether the particles are metallic or nonmetallic, and images of what was recovered, and Moore says customers pass that report to their own vendors, customers and end users as outside confirmation that the criteria were met.

## Hair width is the coarse end of the family

Set against other industries' coding, the hair width benchmark is where counting begins. Infinita Lab's page says the automotive standards ISO 16232 and VDA 19, the inspection of technical cleanliness, report counts in classes such as 50 to 100 microns, 100 to 150 and 200 to 400, and that a single hard particle larger than the clearance between two mating surfaces is enough to start abrasive wear, silting or seizure in a valve or injector. The size that matters is set by the gap in the part, which is why one drawing's limit cannot be read across to another.

Fluid cleanliness codes go finer still. Separator Spares and Equipment, a filtration equipment supplier, says on its own site that NAS 1638, written by the National Aerospace Standards Committee in the 1960s, counts particles in five ranges starting at 5 to 15 microns, that its cleanest class 00 allows 125 such particles per 100 milliliters and none above 100 microns, and that SAE AS4059 replaced it for new designs in May 2001 with automatic counters calibrated to ISO 11171 and size bands starting at 4 microns. Those codes describe hydraulic fluid, the supplier notes, which is why a drawing for a valve body and a specification for the oil inside it can cite different documents.

At the semiconductor and implantable end, the numbers available describe the room rather than the part. American Cleanroom Systems, a cleanroom builder, puts class 3 implantable medical devices, satellite assembly and semiconductor work in ISO 5 rooms on its own site, where particles are counted down to 0.1 micron in the air; those are airborne limits and do not convert into a surface residue level.

Element's answer to the spread is to match the test to the drawing. Not every job needs the laboratory, Moore says: bright white light, UV, wipe and borescope inspection give industrial and medical customers a documented, repeatable check, with UV picking up oils that fluoresce and the borescope, an optical probe fed into internal passages, reaching cavities no exterior inspection covers. The lab methods come in when the specification demands a number.

## Which document the drawing actually names

IEST-STD-CC1246 sets the limits but not the procedures, which it borrows from ASTM F331 and F311, Astro Pak says in its explainer. That split is why a drawing call-out can be short and the work behind it long: the level comes from one document, the method from another, and the customer's own specification may add more.

On its own service page, Astro Pak lists more than 100 industry, government and company specifications it processes to, among them AMS 2700, ASTM F311, F312 and F331, ASTM A967, ISO 14952, SEMI F19, CGA G-4.1 and MIL-STD-1246. Precision Cleaning Group names ISO 14952 as the cleaning verification standard for spacecraft on its own site.

On the recording, Element's speakers refer to the customer's cleanliness requirement and the applicable specification without saying which of these documents a given aerospace drawing invokes, so the engineer should expect the mapping to come from the drawing rather than the lab. Moore says Element starts a program by understanding the component, its end use, the cleanliness requirement, the expected contaminants and the production need before it develops and validates a process.

## Nitric or citric, and what changed in 2018

Passivation is the one process in the webinar that is not strictly cleaning. For stainless steel components, Element uses nitric or citric acid to strip free iron and other contaminants left by machining and handling; with the iron gone, the chromium in the alloy forms a more uniform, protective chromium-rich oxide layer that gives stainless its corrosion resistance. The result is not a coating, Element's team says, and it leaves the part's dimensions unchanged.

Two documents govern the acid. RP Abrasives, a surface finishing company, says on its own site that ASTM A967 is the standard general industry uses to control stainless passivation, that AMS 2700, an Aerospace Material Specification from SAE International, is the aerospace equivalent, and that both set specific combinations of acid concentration, temperature and immersion time for nitric and for citric, along with tests to prove the passive layer formed.

Since March 2018, AMS 2700 F has let a shop use either nitric or citric acid unless the drawing says otherwise. RP Abrasives says revision E, issued in November 2011, said nitric was to be used unless citric was specified, and that revision F dropped that default. The same page says ASTM A967-17, approved July 1, 2017, added a boiling water immersion test and an appendix stating that hydrogen embrittlement restrictions on high strength grades such as 440C apply to nitric acid passivation and not to citric.

Element's speakers say the acid is selected by stainless grade, component condition, the applicable specification and the customer requirement, with concentration, temperature, immersion time, rinsing and drying controlled for a repeatable result. No one on the recording says which industries are driving the shift to citric acid, and the standards revisions do not say either.

## A Class 100 bag is a Federal Standard label

Once a part passes, Element bags it inside the cleanroom. The options the team lists are class 100 cleanroom polybags, vacuum sealing, nitrogen purging to displace air, VCI bags, which release a vapor corrosion inhibitor around the metal, and desiccants against moisture, with multiple bag layers, kitting and tagging when a program calls for them.

Class 100 is a Federal Standard 209E designation; under ISO 14644-1 the same air is ISO 5, American Cleanroom Systems says. The cleanroom builder's own site explains that 209E counts particles per cubic foot, so Class 100 means no more than 100 particles of 0.5 micron or larger per cubic foot, while ISO 14644-1 counts per cubic meter and sets ISO 5 at 3,520 such particles. The page adds that the semiconductor, electronics and aerospace industries still describe rooms in 209E terms because the class number reads directly as a particle count.

Behind an ISO 5 label, the same site lists at least 70 percent ceiling coverage with HEPA or ULPA filters, the high efficiency air filters that strip particles from the supply air, and 300 to 500 air changes per hour. Moore's point in the webinar is that this infrastructure, plus HVAC, environmental monitoring, calibration, training and waste handling, is what a manufacturer takes on when it decides to clean in-house.

## Two labs and the report between them

The verification work the team describes is done on-site in Hartford and at LAX, the Los Angeles lab where Moore works; Legowski runs business operations for Element Enfield and Hartford. Moore says customers at the LAX lab most often ask for quicker turnaround, and that Element designs and builds its own cleaning equipment and tooling, so a fixture can be adapted when a program changes and lead times can be set around a customer's production need.

Legowski's read is that flexibility is the job, because parts reach cleaning late by definition. What Element hands back with the part, Moore says, is a report with particle count and size, metallic or nonmetallic composition and images, built for customers who need objective data to show their own vendors and end users. Element is, as he puts it, the last thing a part sees before it is put on a machine or into space.

## Sources

- [IEST-STD-CC1246E: Product Cleanliness Levels](https://www.iest.org/Standards-RPs/Recommended-Practices/IEST-STD-CC1246) (IEST (Institute of Environmental Sciences and Technology))
- [Precision Cleaning Standards: IEST-STD-CC1246, MIL STD 1246, CGA G-4.1](https://astropak.com/precision-cleaning-standards/) (Astro Pak)
- [Cleanliness Verification and Validation Through Measurement of Particulate Contamination and Nonvolatile Residue](https://www.precgroup.com/cleanliness-verification-and-validation-through-measurement-of-particulate-contamination-and-nonvolatile-residue/) (Precision Cleaning Group)
- [ISO 16232 Technical Cleanliness Testing of Small Parts](https://infinitalab.com/blog/small-part-cleanliness-testing/?srsltid=AU7gw4UqWIH97HQAvtPH-Uwg-VoMc4WzcGuzkkGPKShZrZVvpREbeI2n) (Infinita Lab)
- [Cleanroom Classifications: ISO-14644-1 FED STD 209E and CGMP](https://www.americancleanrooms.com/cleanroom-classifications/) (American Cleanroom Systems)

Tags: Element Materials Technology, precision cleaning, NVR testing, aerospace manufacturing, quality engineering, passivation

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