TL;DR

Key Facts

What Are Impression Materials?

Impression materials are substances used in dentistry to record accurately the three-dimensional form of teeth, edentulous ridges, and adjacent soft tissues. The impression is a negative reproduction of these structures; when poured with dental stone or plaster, it produces a positive cast (model) from which laboratory procedures (fabrication of crowns, bridges, partial dentures, complete dentures, orthodontic appliances, and study models) are performed. The accuracy of the impression directly determines the accuracy of the final restoration — an impression with voids, distortion, or inadequate detail at the preparation margin produces a restoration with poor marginal fit, regardless of the quality of subsequent laboratory work.

Why It Matters

Impression materials are one of the most heavily tested topics in dental board examinations. Questions focus on: the classification of materials; the setting chemistry and why it matters clinically (particularly why condensation silicone must be poured immediately vs. PVS which can be stored); the properties that distinguish materials (dimensional stability, accuracy, wettability, tear resistance, working/setting time); and the clinical selection rationale. Understanding impression materials is also a prerequisite for understanding fixed prosthodontic technique, complete denture construction, and the consequences of impression errors.

Classification of Impression Materials

Elastic Irreversible Impression Materials

Alginate (Irreversible Hydrocolloid)

Composition: Sodium or potassium alginate (soluble); calcium sulphate dihydrate (reactor); trisodium phosphate (retarder — reacts preferentially with Ca²⁺ to extend working time); diatomaceous earth (filler — adjusts consistency and controls water/alginate ratio); zinc oxide or potassium titanium fluoride (hardeners); sodium silicofluoride (pH adjuster). Setting reaction: Ca²⁺ from calcium sulphate displaces Na⁺/K⁺ from alginate chains → insoluble calcium alginate gel (irreversible crosslinked hydrogel). The reaction is first inhibited by trisodium phosphate (retarder) which scavenges Ca²⁺ preferentially — once the retarder is consumed, the main reaction begins.

Working time can be extended by cooling the mixing water (lower temperature slows the reaction). Setting time ranges from ~45 seconds (fast-set, for use with gagging patients or children) to ~3–4 minutes (regular-set). The mix ratio (powder:water) critically affects the final properties — too much water produces a thin mix with reduced accuracy and strength; too little water produces a thick, stiff mix with air inclusions and increased setting stress. Alginate powder must be measured by volume (the canister shaken before measuring to standardise packing), not by mass, for consistent results.

Dimensional stability: Very poor. After removal from the mouth, alginate loses water by syneresis (water expelled from the gel matrix by continuing crosslink contraction) and gains water by imbibition (water absorbed from the environment). Both processes distort the impression. Pour within 15 minutes at room temperature. If unavoidable delay is necessary, wrap the impression in a damp paper towel in a sealed plastic bag — this slows (does not stop) dimensional change.

Polyvinyl Siloxane (PVS / Addition-Cured Silicone / VPS)

Composition: Base paste: polyvinylsiloxane polymers with vinyl end groups + silica fillers. Catalyst paste: polyhydrosiloxane crosslinker + platinum catalyst (chloroplatinic acid). Setting reaction: Vinyl siloxane + hydrogen siloxane → polysiloxane (addition reaction catalysed by platinum). No by-products are formed — this is why PVS has virtually zero post-setting dimensional change and can be stored for weeks before pouring.

Inhibition: Sulphur compounds (from latex rubber gloves, sulphur-containing rubber dam, certain medications, and some hemostatic agents) inhibit the platinum catalyst and prevent or retard setting. The result is a tacky, inadequately cured surface on the impression. Prevention: use vinyl (nitrile) gloves when handling PVS impression material; avoid latex glove contact with the material or the prepared tissue.

Wettability: Inherently hydrophobic. Modern surfactant-modified PVS formulations improve wettability, but moisture control at the preparation margin remains essential. When the impression is poured with dental stone, the stone slurry must also wet the impression surface — stone compatible surfactants (wetting agents) added to modern PVS formulations improve stone pour quality.

Viscosities available: Ultra-low body (wash — injected around the preparation, flows into fine details); light body (slightly more viscous than wash); medium body (for single-phase impressions); heavy body (for tray, provides support for the light body); putty (for custom tray alternative — used in putty-wash two-stage technique). Viscosity is controlled by filler particle size and loading.

Polyether

Composition: Polyether polymer (azetidinium end groups) + filler (silica) + plasticiser + accelerator (benzene sulphonate ester). Setting reaction: Ring-opening copolymerisation initiated by the sulphonate ester — azetidine rings open and crosslink the polymer chains. The reaction produces no volatile by-products.

Hydrophilicity: Polyether is the most hydrophilic of the elastomeric impression materials — contact angle ~13° (PVS modified: ~30–40°; unmodified PVS: ~90°). This makes it the material of choice when moisture control cannot be achieved.

Limitations: Stiff set — the set material has a higher elastic modulus than PVS, making removal from undercuts more difficult and more uncomfortable for patients; impressions can distort more on removal from the mouth if undercuts are not blocked out. Absorbs water readily — must be poured within 1 hour or stored in anhydrous conditions. Strong unpleasant taste. Available only in a limited range of viscosities (one main consistency — medium body) compared with PVS. Higher cost than alginate.

Condensation-Cured Silicone (C-Silicone)

Composition: Base: polydimethylsiloxane polymer (hydroxyl end groups) + silica filler + tin octoate catalyst. Catalyst paste: orthoalkylsilicate crosslinker. Setting reaction: Tin-catalysed condensation of hydroxyl groups on the silicone polymer chains with orthoalkylsilicate crosslinker → crosslinked polysiloxane + alcohol (ethanol or methanol) by-product. The continued release of the alcohol by-product after the impression has set causes continued crosslinking and progressive shrinkage. Pour within 1 hour.

C-silicone is hydrophobic (similar to unmodified PVS), flexible, and available in multiple viscosities (putty, heavy, regular, light). It was widely used before the introduction of PVS and remains in use where cost is a primary consideration. However, for fixed prosthodontic applications where accuracy and stability are paramount, addition-cured silicone (PVS) is superior in all dimensions that matter clinically.

Elastic Reversible Impression Materials

Agar (Reversible Hydrocolloid)

Composition: Agar (a polysaccharide extracted from seaweed) + borax (strengthener) + potassium sulphate (accelerates dental stone setting in the presence of agar) + water. Setting: Physical gel transition — agar is a sol at ~63–70°C (fluid, pourable); gels below ~37–45°C (solid, elastic). This thermoreversible gelation allows the material to be reused by reheating (the only true impression material that can be reused). Agar requires special conditioning equipment (water bath at specific temperatures for liquefying, tempering, and setting) and a water-cooled impression tray to control setting temperature in the mouth.

Agar is the most accurate impression material for crown and inlay impressions historically — its flow characteristics at the point of setting and its hydrophilicity make it superior to older materials. However, the requirement for specialised conditioning equipment and the development of PVS with equivalent accuracy and far greater convenience has made agar largely obsolete in fixed prosthodontics in contemporary practice. It retains niche use in some jurisdictions where laboratory preferences favour it for casting accuracy.

Inelastic (Rigid) Impression Materials

Zinc Oxide–Eugenol (ZOE) Impression Paste

Composition: Paste 1 (base): zinc oxide + fixed vegetable oil (for consistency) + zinc acetate (accelerator). Paste 2 (accelerator): eugenol + filler (inert) + olive oil. Setting reaction: ZnO + eugenol (+ H₂O + Zn acetate) → zinc eugenolate chelate + unreacted ZnO. Water (from saliva) is essential for the reaction; the zinc acetate in the base paste accelerates the reaction.

ZOE impression paste is rigid (inelastic) when set — it cannot pass over undercuts without fracturing. Its primary use is for final impressions of edentulous ridges in complete denture construction (no undercuts on an edentulous ridge — or undercuts are blocked out in the custom tray). It produces minimal tissue displacement (mucostatic impression technique) and excellent detail of the soft tissue surface.

Eugenol allergy/sensitivity: a small proportion of patients are sensitive to eugenol (primarily patients with a history of multiple dental treatments where eugenol-containing materials were used). Eugenol-free alternatives (e.g., methacrylate-based bite registration pastes) are available. The eugenol content of ZOE cement is also relevant when ZOE-based temporary cements are used before resin cementation — eugenol residue on the preparation inhibits resin polymerisation (eugenol inhibition of free radical polymerisation).

Impression Compound (Modelling Compound)

Composition: Natural resins (shellac, copal resin) or synthetic resins; waxes; filler (talc or chalk); plasticiser (stearic acid). Setting: Thermoplastic — softens on heating (~55–65°C), plastic above ~43°C, rigid below body temperature. Reversible (can be reheated and resoftened). Used in water baths or direct flame (direct flame risks overheating and burning the compound — always use water bath for clinical use).

Impression compound is inelastic when set. Its primary uses: (a) border moulding — applying small increments of softened compound to the periphery of a special tray and moulding to the vestibular depth during functional muscle movements; (b) primary (preliminary) impressions for complete dentures — provides reasonable mucosal recording with some functional border moulding built in; (c) stock tray impressions for alginate study models (used to customise tray borders). Impression compound is poor at recording fine surface detail — it is a rough-capture material, not a detail-capture material.

Impression Plaster

Composition: Calcium sulphate hemihydrate (plaster of Paris) — the same base material as dental stone but used in a lower-water-ratio impression application. Setting: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum). Rigid, brittle, dimensionally stable. Historically used for edentulous (mucostatic) impressions — minimal tissue displacement, excellent surface detail, very accurate. Falls from favour due to brittleness (must be cracked off the cast by careful fracture of the impression) and the development of more convenient alternatives.

Clinical Comparison Table

Key Properties of Impression Materials

Dimensional Stability

Dimensional stability refers to the ability of the set impression material to maintain its dimensions (i.e., not shrink, swell, or distort) after removal from the mouth. It is the most clinically critical property for fixed prosthodontic impressions because even small dimensional changes produce crowns that are too large (won’t seat) or too small (will rock on the die). Setting reactions that produce by-products (condensation silicone → ethanol; alginate → ongoing gelation) cause progressive dimensional change; setting reactions with no by-products (addition silicone/PVS) have minimal dimensional change.

Tear Resistance and Elastic Recovery

Tear resistance is the ability to withstand tearing at thin cross-sections — particularly relevant at crown margins, interproximal areas, and sulcular depths where the impression material is very thin. PVS has the highest tear resistance of all elastomeric impression materials; alginate has the lowest. Elastic recovery (the percentage of deformation that is recovered after removing the impression from undercuts) determines how much permanent distortion occurs during impression removal. Perfect elastic recovery (100%) means the impression returns exactly to its original dimensions. All elastic impression materials have some permanent deformation on removal; PVS and polyether have the best elastic recovery.

Wettability and Hydrophilicity

Wettability is measured by the contact angle of water on the set impression material surface — lower contact angle = more hydrophilic = better wetting by aqueous fluids (saliva, GCF, blood) in the mouth, and better wetting by the dental stone slurry when pouring the cast. Contact angles: unmodified PVS ~90° (hydrophobic); surfactant-modified PVS ~30–40°; polyether ~13° (most hydrophilic elastomer); alginate (inherently hydrophilic — contact angle not meaningful as it is aqueous itself); ZOE paste moderate wettability. A more hydrophilic impression material records surface detail in the presence of a moisture film without voiding, and accepts dental stone pour more readily.

Impression Techniques

Two-Phase / Double-Mix Technique

Two materials of different viscosities are used simultaneously (or in sequence) to record different aspects of the impression:

• Two-phase simultaneous (double-mix): Light body (low viscosity) is injected around and over the prepared teeth by an assistant while the heavy body/putty-loaded tray is simultaneously seated. The light body flows into fine detail (margin, interproximal); the heavy body supports the light body and records the surrounding arch. This is the most widely used technique for fixed prosthodontic impressions. The two materials are compatible (same setting chemistry) and bond to each other during the simultaneous setting reaction.
• Two-phase sequential (putty-wash): Stage 1: putty impression taken in a stock tray, allowed to set, removed. Stage 2: relief space created in the set putty (by removing a thin layer of material or placing a plastic spacer before the first set); wash (light body) reloaded into the relief spaces; re-seated for the final impression. Advantage: custom tray not required. Disadvantage: the putty impression has already set and undergone some dimensional change before the wash is added; the two-stage process adds complexity and potential for error at the re-seating step; not recommended for high-precision crown preparations.

Single Phase (Monophase) Technique

A single medium-viscosity material is loaded into both the syringe (to inject around the preparation) and the custom tray. The same material fills the tray and is injected directly — no two-material interface to compromise. Medium-body PVS or polyether medium body is typically used. Advantage: no risk of material interface failure; simpler technique. Disadvantage: the medium viscosity is a compromise — less flow into fine details than light body; less support from a stiffer heavy body layer. Commonly used with polyether (which is available primarily in medium body) and for fixed prosthodontic impressions where the preparation is well-defined and moisture control is excellent.

Common Impression Errors

Clinical Considerations

• Digital impressions (intraoral scanners) are transforming impression-taking in fixed prosthodontics but have specific limitations: Intraoral scanners (IOS — CEREC Primescan, iTero Element, 3Shape TRIOS) capture the preparation as a digital point cloud that is directly imported into CAD/CAM software, eliminating impression materials, trays, and stone models entirely. Advantages: immediate feedback on scan completeness; no dimensional change; eliminates laboratory model pouring errors; single-visit crown fabrication possible (in-office CEREC workflow). Limitations: accuracy equivalent to PVS for single units and short-span bridges, but less established for full-arch scans and implant-supported multi-unit frameworks; cannot capture subgingival margins without the gingival retraction and moisture control still required; more sensitive to blood and moisture than PVS in terms of scan quality (haemorrhage at the margin creates reflective surface that the scanner cannot resolve); learning curve for technique; capital cost of equipment. For board purposes: digital impressions and PVS have equivalent accuracy for single crowns; PVS remains superior or equivalent for full-arch multi-unit cases based on current systematic review evidence.
• Tray adhesive is mandatory for all elastomeric impression materials: Elastomeric impression materials (PVS, polyether, C-silicone) do not bond to acrylic, metal, or plastic trays without an adhesive. If the set material debonds from the tray during removal, the impression is distorted (the material stretches away from the tray during removal, then contracts, producing a smaller-than-actual dimension). Tray adhesive must be applied to the entire internal surface of the tray and allowed to dry before material loading (typically 2–5 minutes). The adhesive is specific to the material chemistry (PVS adhesive for PVS; polyether adhesive for polyether — do not cross-use). Neglecting the tray adhesive is the single most underappreciated technical error in clinical impression-taking.
• Gypsum products used to pour impressions must be compatible with the impression material: Dental stone (Type III) and die stone (Type IV — high-strength, low-expansion) are the standard pouring materials for fixed prosthodontic impressions. Alginate impressions inhibit the setting of gypsum by the alginate’s potassium alginate component (potassium ions disrupt calcium sulphate crystallisation) — this is why the surface of alginate impressions treated with gypsum shows a powdery, smooth surface layer and may not reproduce fine details as clearly as PVS. The potassium sulphate accelerator added to alginate (and the chemical wash or rinse of alginate impressions with a dilute sodium sulphate solution before pouring) counteracts this inhibition to some extent.
• Disinfection of impressions before sending to the laboratory is a legal and ethical requirement: All impressions must be disinfected before sending to the dental laboratory — the laboratory technician handling a non-disinfected impression is exposed to the patient’s oral microorganisms (blood-borne viruses, bacteria). Disinfection method must be compatible with the impression material: immersion in 2% glutaraldehyde or sodium hypochlorite 1:10 for 10 minutes — appropriate for PVS, polyether, and condensation silicone; alginate should be sprayed (not immersed — imbibition causes dimensional change) with 2% glutaraldehyde or phenol-based spray for 10 minutes. ZOE paste impressions are relatively resistant to disinfection-associated dimensional change and can be immersed. The laboratory should be notified of the disinfection method used.
• The choice between stock tray and custom tray affects impression accuracy: A custom (special) tray, fabricated on the primary cast from the study model with a uniform 2–3 mm spacer, provides uniform thickness of impression material throughout the impression. Uniform material thickness produces more uniform setting shrinkage and distortion, and allows the material to express optimum elastic recovery. Stock trays produce variable material thickness — thick where the tray is far from the teeth, thin where the tray is close. Variable thickness means variable elastic recovery and distortion. For complete denture master impressions and for any impression requiring the highest accuracy, a custom tray is mandatory.

Common Mistakes & Misconceptions

• Misconception: “Alginate is suitable for crown impressions if it is poured quickly.”
  Correction: Even when poured within 15 minutes, alginate does not produce sufficient surface detail, elastic recovery, or dimensional accuracy for fixed prosthodontic final impressions. The 15-minute rule for alginate refers to the preservation of adequate accuracy for study models and opposing arch impressions — not for crown preparations. The inherent tear resistance of alginate is too low to reproduce the thin flash of material at a subgingival chamfer without tearing, regardless of pouring time.
• Misconception: “Condensation silicone and addition silicone (PVS) are interchangeable — both are silicones.”
  Correction: While both are organosiloxane-based materials, their chemistry, by-products, and post-setting dimensional stability are fundamentally different. Condensation silicone produces ethanol as a by-product and undergoes post-setting shrinkage — it must be poured within 1 hour. Addition silicone (PVS) produces no by-products and has negligible post-setting dimensional change — it can be stored for 14+ days. Using condensation silicone with the assumption it behaves like PVS (i.e., delaying pouring) is a common cause of ill-fitting crowns.
• Misconception: “PVS impressions can be taken without tray adhesive if the material is heavily loaded.”
  Correction: Heavy material loading does not prevent debonding of PVS from the tray. Tray adhesive creates a chemical bond between the silicone material and the tray surface — without it, the material relies only on mechanical interlock from the tray perforations (perforated trays) or a rim of material at the tray edge. This mechanical retention is insufficient to prevent detachment during the forceful impression removal from undercuts. Tray adhesive is non-negotiable for PVS impressions.
• Misconception: “Polyether is always preferable to PVS because it is more hydrophilic.”
  Correction: Polyether’s hydrophilicity is an advantage when moisture control is suboptimal — but polyether’s stiffer set, shorter working time, stronger taste, and tendency to absorb water from storage are disadvantages compared with PVS in situations where moisture control is achievable. The clinical decision should be: if reliable moisture control can be achieved at the preparation margin, PVS is preferred (better dimensional stability on storage, greater flexibility, better patient tolerability). If moisture control is genuinely difficult, polyether’s superior hydrophilicity justifies its use.
• Misconception: “Reversible hydrocolloid (agar) is reversible because it can be reused indefinitely.”
  Correction: Agar is called reversible because the physical gel→sol transition is temperature-reversible — the material can be reheated and re-liquefied. However, repeated heating cycles degrade the agar polysaccharide chains, reducing the material’s accuracy and mechanical properties. Most manufacturers recommend a limited number of reuse cycles (typically 3–5 cycles maximum). After this, the impression quality deteriorates and the material should be discarded. “Reversible” refers to the phase transition, not to infinite reusability.

Related Topics

References & Sources

1. Craig RG, Powers JM (eds) (2012). Restorative Dental Materials, 13th ed. Mosby/Elsevier. [Standard dental materials reference — classification, chemistry, properties, and clinical use of all impression materials]
2. Anusavice KJ, Shen C, Rawls HR (eds) (2013). Phillips’ Science of Dental Materials, 12th ed. Saunders/Elsevier. [Comprehensive dental materials science — setting reactions, dimensional stability data, wettability]
3. Chee WW, Donovan TE (1992). Polyvinyl siloxane impression materials: a review of properties and techniques. Journal of Prosthetic Dentistry, 68(5):728–732. [Properties of PVS — setting chemistry, dimensional stability, wettability, sulphur inhibition]
4. Lim MV, Duncanson MG Jr, Co HC Jr, Ellison JA (2002). Polyether and addition silicone working times with use of warm temperatures. Journal of Prosthetic Dentistry, 88(3):324–331. [Effect of temperature on PVS and polyether working/setting times]
5. Endo T, Finger WJ (2006). Dimensional accuracy of a new polyether impression material in comparison to polyvinyl siloxane. American Journal of Dentistry, 19(4):228–232. [PVS vs. polyether dimensional accuracy comparison]
6. Mandikos MN (1998). Polyvinyl siloxane impression materials: an update on clinical use. Australian Dental Journal, 43(6):428–434. [Clinical review of PVS — technique considerations, tray adhesive importance]
7. Caserio M, Fabiano A (2013). Digital impression versus polyvinylsiloxane impression: direct comparison in fixed prosthetics. ISRN Dentistry, Article 408317. [IOS vs. PVS accuracy comparison for fixed prosthodontics]
8. Donovan TE, Chee WW (2004). A review of contemporary impression materials and techniques. Dental Clinics of North America, 48(2):vi-vii, 445–470. [Comprehensive clinical review of all impression materials — clinical selection rationale]

Summary

Impression materials are classified as elastic irreversible (alginate, PVS/addition silicone, polyether, condensation silicone), elastic reversible (agar), or inelastic/rigid (ZOE paste, impression compound, impression plaster). PVS (addition-cured silicone) is the gold standard for fixed prosthodontic impressions — no by-products from the platinum-catalysed addition reaction means negligible dimensional change (pour up to 14 days later). Alginate is hydrophilic and inexpensive but dimensionally unstable (pour within 15 minutes) — appropriate for study models and opposing arch impressions only, not for crown preparations. Condensation-cured silicone produces ethanol as a by-product during and after setting, causing progressive post-setting shrinkage — pour within 1 hour. Polyether is the most hydrophilic elastomer (contact angle ~13°) — the choice for moisture-difficult preparations, but stiffer and with shorter working time than PVS. ZOE paste is inelastic, dimensionally stable, and used for edentulous master impressions in complete denture construction. Common impression errors include: voids at margins (moisture contamination — prevented by retraction cord, moisture control, immediate injection post-cord removal); tear/drag at margins (inadequate set time); tray movement/debonding (use tray adhesive — mandatory for all elastomers). Tray adhesive is not optional — it is required for all elastomeric impression materials to prevent tray debonding and impression distortion.

About the Author

Dr. Andries Smith

Founder, Dental Panda

Dr. Andries Smith founded Dental Panda in 2020. As an immigrant to the United States, he had to take the INBDE exam, even though he was practicing dentistry for over 10 years. This revealed an opportunity. Andries noticed that INBDE prep course companies were putting profit over students. With his expertise and experience in dentistry, he created free dental wiki resources for students and the general public to have access to.