Metal finish sample approval and batch variation

At a glance

Causes the sources name. None gives a typical amount of variation or promises anything about a batch.

A control sample and two limit samples

A single signed sample fixes the aim but not the edges. The sources we read all accept that some variation will happen. ASSDA says some variation in surface appearance is inevitable, and Euro Inox says electrochemically coloured stainless shows slight variations in colour. So the useful question is not whether two pieces match, but how far apart they may be before one is rejected.

The answer is three samples. The control sample, signed and dated, is the aim. A light limit and a dark limit (or coarse and fine, or high and low sheen, whichever property varies) mark the edges of what will be accepted. One aluminium finisher calls these range samples; the trade also says limit or border samples. For coloured stainless, Euro Inox says larger samples showing the typical colour range can serve as the visual standard for the project once specifier and supplier agree them.

This sheet is the metal companion to our sheet on the sample hierarchy, which explains the seven kinds of sample and who signs. Ask the partner finisher early whether it can make limit samples for your finish. For a one-off hand finish such as a patina it may only be able to show a range, and you should know that before you sign.

The approval checklist

Before the samples are made (agree with the partner finisher)

• Name the finisher and the process, for the sample and for production. A sample made by a different finisher proves little.
• State the metal: alloy and EN designation, thickness and temper. ASSDA says different grades will not match exactly and that 2B cannot match across thicknesses. An aluminium finisher says sheet and extrusion always differ.
• State the finish by code or process (see stainless finish codes) and the values to be measured on the sample: Ra and the direction measured, gloss in GU at 60 degrees, colour if it matters.
• Agree who makes the limit samples, from what, and by which process.

At approval

• View the control and limit samples together under the conditions you will write down: the lights, the distance, the angle and the sample size. ASSDA names these four; Euro Inox says to view under the lighting at the building. The sample hierarchy sheet explains why the lamp type matters.
• Mount the sample as it will be fitted: vertical on a wall, flat on a top, with the grain arrowed.
• Try the real use: handling, and the cleaner the client will use.
• Record the measured values beside the signature. Sign and date the control sample and both limit samples, and give each party a matching set.
• For a living finish or a patina, approve an aged sample as well as a new one. See patination of brass, bronze, copper and steel.

In production

• Make every piece that will be seen together from one lot of metal, in one run, by one finisher, and order the whole visible run at once. An aluminium finisher advises one lot and one shipment where possible, and says this is not always feasible.
• Treat a new lot, supplier, thickness, process or finisher as a new sample.
• Ask for the first production piece beside the control sample before the rest are finished.

On delivery and fitting

• Check each piece against the control and limit samples, under the written conditions, before it is fixed. The same finisher advises checking incoming shipments for variation before installation.
• Fit pieces in order of tone so that neighbours match, keep dissimilar pieces apart, and hold the grain direction.
• Photograph anything outside the range, with a scale and the measured values, and record it as a snag (see snagging and acceptance criteria).

The approval record, line by line

One page, one record for each control sample. The drawing shows the layout; the table says why each field is there. The reference matches the one in the specification, as the sample hierarchy sheet describes.

Acceptance tolerances: what is written down, and what is not

Designers often ask what colour difference (delta E) is acceptable between batches. None of the documents we read gives a pass or fail figure for that, so we do not either. What they do give is below, with how to state each property.

If a number matters, ask the finisher what it holds its own production to and how it measures it, then write that figure and the method on the record. Where something cannot be measured, such as texture, sparkle, grain or colour that shifts with the angle, the limit samples decide.

Keeping a visible run together, and what to do when it drifts

• One finisher for the whole run. An aluminium finisher advises one finisher for all set-ups, and the same colour sample for every finisher where metal is matched to existing work.
• Plan the whole run early, so the metal is ordered once. That finisher says a single lot is not always possible, especially across different gauges and widths.
• Do not mix forms. Sheet and extrusion are different alloys, and the finisher says each should match within its own type.
• Heat changes a finish. Welding and grinding can change the surface and leave a halo on anodised aluminium. IMOA says a weld should not be rougher than its surroundings, and ASSDA says weld heat tint must be treated.
• Matching old metal. New metal will not match aged metal on the day it is fitted. Judge the sample against the existing piece in place, and say whether the new work is to be aged.
• When a piece is outside the range, compare it with the control and limit samples under the written conditions, photograph it with a scale, and record the measured values. Whether it can be put right depends on the finish. Euro Inox says coloured stainless cannot be repaired locally if it is scratched; for the rest, ask the partner what is possible and how long it takes.

Finish to [element] shall match the control sample [S-01], approved and signed by [designer], [maker] and [client] on [date]. The accepted range lies between the limit samples [S-02, lighter] and [S-03, darker]. A piece outside the limit samples is not accepted. Samples were made by [finisher] by the production process, in [alloy and thickness]. A matching set is held by [maker] and [designer].

Roughness [Ra 0.5] micrometres maximum, measured [in the direction stated on the record]. Gloss [30] GU [plus or minus 5] at 60 degrees to [BS EN ISO 2813, edition to be confirmed]. [Colour: instrument, light source, observer and colour-difference formula named by the finisher; limit proposed by the finisher and agreed before production.] Measured values for the control sample are recorded on approval record [S-01]. Where a property cannot be measured, the limit samples govern.

Samples and production are judged [vertical / flat] as fitted, from [distance] m, at [angle], under [daylight] and [lamp type of the room], at [light level / as agreed]. The conditions are written on the approval record.

All pieces of [element] seen together are made from one lot of [alloy, thickness], finished in one run by [finisher] and delivered in one shipment [where practicable]. Pieces are marked with lot and run, and fitted in order of tone with the grain as drawn. A change of metal lot, supplier, thickness, process or finisher after approval needs a new sample and written approval before use. The first production piece is presented beside the control sample before the remaining [number] are finished.

Standards and sources

• IMOA: Which Stainless Steel Should Be Specified for Exterior Applications? (ABC 00/03, 2003; via BSSA)
• Euro Inox: Colouring Stainless Steel, Vol. 16, 2011 (via BSSA): sections 2, 2.3 and 9.
• Euro Inox: Guide to Stainless Steel Finishes (via BSSA)
• ASSDA: Mill forms (2023)
• ASSDA: Technical FAQ 6, Preventing Coastal Corrosion
• PCI: Architectural Specification Comparison (March 2021)
• Canadian Conservation Institute, CCI Notes 9/3 (2007)
• An aluminium finisher's technical guides on colour variation and range samples: US practice.
• BS EN ISO 2813, ISO/CIE 11664, BS EN 10088-2, AAMA 611: named, not read; confirm current editions.

Questions we are asked

How many samples should a metal finish approval include?

At least three: a signed control sample and a light and a dark limit sample, with a matching set held by each party. Euro Inox describes samples showing the colour range serving as the project's visual standard, and an aluminium finisher makes range samples for the same reason. For a hand finish the finisher may only be able to show a range.

What colour difference (delta E) is acceptable between batches?

We found no pass or fail figure for batch matching in the documents we read. The powder-coating comparison we read gives gloss tolerances and, for colour, an established colour range, and states colour difference only for weathering. Ask the finisher what it holds production to, name the instrument, light and formula, write the limit on the record, and let the limit samples settle what the instrument cannot.

Why does anodised aluminium vary in colour from batch to batch?

One aluminium finisher names the alloy, temper, mill lot, coating thickness and process, and adds that sheet and extrusion will always differ. Its advice is one lot and one shipment where possible, range samples, and sorting panels by tone on site.

Will two stainless panels with the same finish code match?

Not always. ASSDA says finishes from different suppliers are unlikely to match, that different grades will not match exactly, and that a 2B finish cannot match across different thicknesses. Take a visible run from one supplier at one thickness and approve a sample.

Who keeps the control sample?

Each party keeps a matching copy, and the maker's copy stays where the work is made, so that production and inspection use the same benchmark. The sample hierarchy sheet sets out the register.

Does an approved sample cover a patinated finish?

Only as a range. A fabricator says patina results depend heavily on the conditions and the surface preparation, and that patina applied on site is very hard to control. Approve an aged sample too, and say whether the finish is sealed.