Stained Glass Vs Mineral Deposit - Decision Tree

Why this matters

The single most expensive mistake a window cleaner makes is treating an etched (stained) glass pane as if it were a removable mineral deposit. Aggressive acid treatments on already-etched glass deepen the damage; aggressive mechanical polish on a thin coating like Low-E or pyrolytic film destroys an irreplaceable surface. A 36 by 48 inch insulated glass unit with a hard-coat Low-E layer can cost more than a thousand dollars to replace, and the property owner is going to send you that invoice if you cannot prove which condition you started with. This decision tree tells you what you are looking at before you touch it, and tells you what to write in the customer waiver if you proceed.

Definitions

• Mineral deposit: a layer of calcium carbonate, calcium sulfate, or silica that has dried ON the glass surface. Removable mechanically and chemically without altering the glass itself.
• Etched glass: micro-pitting of the glass surface caused by prolonged hard-water contact, alkaline runoff from concrete or stucco, or acidic cleaning chemicals. The pits ARE the glass. They are not removable; you can only polish the high points down to the depth of the pits, which thins the glass.
• Coating damage: physical or chemical damage to a manufacturer-applied film (Low-E, reflective, anti-glare, security film). Removable only by replacing the IGU.

Step 1 - The water-bead test

Wet a small area at the edge of the suspect zone with distilled water. Observe how the water behaves:

• Beads up and runs off cleanly: glass surface is intact. The deposit is on top.
• Spreads out into a continuous film with no beading: glass is etched. The film clings to micro-pits.

Confirm by wiping the wetted area with a clean dry microfiber. If the residue transfers to the cloth, it is a deposit. If nothing transfers and the cloudy appearance returns when the water dries, it is etching.

Step 2 - The razor test (corner only)

Pull a fresh single-edge razor across a half-inch corner of the suspect zone at a 30 degree angle. CAUTION: never razor tempered glass with fabrication debris (see Step 3) and never razor a coated surface.

• Material lifts and scrapes off, leaving clear glass below: mineral deposit. Proceed to chemical removal.
• Razor glides across without lifting anything: etching OR coating. Stop. Do not proceed with abrasive treatment.

Step 3 - Identify the glass type before razoring anything full-size

Per the Glass Association of North America, tempered glass produced after 2002 may have fabrication debris (small particles of glass or tin) embedded at the surface. Razoring tempered glass with debris causes long, visible scratches. The protocol:

• Check for a tempering stamp in the corner. If present, glass is tempered.
• Inspect for distortion under polarized sunglasses. Tempered glass shows iridescent stress patterns.
• Run a razor only on the upper outside corner first, as the scratch test. If you see a single scratch line, the glass has fabrication debris. Stop. Inform customer in writing.

The IWCA position is that tempered glass scratches are the manufacturer's defect, not the cleaner's negligence - but you must document the conversation BEFORE you razor.

Step 4 - Identify the coating type

Low-E coatings come in two forms: pyrolytic (hard-coat, applied during float-glass manufacture, on surface 3 of an IGU, generally cleanable) and sputtered (soft-coat, applied post-manufacture, almost always on surface 2 or 3 inside the IGU and inaccessible from outside). Anti-reflective and self-cleaning coatings can be on the outside surface.

• If the customer cannot tell you the glass spec, treat all coated surfaces as soft-coat: water and microfiber only.
• Look for a coating identifier mark printed at the corner or visible by tilting the glass at an angle - reflective coatings show a different reflectance than uncoated glass.

Step 5 - Match the treatment to the diagnosis

For mineral deposits on uncoated, non-tempered glass: white vinegar (5% acetic acid) soak for 10 minutes, then bronze wool grade 0000 with steady pressure, then squeegee clean. For heavier carbonate deposits, escalate to oxalic acid 5% solution with PPE per the SDS. Always test a one-inch square first.

For etched glass: Cerium Oxide polishing slurry with a felt pad on a variable-speed buffer at 1200 rpm. Works on uncoated single-pane glass only. Removes 0.001 to 0.005 inch of glass thickness, which is acceptable on float glass and unacceptable on tempered glass (alters the temper). Documents and warranty disclaimer required.

For coated glass with any visible damage: replacement only. Pure water and microfiber maintenance going forward.

Step 6 - The waiver

Before any restoration treatment, the customer signs a waiver acknowledging: (a) the glass condition pre-existed your arrival, documented with photos; (b) the treatment may not fully restore visual clarity; (c) the treatment carries a known risk of failure mode X; (d) the customer holds you harmless for outcomes attributable to the pre-existing condition. Capture the signature in the Manuall Signatures module against the job record.

References

• Glass Association of North America, GANA Bulletin GANA TD-02-0303, Tempered Glass Fabrication Debris.
• ASTM C1036-21, Standard Specification for Flat Glass, Section 6 surface quality.
• ASTM C1048-18, Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass.
• IWCA I-14.1-2001, Annex on Glass Identification and Restoration.
• Cardinal Glass Industries Technical Service Bulletin, Cleaning Low-E Coated Glass.