🦷 Alginate (Irreversible Hydrocolloid)

Alginate is an irreversible hydrocolloid impression material. It is derived from alginic acid (found in brown seaweed). Once set, it cannot be returned to its original state.

Potassium alginate - it reacts with calcium sulfate (reactor) to form an insoluble calcium alginate gel.

Potassium alginate + Calcium sulfate (dihydrate) → Calcium alginate (gel) + Potassium sulfate The trisodium phosphate acts as a retarder - it reacts with calcium sulfate first to give more working time.

Typically 1 scoop powder : 2 measures water (follow manufacturer instructions). Water temperature affects setting time - cold water = slower set, warm water = faster set.

• Use a rubber bowl and a stiff, wide-bladed spatula
• Add powder to water (not water to powder)
• Mix with a wiping/stropping motion against the sides of the bowl for 60 seconds
• The mix should be smooth, homogeneous, and creamy with no lumps

Smooth, creamy, lump-free. It should be able to hold its shape in the tray without slumping excessively.

• Potassium alginate (12-15%) - reacts to form gel
• Calcium sulfate dihydrate (8-12%) - reactor/cross-linker
• Trisodium phosphate (2%) - retarder
• Diatomaceous earth / silica (70%) - filler (improves stiffness)
• Potassium titanium fluoride - improves surface hardness of poured stone
• Coloring and flavoring agents

• Syneresis: Contraction of the gel with expulsion of fluid - alginate shrinks if left in air
• Imbibition: Absorption of water - alginate swells if stored in water
• Therefore, alginate impressions must be poured immediately or stored in a humid atmosphere for no more than 1 hour

• Study models / diagnostic casts
• Opposing arch impressions
• Preliminary impressions
• Orthodontic models
• Fabrication of custom trays

• Working time: ~1-2 minutes (time to mix and load tray)
• Setting time: ~2-4 minutes in the mouth (from start of mixing)
• Type I (fast set): sets in ~1-2 min | Type II (regular set): ~3-4.5 min

Cold water slows the reaction between potassium alginate and calcium sulfate, extending working time. This is especially useful in warm climates.

• Over-mixing: breaks the gel structure, weak and inaccurate impression
• Under-mixing: lumpy, voids present, inaccurate impression with trapped air

⚕️ Zinc Oxide Eugenol (ZOE)

ZOE is a two-paste system (base + accelerator/catalyst). It is used as a luting cement, temporary restoration, impression material (non-rigid), pulp capping agent, and cavity liner.

• Base paste: Zinc oxide + fixed oil (e.g., olive oil) + resin
• Accelerator/Catalyst paste: Eugenol + filler (zinc acetate, calcium chloride accelerates the reaction)

Zinc oxide + Eugenol → Zinc eugenolate (chelate complex) + water ZnO + H₂O → Zn(OH)₂ → reacts with eugenol → zinc eugenolate

• Dispense equal lengths of base and accelerator onto a glass slab (or paper pad)
• Mix with a metal spatula using a folding/stropping motion
• Mix for approximately 60 seconds until a uniform color is achieved (no streaks)
• Correct consistency: smooth, homogeneous paste

The mix is uniform in color - no streaks of white (base) or yellow (eugenol). The color should be consistent throughout.

• Humidity/water: Accelerates setting (moisture speeds up reaction)
• Temperature: Higher temperature = faster set
• Zinc acetate: Acts as an accelerator
• Powder:liquid ratio: More ZnO = faster set
• Particle size: Finer ZnO = faster set

• Temporary luting cement (temporary crowns)
• Temporary restorations
• Cavity liner / base (under restorations - provides sedative effect)
• Pulp capping (direct/indirect)
• Root canal sealer
• Impression paste (for edentulous ridges)
• Periodontal dressings
• Surgical dressing (after extractions)

Eugenol has obtundent (sedative/palliative) properties - it reduces pulpal inflammation and sensitivity. However, it can be irritating in high concentrations and is contraindicated near vital pulps in some formulations.

Ethoxybenzoic acid (EBA) is added to ZOE to improve its strength and hardness. EBA replaces part of the eugenol. This is called reinforced ZOE or EBA cement.

• Should not be placed under composite resin - eugenol inhibits the polymerization of resin-based composites
• Some patients have eugenol allergy

• Low compressive strength (brittle)
• Good thermal insulation
• Good sealing ability
• Antibacterial properties
• Biocompatible / obtundent effect on pulp

Bonus - Common Examiner Questions

• Reversible (agar): can be re-melted and re-used (physical change)
• Irreversible (alginate): cannot be reversed once set (chemical change)

Due to syneresis and imbibition - alginate dimensionally changes rapidly after removal from the mouth. Delaying pouring causes inaccurate models.

Perforated stock trays - perforations provide mechanical retention for the alginate.

🟢 Green Stick Compound (Impression Compound Type II)

It is Type II impression compound (stick form), also called tracing stick compound or low-fusing compound. It is a thermoplastic material that softens on heating and hardens on cooling (reversible physical change).

• Thermoplastic resins (copal resin, dammar resin)
• Waxes
• Filler (talc/soapstone)
• Plasticizers
• Coloring agents (gives the green color)

Softens at 55-60°C (in warm water). It becomes plastic and moldable. It re-hardens below this temperature.

• Border molding / muscle trimming of custom trays before final impression
• Beading the periphery of impression trays
• Tray modification to extend or build up tray borders
• Making individual tooth impressions (limited use today)

It is the process of shaping the peripheral borders of an impression tray using green stick compound so that the borders accurately reflect the functional depth and width of the vestibule. Done for complete denture impressions.

Physical/reversible change - it softens on heating and hardens on cooling repeatedly without any chemical change.

• Type I (Tray compound - cake/sheet form): Higher fusing, used to make custom impression trays; softens at ~70°C
• Type II (Stick/green stick compound): Lower fusing, used for border molding; softens at ~55-60°C

🟡 Yellow Stick Wax (Periphery Wax / Tenax Wax)

Yellow stick wax, also known as periphery wax or Tenax wax, is a soft, pliable wax used to extend or add to the borders of impression trays.

• Beeswax
• Paraffin wax
• Soft waxes and resins
• Yellow coloring pigment

• Extending the borders of stock trays (to get an adequate peripheral seal)
• Adding bulk to tray borders before alginate impression
• It is softer and more pliable than green stick compound

• Yellow stick wax is softer and more flexible - used to simply extend tray borders
• Green stick compound is stiffer - used for active border molding with muscle movements
• Yellow wax does not require water bath; green stick needs warm water to soften

🔵 Alginate (Irreversible Hydrocolloid)

Alginate - an irreversible hydrocolloid impression material.

Alginate powders are available in various colors (white, pink). Some are color-changing (chromatic alginates): bright colored when freshly mixed → change to pale/white when set, indicating the correct setting time.

Alginate is hygroscopic - absorbs moisture from the air. Must be stored in a dry, sealed container. Excessive moisture absorption degrades the material before use.

🧱 Custom Tray (Individual Tray)

A custom tray (individual/special tray) is a tray fabricated specifically for a single patient's mouth to provide uniform thickness of impression material and accurate adaptation to the patient's arch.

• Self-cure acrylic resin (PMMA) - most common
• Light-cure resin (visible light cure resin)
• Shellac base plates
• Thermoplastic materials

• Uniform space for impression material
• Better accuracy and fit
• Less material needed → less distortion
• Can be made with appropriate relief (spacer) and handles

Usually 2 sheets of wax (~3-4 mm) placed over the cast before making the tray. The spacer creates a uniform space for the impression material. It is removed before use.

Small openings left through the spacer so that acrylic contacts the cast in 2-3 areas. These form stops that prevent the tray from seating too deeply, ensuring a uniform thickness of impression material.

• Final/definitive impressions for complete dentures
• Final impressions for fixed partial dentures with elastomers
• Used with ZOE impression paste, elastomers, or alginate

🔵 Blue Wax (Inlay Casting Wax / Base Plate Wax?)

Type I Inlay casting wax (blue color) is used for direct technique - wax pattern carved directly in the patient's mouth. It is harder and more brittle.

• Type I (Hard/Blue): Direct technique - carved in patient's mouth
• Type II (Soft): Indirect technique - wax pattern made on die in the lab

• Paraffin wax (major component, 40-60%)
• Carnauba wax (hardener, raises melting point)
• Ceresin wax
• Beeswax
• Coloring agents

• Should melt/soften at mouth temperature for adaptability
• No residue on burnout (burns out completely at 500°C)
• Dimensional accuracy (low thermal expansion)
• Should not flake or chip when carved

Flow is the ability of wax to deform under stress. At 37°C (mouth temp), it should have enough flow to adapt, but minimal flow at room temperature to avoid distortion during handling.

Base plate wax (pink/blue) is used to construct occlusal rims on record bases for recording jaw relations in complete/partial denture fabrication.

• Type I (Soft): For building occlusal rims in tropical/warm climates
• Type II (Medium): General purpose
• Type III (Hard): For use in very warm/tropical conditions

🧠 General Wax Questions (Any Wax Spotter)

1. Pattern waxes: Inlay wax, casting wax, baseplate wax
2. Processing waxes: Boxing wax, utility wax, sticky wax
3. Impression waxes: Bite registration wax, corrective impression wax

• Thermal dimensional change (expand/contract with temperature)
• Residual stress and springback on removal
• Distortion if not handled properly

Distortion occurs due to stress release in wax. Minimize by:

• Investing the wax pattern immediately after fabrication
• Storing in a cool environment
• Avoiding unnecessary handling

Quick Identification Tips for Spotters

🏗️ Investment Material

A material used to surround and support a wax pattern during the casting process. It must withstand high burnout temperatures and provide a mold for molten metal to flow into.

1. Gypsum-bonded investment - for gold alloy castings (up to 700°C)
2. Phosphate-bonded investment - for high-fusing alloys, base metals, ceramics (up to 1000°C+)
3. Silica-bonded (ethyl silicate) investment - for very high fusing alloys (cobalt-chromium)
4. Alumina/oxide-bonded - for casting titanium

• Alpha-hemihydrate (binder) - 25-35%
• Silica (quartz/cristobalite) (refractory) - 60-65%
• Modifiers (boric acid, sodium chloride) - regulate setting and thermal expansion

Silica undergoes thermal expansion on heating, which compensates for the casting shrinkage of the metal alloy. Without it, the casting would be undersized.

• Quartz: Undergoes inversion at 573°C (small expansion)
• Cristobalite: Undergoes inversion at 200-270°C (larger expansion, preferred) Cristobalite gives more uniform and useful thermal expansion.

Investment expands slightly on setting (0.4-0.6%). This expansion, along with thermal expansion during burnout, compensates for wax shrinkage and metal casting shrinkage to give an accurately fitting casting.

Above 700°C, calcium sulfate (from gypsum) decomposes, releasing sulfur gases that contaminate and weaken the metal casting. So it is only used for low-fusing alloys (gold alloys).

Typically 0.35-0.40 ml water per gram of powder. Increasing water increases setting time and decreases expansion.

If investment is allowed to set while submerged in water, it absorbs water and expands significantly more than normal setting expansion. This is called hygroscopic technique and is used to achieve greater expansion.

The process of heating the invested mold in a furnace to:

• Melt and vaporize the wax pattern (leaving a mold cavity)
• Achieve thermal expansion of the investment
• Preheat the mold for metal casting

🦷 GIC (Glass Ionomer Cement)

Glass ionomer cement is a fluoride-releasing, adhesive dental cement that bonds chemically to tooth structure (enamel and dentine). Introduced by Wilson and Kent in 1972.

Powder:

• Calcium fluoroaluminosilicate glass (acid-soluble glass)
• Aluminum oxide, silicon dioxide, calcium fluoride

Liquid:

• Polyacrylic acid (polyalkenoic acid) - main component
• Itaconic acid, maleic acid (copolymers to reduce viscosity)
• Water

Three stages:

1. Dissolution: Polyacrylic acid attacks glass → releases Ca²⁺, Al³⁺, F⁻ ions
2. Gelation: Ca²⁺ ions cross-link polyacid chains quickly (initial set - within minutes)
3. Maturation: Al³⁺ ions slowly cross-link chains over 24 hours → full strength

• Type I: Luting cement (crown/bridge cementation)
• Type II: Restorative (aesthetic restorations)
• Type III: Liner/base (under restorations)
• Type IV: Pit and fissure sealant
• Type V: Orthodontic cement
• Type VI: Core build-up
• RMGIC: Resin-modified GIC (with HEMA/resin added for improved properties)

• Chemical adhesion to enamel and dentine (no etching needed)
• Fluoride release - anticariogenic (prevents secondary caries)
• Biocompatible with pulp
• Similar thermal expansion to tooth structure
• Anticariogenic - can take up fluoride and re-release it (fluoride reservoir)

• Moisture sensitive during initial setting (first 24 hours - must be protected)
• Low fracture toughness and brittleness
• Susceptible to dehydration (surface crazing if dried out early)
• Lower aesthetics compared to composite

During early setting (first 24 hours), the calcium cross-links are immature and easily disrupted by water (dissolution). Premature moisture contact causes surface washout and crazing. A varnish or petroleum jelly must be applied immediately after setting.

Typically 1.5-2 parts powder to 1 part liquid (depending on type). Luting cements are more fluid; restorative mixes are stiffer.

• Dispense on a paper pad or glass slab (cooled glass slab for longer working time)
• Divide powder into two halves
• Incorporate first half quickly (~10 sec), then second half
• Total mix time: 30-45 seconds
• The mix should be glossy and homogeneous (loss of gloss indicates over-mixing or expired material)

The GIC mix is past its working time - gelation has begun. A glossy surface means polyacid chains are still free to bond to tooth. Dull surface = setting reaction too advanced = poor adhesion.

Resin-Modified GIC contains additional HEMA (hydroxyethyl methacrylate) and a photoinitiator. Benefits:

• Dual/triple cure (acid-base + light cure)
• Moisture tolerant - less sensitive to early moisture
• Better tensile and flexural strength
• Better translucency and aesthetics

⚙️ Articulator

An articulator is a mechanical instrument that simulates the temporomandibular joints and jaw movements. It holds casts of the upper and lower jaws in a fixed spatial relationship to reproduce mandibular movements outside the mouth.

1. Class I - Simple hinge: Only opens and closes (hinge movement only). Most basic. Used for simple restorations.
2. Class II - Average value: Simulates average jaw movements. Fixed condylar guidance (no individual adjustment).
3. Class III - Semi-adjustable: Can be adjusted to approximate individual patient movements using face bow records.
4. Class IV - Fully adjustable: Reproduces all jaw movements precisely. Requires extensive jaw recordings.

A face bow is a caliper-like instrument used to transfer the relationship of the maxillary arch to the hinge axis (TMJ) to the articulator. Without a face bow, the maxillary cast is mounted arbitrarily, leading to errors in occlusion.

The lateral bodily shift of the mandible toward the working side during lateral jaw movement. Semi-adjustable and fully adjustable articulators can be set to accommodate Bennett angle.

The angle formed by the path of the condyle on the articular eminence relative to the horizontal plane during protrusive movement. Average value is 25-33° (used in average value articulators).

The most retruded, unstrained position of the mandibular condyles in the glenoid fossa from which lateral movements can be made. It is a reproducible reference position used to mount casts.

• Centric relation (CR): A joint position (retruded condyle position) - not dependent on teeth
• Centric occlusion (CO): Maximum intercuspation of teeth
• In ideal cases they should coincide; the discrepancy is the CR-CO slide

🟫 Impression Compound (Type I - Tray Compound)

Impression compound is a thermoplastic impression material - it softens on heating and hardens on cooling without any chemical change (reversible physical change). Type I is used for making primary/preliminary impressions.

• Thermoplastic resins (copal resin, dammar resin) - 30-40%
• Waxes (paraffin, carnauba) - 30-40%
• Filler (talc/chalk) - 15-30%
• Plasticizer (stearic acid)
• Coloring agent (red/brown/green)

• Softening range: 55-65°C (tempered in water bath at 55-60°C)
• Becomes plastic and moldable above this temperature
• Hardens below this temperature

• Primary/preliminary impressions for edentulous patients (complete denture fabrication)
• Making stock tray impressions for edentulous ridges
• Border molding (stick form - green stick, Type II)
• Making custom trays for edentulous patients

ADA Specification No. 3

• Type I: Impression compound (cake/stick/cone)
• Type II: Tray compound (for special trays)

• Rigid - poor elasticity (cannot reproduce undercuts)
• Dimensionally inaccurate - high thermal contraction on cooling
• High viscosity - does not record fine details
• Should not be used where undercuts are present (may fracture on removal)

Flow is the percentage deformation at a given temperature and load. At 45°C, flow should be ≥ 85% (so it's moldable). At 37°C, flow should be ≤ 6% (so it's stable in the mouth).

🟤 Modelling Wax (Utility Wax / Boxing Wax / Baseplate Wax)

Modelling wax is a pattern wax used in the dental laboratory for constructing and modifying denture bases, occlusal rims, and wax-ups of dental restorations. Most commonly this refers to baseplate wax (pink sheets) or a general purpose wax.

• Paraffin wax (major component) - 70-80%
• Carnauba wax (hardener, raises melting point) - 5%
• Ceresin wax - stiffens the mixture
• Beeswax - improves texture
• Coloring agents (usually pink)

• Building occlusal rims on record bases
• Wax-ups of denture teeth positions
• Try-in stages of denture fabrication
• Lab pattern work

Boxing wax is a strip of wax used to build walls around an impression before pouring the stone/plaster, creating a box shape. This prevents plaster from spreading and gives a neat cast with uniform base height.

A soft, pliable wax (rope-like or sheet form) used to:

• Bead (outline) the borders of an impression
• Adapt or modify trays
• Seal around orthodontic bands
• Easy to manipulate at room temperature

A brittle wax that becomes very tacky/adhesive when melted. Used to:

• Join broken fragments of a denture temporarily
• Hold parts together on the articulator during lab procedures
• Low melting point, very adhesive on cooling

• Wax distorts due to release of internal stresses and thermal dimensional changes
• Prevent by: investing immediately, storing in cool conditions, handling minimally, and using Type III stone for poured models quickly

🧠 Quick Summary Table for All Spotters

🏗️ Investment Material — Advanced

1. Setting expansion (0.4-0.6%): Occurs as investment sets due to growth of gypsum crystals
2. Hygroscopic expansion (1.2-2.2%): Enhanced expansion when investment sets in contact with water
3. Thermal expansion (1.0-1.6%): Expansion during burnout heating due to silica inversion

Together they must equal the casting shrinkage of the alloy (~1.5-2% for gold alloys) so the final casting fits accurately.

• Quartz inversion: Occurs at 573°C - sudden, rapid expansion (0.8-1.4%)
• Cristobalite inversion: Occurs at 200-270°C - earlier, more gradual expansion (1.0-2.0%)

Cristobalite is preferred because its expansion occurs at a lower, more manageable temperature and is larger in magnitude, giving better compensation.

• Increased setting time
• Decreased setting expansion and thermal expansion
• Weaker investment (more porous)
• Resulting casting will be undersized (under-compensation)

• Casting shrinkage: Metal alloy contracts ~1.5-2% as it cools from molten to solid state → investment expansion must compensate
• Wax shrinkage: Wax pattern contracts as it cools from mouth temperature to room temperature → this is a source of inaccuracy BEFORE investing

A wet asbestos substitute liner (ceramic fiber paper) is placed inside the casting ring:

• Allows the investment to expand freely during setting and burnout
• Prevents the rigid metal ring from restricting expansion
• Without liner, the ring restricts investment expansion → undersized casting

• Burnout is heating the invested mold to 450-500°C to completely vaporize and remove the wax pattern
• For gypsum-bonded investment: maximum 700°C (above this, CaSO₄ decomposes)
• For phosphate-bonded investment: up to 1000-1100°C (for base metals/ceramics)

• Binder: Magnesium ammonium phosphate (formed from MgO + NH₄H₂PO₄ + water)
• Refractory: Silica (quartz + cristobalite)
• Used for:
  • Base metal alloys (Ni-Cr, Co-Cr) - casting bridges and partial dentures
  • Ceramic fusing investments
  • Titanium casting (special variants)
• Withstands temperatures up to 1100°C+

🦷 GIC — Advanced

• The polyacrylic acid (weak acid) attacks the fluoroaluminosilicate glass (base)
• Ca²⁺ ions are released first → rapid gelation within 2-3 minutes (initial set)
• Al³⁺ ions are released slowly → form stronger, more extensive cross-links over 24 hours (maturation)
• F⁻ ions are also released → anticariogenic effect
• The set material consists of glass particles surrounded by a silica gel, embedded in a polyacrylate matrix

• Chemical adhesion via ionic bonding
• The carboxylate groups (-COO⁻) of polyacrylic acid bond to Ca²⁺ ions in hydroxyapatite of enamel and dentine
• No etching needed - it is a direct chemical bond (not micromechanical like composite)
• Bond to enamel is stronger than to dentine (more mineral content in enamel)

• GIC releases fluoride ions continuously from the glass particles
• Fluoride inhibits bacterial metabolism and remineralizes early carious lesions
• GIC can also take up fluoride from topical fluoride applications and re-release it later (called fluoride reservoir effect)
• This makes GIC ideal for high caries-risk patients and ART (Atraumatic Restorative Technique)

Atraumatic Restorative Technique (ART) is a minimal intervention technique:

• Remove caries with hand instruments only (no bur, no electricity)
• Restore with high-viscosity GIC
• Used in remote areas, pediatric patients, anxious patients
• GIC is ideal because: adhesive, fluoride releasing, no need for dry field machinery

GIC has low fracture toughness (~0.3 MPa·m½) and is brittle. Under occlusal loading it fractures. Therefore it is not used in Class IV restorations or high-stress posterior restorations without modification (RMGIC or hybrid ionomer).

A technique where:

• GIC is placed first as a base/liner to replace dentine (adheres chemically, releases fluoride)
• Composite resin is placed on top to replace enamel (better aesthetics)
• GIC and composite bond via chemical adhesion and micro-mechanical bonding
• This combines the fluoride release of GIC with the aesthetics and strength of composite

⚙️ Articulator — Advanced

• Arcon (Articulated Condyle): Condyle element is on the lower member (like the human TMJ - condyle moves with mandible). More anatomically accurate.
• Non-Arcon: Condylar element is on the upper member. Less anatomical but simpler design.
• Arcon articulators give more accurate simulation during lateral and protrusive movements.

The hinge axis (transverse hinge axis) is the imaginary axis around which the mandible rotates in pure rotation (first 20-25mm of opening). It passes through both condyles.

• Locating this axis is essential for accurate face bow transfer
• If the hinge axis is wrongly located, the arc of closure is incorrect → occlusal errors in the restoration

• Patient protrudes mandible ~6mm and bites into wax/bite registration material
• This record is used to set the condylar guidance angle on the articulator
• The condylar guidance angle is adjusted until the condylar elements follow the same path as recorded in the protrusive record

During lateral mandibular movement:

• Working side: Side toward which mandible moves (chewing side). Working condyle rotates.
• Non-working / Balancing side: Opposite side. Balancing condyle moves forward and medially (Bennett movement occurs here).

The occlusal plane is the imaginary surface touching the incisal edges and cusp tips of the teeth. It should be parallel to certain reference planes (Frankfort horizontal plane / ala-tragus line). Without a face bow, the occlusal plane is arbitrarily mounted → arc of closure is wrong → restorations will have occlusal discrepancies.

• Anterior reference point: Corner of the eye (Frankfort plane) OR base of nose (ala-nasion line)
• Posterior reference point: Kinematic hinge axis (true) or anatomical hinge axis (approximate: 13mm anterior to tragus of ear on tragus-corner of mouth line)

🟫 Impression Compound — Advanced

• It has a high coefficient of thermal expansion/contraction (~7 × 10⁻⁴/°C)
• As it cools from 55°C (softening) to 37°C (mouth) to room temperature, it contracts significantly
• This gives a smaller impression than the actual tissue dimensions
• Also, it does not record fine detail due to high viscosity

• At 45°C: Flow ≥ 85% (must be plastic and moldable)
• At 37°C: Flow ≤ 6% (must be stable and rigid in the mouth) This ensures it is workable when warm but dimensionally stable when set.

After seating the impression in the mouth, it is chilled with cold water spray to ensure uniform, rapid hardening throughout the bulk of material. This minimizes dimensional change and internal stress.

Technically it can be re-softened (thermoplastic), BUT:

• Repeated heating degrades the resins and waxes (loss of plasticizer)
• Becomes more brittle and inaccurate
• Hygiene risk (contamination from mouth)
• In clinical practice, it is effectively single-use

🟤 Modelling / Pattern Waxes — Advanced

1. Accurate adaptation to the die at mouth/room temperature
2. Smooth, glossy surface (reflects accuracy)
3. Minimum residue on burnout (complete elimination at 500°C)
4. Low coefficient of thermal expansion (minimize distortion)
5. No flaking or chipping when carved
6. Adequate flow at slightly elevated temperature for adaptation

• Loss of low-fusing components (volatiles evaporate)
• Increased brittleness and chipping
• Loss of accuracy - residual stress is harder to relieve
• The wax composition changes with each heating

When wax is heated and adapted:

• Molecules are aligned under stress
• On cooling, stress is "locked in" (frozen stress)
• At room temperature or on slight warming, this stress releases slowly → wax pattern distorts and warps
• Result: casting does not fit

Prevention: Invest immediately, use uniform cooling, warm the die before adapting wax

Sticky wax is a pattern/processing wax composed mainly of beeswax + rosin.

• Unique property: When melted and applied, it becomes very adhesive on cooling
• Sticks firmly to metal, plaster, and stone surfaces
• Brittle at room temperature
• Use: Temporarily joining broken denture parts, holding parts on the articulator, soldering procedures

🔗 Cross-Linking Examiner Questions (They LOVE these)

• ZOE impression paste (for edentulous, rigid, good detail)
• Alginate (for dentate/edentulous study models)
• Elastomers (polyether, polyvinyl siloxane) - best detail and dimensional stability
• Impression compound (for preliminary impressions)

Best detail: Polyvinyl siloxane (addition silicone) - lowest viscosity, excellent detail reproduction, dimensionally stable.

This is about syneresis of alginate - when alginate loses water, it forms droplets on the impression surface. If you pour a GIC-based material into an alginate impression with moisture, it is actually favorable for a stone cast, but GIC restorations are placed directly in the tooth - no connection here. However, the concept of moisture contamination during initial GIC placement is critical (see Q17 GIC section).

⭐ Quick-Fire Round (Examiner's Favorite Short Questions)

🔩 DENTAL CASTING PROCEDURE — Complete Viva Guide

📌 DEFINITION

Casting is the process of pouring or forcing molten metal into a mold to produce a metal restoration (inlay, onlay, crown, bridge, partial denture framework) that replicates the shape of a wax pattern.

The Lost Wax Technique (cire perdue) is the fundamental principle of dental casting:

1. A wax pattern of the restoration is made
2. It is invested (surrounded by investment material)
3. The wax is burned out (lost) leaving a mold cavity
4. Molten metal is forced into the cavity
5. Investment is broken away to reveal the metal casting

📋 STEPS OF CASTING PROCEDURE (In Order)

STEP 1 — Wax Pattern Fabrication

• Direct technique: Wax pattern is fabricated directly in the patient's mouth on the prepared tooth. Uses Type I (hard/blue) inlay wax. More accurate fit but time-consuming for patient.
• Indirect technique: Impression taken → die poured → wax pattern made on the die in the laboratory. More common clinically.

A die is a positive replica of the prepared tooth made from a hard stone (Type IV dental stone). The wax pattern is carved on the die.

Type IV (Die stone / Improved stone) or Type V (High strength, high expansion stone):

• Very high compressive strength
• Low W:P ratio (0.19-0.24) → very dense, hard surface
• Resists abrasion when carving wax
• Accurate surface detail reproduction

• Die hardener: Cyanoacrylate or resin applied to the surface to harden it and prevent abrasion
• Die spacer (silver paint or resin): Applied to create space for cement luting - typically 20-40 μm thick, not applied to the margin area

STEP 2 — Sprue Attachment

A sprue is a wax rod/cone attached to the wax pattern that forms the channel through which molten metal flows into the mold cavity after wax burnout. It becomes the sprue channel/runner in the investment.

• Diameter: Slightly larger than the thickest part of the wax pattern (typically 2-2.5 mm for inlays/crowns)
• Too narrow: Metal freezes before completely filling the mold → incomplete casting (misrun)
• Too wide: Last to solidify → creates shrinkage porosity in the casting

• At the thickest, bulkiest area of the wax pattern
• Away from margins and delicate areas
• At a 45° angle ideally, not directly perpendicular (to avoid turbulence)
• The area should be waxed smooth at the junction (no sharp angles → reduces turbulence)

• 6-8 mm from the pattern to the base of the crucible former
• Too short: Pattern too close to the heat of crucible → overheating and oxidation
• Too long: Pattern too far from casting force → incomplete filling

The base (wax or metal cone/button) on which the sprue is mounted. It sits at the open end of the casting ring and creates the crucible/reservoir into which metal is placed for casting.

A wax bead placed on the sprue near the pattern. It acts as a reservoir of molten metal that feeds the casting as it solidifies and contracts, preventing shrinkage porosity.

STEP 3 — Investing the Wax Pattern

The process of surrounding the wax pattern with investment material to create a rigid mold that can withstand burnout and casting forces.

A metal cylinder (usually stainless steel) that contains the investment during setting and casting. Sizes vary - typically:

• Small: 1-2 unit inlays/crowns
• Large: Multiple unit castings
• A ring liner (wet paper/ceramic fiber) is placed inside before investing

• Allows free setting expansion and thermal expansion of investment
• Without it, the rigid metal ring restricts expansion → undersized casting
• Asbestos was used historically; now ceramic fiber paper or paper liner is used

• Measure exact W:P ratio per manufacturer's instructions
• Mix by hand for 30 seconds, then vacuum mix for 60 seconds (vacuum spatulator)
• Vacuum mixing: Removes air bubbles → prevents voids on casting surface
• Consistency: Smooth, creamy, no lumps

Mixing investment under vacuum:

• Eliminates air bubbles from the mix
• Prevents nodules (blebs) on the internal surface of the mold
• Nodules in the mold → corresponding pits in the casting → poor fit
• Results in a smoother, more accurate casting surface

1. Paint first coat of investment over the wax pattern with a brush (to capture all detail, eliminate bubbles)
2. Vibrate gently to settle investment around the pattern
3. Place casting ring with liner over the sprue former
4. Fill the ring with investment mix using a vibrator
5. Allow to bench set for minimum 30-60 minutes before burnout

Vibration:

• Helps investment flow into fine details of the wax pattern
• Dislodges air bubbles from the surface
• Produces a denser, smoother mold surface

STEP 4 — Wax Elimination (Burnout)

Heating the invested ring in a burnout furnace:

1. Eliminate the wax (vaporize completely) leaving a clean mold cavity
2. Thermally expand the investment to compensate for casting shrinkage

• 450-480°C for wax elimination (holds for 30-60 minutes)
• Some protocols go to 700°C maximum for additional thermal expansion
• NEVER exceed 700°C → CaSO₄ decomposes releasing SO₂ → contaminates casting

• Place ring in cold furnace → slowly raise temperature (avoid thermal shock cracking investment)
• Sprue hole should face DOWN initially so molten wax flows out (not absorbed by investment)
• Hold at burnout temperature until mold is uniformly red/orange and no carbon traces remain
• A gray/black ring inside the sprue hole indicates incomplete burnout → carbon contamination

• Carbon residue remains in the mold
• Carbon contaminates the molten metal → carbon inclusions, porosity, rough surface
• Casting appears dark/black and has poor surface quality

STEP 5 — Melting the Alloy

1. Torch melting (gas-air / gas-oxygen flame):
   • Gold alloys: gas-air flame
   • Base metals: gas-oxygen (higher temperature)
   • Simple, inexpensive
2. Electric resistance melting:
   • Electrical coils heat a crucible
   • Limited to low-fusing alloys
3. Induction melting:
   • High-frequency electromagnetic current induces heat in the metal itself
   • Very fast, uniform, less oxidation
   • Used for base metal alloys in centrifugal casting machines
4. Arc melting:
   • Electric arc used for very high-fusing alloys (titanium)

A slightly reducing (carburizing) flame - blue, luminous inner cone with a well-defined outer envelope.

• Oxidizing flame (too much air) → oxidizes the gold alloy → rough, porous casting
• Reducing flame protects the melt from oxidation

• The metal should be "spinning" and appear bright, mirror-like, molten
• A "whirling" motion indicates molten state
• Surface should look like a rotating liquid bead (surface tension effect)
• Do not overheat → excessive oxidation, porosity, grain growth

STEP 6 — Casting (Forcing Metal into Mold)

1. Centrifugal casting:
   • Most common in dentistry
   • Spring-loaded or electric centrifuge spins the mold
   • Centrifugal force drives molten metal into the mold
   • Good for all types of alloys
2. Air pressure casting:
   • Compressed air forces metal into the mold from above
   • Used for some automatic casting machines
3. Vacuum-assisted casting:
   • Vacuum applied to the opposite end of the mold pulls metal in
   • Less turbulence, good for thin castings
4. Gravity casting:
   • Metal flows by gravity only
   • Least accurate, rarely used

1. Spring is wound 3-4 turns (stored energy)
2. Invested ring is placed in the cradle/arm
3. Metal is melted in the crucible attached to the arm
4. Spring is released → arm spins rapidly
5. Centrifugal force drives metal through the sprue into the mold
6. Arm is allowed to spin freely until it stops naturally (do not stop it manually)

If stopped prematurely, the metal has not completely solidified. Stopping abruptly causes turbulence → porosity, distortion, or incomplete filling of the casting.

The sprue end faces the center of rotation (toward the crucible). Centrifugal force drives metal outward from center → through sprue → into the mold cavity at the periphery.

STEP 7 — Cooling and Devesting

1. Slow cooling (bench cooling):
   • Allow ring to cool completely at room temperature
   • Recommended for base metal alloys and most gold alloys
   • Prevents thermal shock cracking and residual stress
2. Rapid cooling (quenching):
   • Plunge hot ring into cold water while still red hot (for certain gold alloys only)
   • Causes thermal shock in investment → investment cracks/fragments → easier to remove
   • Also stress-relieves and softens gold alloys (annealing effect)
   • NOT for base metals → makes them brittle or causes cracking

Removal of the investment from around the casting:

1. Tap the ring gently to loosen investment
2. Remove the bulk of investment with an instrument
3. Sandblast with aluminum oxide (50-100 μm particles) to remove remaining investment
4. Gives a clean metal surface for finishing

STEP 8 — Finishing and Polishing

1. Pickling - Removes surface oxides
2. Sprue removal - Cut sprue with a separating disk
3. Finishing - Remove nodules, blebs with carbide burs/stones
4. Polishing - Progressive polishing: coarse → medium → fine → ultrafine abrasives
5. Evaluation - Check fit on die, margins, occlusion

Pickling is immersing the hot casting in an acid solution to remove surface oxides and investment debris:

• Gold alloys: 50% hydrochloric acid or 10% sulfuric acid
• Base metals: Special pickling solutions
• The casting is heated (not red hot) then placed in the pickle solution
• Never use steel tweezers in pickle solution → contamination → copper plating on casting
• Use plastic or wooden/quartz tweezers

• Nodules: Round bumps on the casting surface
  • Caused by air bubbles trapped against the wax pattern during investing → air bubble becomes a cavity → metal fills it
  • Prevented by: vacuum mixing, careful painting of first coat, vibration
• Blebs: Irregular protrusions
  • Caused by cracks in the investment (thermal shock during burnout)

⚠️ CASTING DEFECTS — Must Know

1. POROSITY

2. INCOMPLETE CASTING (Misrun / Short casting)

• Insufficient casting pressure
• Metal solidified before filling the mold
• Mold too cold (sprue too long / incorrect preheating)
• Sprue too narrow
• Alloy not fully melted

3. DISTORTION

• Wax pattern distorted before investing
• Premature removal from die
• Incorrect handling of wax pattern
• Residual stress release in wax

4. ROUGH SURFACE

• Oxidizing flame during melting
• Investment breakdown at high temperature
• Incomplete wax elimination (carbon residue)
• Overheating the alloy

5. FINS AND SPINES

• Cracks in investment (thermal shock during burnout)
• Metal flows into cracks → fins on casting

6. UNDERSIZED CASTING

• Insufficient investment expansion
• Too much water in investment mix
• Metal alloy not matching the expansion design of investment

7. OVERSIZED CASTING

• Excessive investment expansion
• Hygroscopic technique overcorrection

🔄 COMPLETE CASTING SEQUENCE — Quick Revision

Tooth preparation
      ↓
Impression + die fabrication
      ↓
Wax pattern fabrication (direct/indirect)
      ↓
Sprue attachment
      ↓
Investing (vacuum mix → paint → fill ring)
      ↓
Bench set (30-60 min)
      ↓
Burnout (450-700°C)
      ↓
Alloy melting (reducing flame/induction)
      ↓
Casting (centrifugal/pressure)
      ↓
Cooling (bench/quench depending on alloy)
      ↓
Devesting + sandblasting
      ↓
Pickling (acid - oxide removal)
      ↓
Sprue cutting
      ↓
Finishing and polishing
      ↓
Fit evaluation on die
      ↓
Cementation

⭐ Quick-Fire Casting Questions