Heritage Conservator's Guide to Dry Ice Cleaning
A 4,000-word guide for heritage architects, council asset managers and conservators. Heritage authority approval pathway, sample-test protocol, tender preparation, monument and facade cleaning.
This guide is for heritage architects, council conservators, asset managers responsible for listed buildings, monuments and heritage estates, and contractors bidding on heritage cleaning tenders. We’ll cover the heritage authority approval pathway, sample-test protocols, substrate-specific technique, monument and facade cleaning, and tender preparation for dry ice cleaning work.
If you’ve worked in heritage cleaning for a decade you’ve experienced the frustration of method limitations: sandblasting damages soft stone, acid wash etches sandstone and saturates mortar, pressure-washing forces water into joints, chemical poultices stain or weaken some substrates. Dry ice is recognised across most Australian heritage jurisdictions as an approved physical-removal method. This guide covers exactly when, where and how it applies.
Heritage cleaning is constrained by what’s approved
The conservator’s role isn’t to find new cleaning methods — it’s to find methods that can be approved by the heritage authority and that don’t compromise the substrate. Most cleaning methods fail one or both tests:
Sandblasting removes microns of substrate per cycle. On heritage sandstone or limestone, even fine-grit blasting damages original carving detail and tooling marks. Universally rejected by Australian heritage authorities for valuable substrates.
Acid wash etches the surface chemically. Saturates mortar joints. Produces effluent requiring careful disposal. Approved in some jurisdictions for specific substrates (limited use on Victorian-era brickwork) but increasingly rejected.
Chemical poultices can be effective for specific stains but introduce moisture and chemistry into porous substrates. Risk of permanent staining or weakening. Conservator-led case-by-case approval required.
Pressure washing soaks mortar joints, drives water through the substrate, can mobilise salts. Particularly damaging in older masonry where cement-mortar repointing has trapped moisture inside the historic lime mortar.
Hand cleaning is slow and labour-intensive. For specific high-value items (statuary, architectural sculpture) it remains the right choice.
Dry ice cleaning addresses the gaps. It’s:
- Non-abrasive at heritage-appropriate pressure
- Non-saturating — no water introduced
- Chemistry-free — no residue, no environmental concern
- Reversible in effect — only the contamination layer is removed; substrate remains as-was
That’s the case for heritage approval. The case has been accepted across most Australian jurisdictions.
The heritage authority approval pathway
Most heritage projects we work on follow this approval pathway:
1. Conservator engagement. The heritage architect or conservator scoping the project engages with us early. We discuss the substrate, the contamination, the desired outcome, and the conservation requirements.
2. Sample-area test. On an inconspicuous area (rear elevation, bottom corner, hidden detail) we run a small-scale test. Pressure tuned to the substrate. Conservator on-site for assessment. Photographic record produced.
3. Method statement. We produce a method statement covering the specific substrate, the proposed pressure, the cleaning sequence, the masking and protection requirements, and the documentation pack. The conservator reviews and signs off.
4. Heritage authority submission. The conservator submits the method statement, sample-test results, and any required reports to the heritage authority. We provide supplementary technical documentation as needed.
5. Approval and start. Once approved, we start work to the agreed methodology. Conservator on-site at agreed milestones.
Approval timelines vary by jurisdiction:
- Council heritage — typically 4-8 weeks
- State heritage register — typically 8-16 weeks
- National Trust / Commonwealth — typically 12-24 weeks
For preferred-supplier councils where we’ve previously been approved, subsequent approvals are faster.
Substrate-specific technique
| Substrate | Pressure | Notes |
|---|---|---|
| Sandstone (soft) | 50-70 PSI | Sample-test on grain pattern; preserve weathering |
| Sandstone (hard) | 70-90 PSI | Standard heritage pressure |
| Limestone | 50-70 PSI | Particularly soft; sample-test mandatory |
| Granite | 80-120 PSI | Hard substrate; tolerates higher pressure |
| Heritage brick | 60-80 PSI | Preserve mortar joints |
| Render and parging | 50-70 PSI | Test on each batch |
| Heritage timber | 50-70 PSI | Conservator-coordinated |
| Bronze (sculpture) | 50-70 PSI | Preserve patina |
| Copper (decorative) | 60-80 PSI | Preserve patina |
| Lead (flashings, statuary) | 50-70 PSI | Soft metal — particularly cautious |
| Painted heritage finish | 50-70 PSI | Sample-test mandatory |
The pressure tuning is the dominant variable. Our crews are trained to recognise substrate condition and adjust pressure accordingly. Sample-testing is the procedural safeguard.
Monument and statuary cleaning
Statuary requires particularly careful work because the value is in the detail — original tooling marks, inscriptions, decorative carving — that aggressive cleaning would erase.
Standard monument cleaning protocol:
Pre-work documentation. Comprehensive photo record of every surface, with detailed photography of inscriptions, tooling marks, decorative elements, and any damage or weathering pattern.
Conservator collaboration. The conservator is on-site for sample-test sign-off and for any judgement calls during cleaning. Particularly important for sculpture where individual surfaces may need different treatment.
Pressure calibration. Per-surface pressure tuning. Inscriptions and fine detail at lower pressure; broader surfaces at higher.
Selective cleaning. Sometimes we leave aged patina in place and only remove specific contamination (graffiti, biological growth, specific staining). Selective cleaning preserves the historical character while addressing the issue.
Patina preservation. Particularly on bronze, copper and weathered stone, the aged patina is part of the historical value. We preserve it actively rather than incidentally.
Final documentation. Photo record of every surface post-cleaning. Comparison shots with pre-work photos. Conservator sign-off.
Typical monument projects: 1-3 days for individual statuary, 1-2 weeks for war memorials and civic monuments.
Facade cleaning
Listed-building facade cleaning is a substantial program of work. Standard structure:
Phase 1 — Survey. Full facade survey with conservator. Substrate identification, contamination type per area, damage and existing repairs identified. Output: cleaning scope document.
Phase 2 — Sample testing. 2-4 sample areas across substrate variations. Conservator sign-off on each. Output: validated cleaning specification per substrate.
Phase 3 — Heritage authority approval. Method statement + sample test results submitted. Approval issued.
Phase 4 — Cleaning campaign. Phased work across the facade. Scaffolding or aerial platform per facade. Crew on-site for the duration. Daily photo log.
Phase 5 — Inspection and sign-off. Conservator and heritage authority walk-through. Snag list addressed. Final documentation pack delivered.
Typical facade campaigns: 2-12 weeks depending on facade size and complexity. Major listed buildings can run 12-24 weeks.
Graffiti removal — heritage context
Heritage graffiti removal is one of our most-requested services. Standard process:
- Rapid response. Within 48-72 hours of report (for council preferred-supplier customers).
- Sample-area test. Inconspicuous test on adjacent stone before any visible-area work.
- Pressure-calibrated cleaning. Lower pressure than industrial graffiti work. Pellets lift the spray-paint off the substrate without driving paint deeper.
- Ghosting management. On highly porous heritage stone, some shadowing is occasionally unavoidable. We discuss with conservator before commencement; alternative approaches (poultice + dry ice combination) may be used for particularly porous substrates.
- Photo log. Pre/during/post documentation for council records.
Most heritage stone can be cleaned without ghosting. Sample-test results inform the conservator’s go/no-go decision.
Council preferred-supplier programs
For council heritage cleaning programs, we operate as preferred supplier where appropriate qualifications align:
- Public liability $20m + sub-trade endorsement
- Heritage method documentation
- Conservator partnerships demonstrated
- Tender-ready case studies with council references
- Insurance certificates current in all states we operate
To apply for preferred-supplier inclusion, email office@dryiceblasters.com.au with the council and program details. Most councils we’ve engaged with have approved our inclusion within 6-12 weeks.
Tender preparation
For heritage tender submissions we provide:
- Method statement specific to the substrate and tender scope
- Case studies from comparable heritage projects
- Insurance certificates and qualifications
- Conservator references
- Pricing schedule per scope
- Compliance statements
Most tender packs we prepare are standard within 5 business days. Specialised tenders (national heritage register, commonwealth-funded restoration programs) may take longer to prepare.
Pricing benchmarks
Heritage cleaning pricing depends substantially on access, scaffolding, conservator coordination requirements and substrate complexity:
| Project type | Typical price | Duration |
|---|---|---|
| Small monument / sculpture | $4,500-$12,000 | 1-3 days |
| War memorial / civic statuary | $8,000-$25,000 | 1-2 weeks |
| Heritage building facade (small, ground floor) | $15,000-$45,000 | 2-4 weeks |
| Heritage building facade (multi-storey) | $40,000-$180,000 | 6-16 weeks |
| Council graffiti removal (per incident) | $1,200-$3,500 | Same-day or 1-2 days |
| Council facade clean (smaller asset) | $8,000-$25,000 | 1-3 weeks |
Listed-building work attracts heritage-specialist pricing rather than industrial rates. The pricing reflects the conservator coordination, sample-test requirements, and documentation overhead.
Common questions for heritage projects
Will the heritage authority approve dry ice for our facade? In most jurisdictions, yes — but it requires the conservator’s submission and (often) sample-test results. We support the submission with technical documentation.
Can you work with our existing conservator? Yes — every heritage project is conservator-led on scope, methodology and sign-off.
Does it work on our specific stone type? Almost all common heritage substrates clean well at appropriate pressure. Sample-testing is the verification step.
Will our mortar joints be damaged? Sound mortar is unaffected. Existing failed joints become visible — not damaged by us, just no longer hidden.
Can you handle the heritage register documentation requirements? Yes — per-zone photo logs, area maps, methodology records, and conservator sign-off pack are produced.
What about access for a multi-storey facade? Scaffolding or aerial platform. We coordinate with scaffolding contractors or use elevated work platforms.
Do you work with grant-funded restoration programs? Yes — we’ve worked on multiple state and federal grant-funded heritage projects.
How to engage
Three paths:
-
Specific project quote. Send brief via 60-second quote tool with project details (substrate, location, conservator, heritage status). Fixed quote within 24-48 hours.
-
Heritage Conservation Guide download. 22-page PDF covering heritage authority approval pathway, sample protocols, tender preparation. Use for internal review.
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Council preferred-supplier application. Email office@dryiceblasters.com.au with council details. We’ll provide insurance certificates, qualifications, and case study references for inclusion process.
Final thoughts
Heritage cleaning is an area where method choice has irreversible consequences. Dry ice cleaning, applied with heritage-appropriate pressure and conservator coordination, is one of the few methods that can address contamination on valuable substrates without compromising what makes them valuable.
For council and listed-building work, the approval pathway is straightforward when method documentation and sample-test results are provided. For monument and statuary work, the conservator-led process is the standard. For graffiti removal on heritage assets, our rapid-response capability is particularly valuable for councils.
Download the Heritage Conservation Guide for the full operational framework, or request a project quote for specific scopes.