Impression taking looks routine from the dental chair. You mix, load, seat, wait and remove. For a dental nurse revising it as a post-registration additional skill, there is a lot more to it. Every step affects whether the model that reaches the lab is accurate. Assessments test that chain of cause and effect far more than the mixing itself.
This post follows the order of a typical impression appointment: bite registration, upper and lower alginate impressions, then the work done after the patient has gone. The source video was made in a US practice, so where UK expectations differ, they are spelled out.
Who does what: scope of practice comes first
In the video, the “dental provider” takes the impressions. In the UK, the GDC Scope of Practice sets out who may do this. Taking impressions to the prescription of a dentist is an additional skill that a dental nurse can develop. So is pouring, casting and trimming study models. You need appropriate training and must be competent before doing either. A blog post is not that training.
The scope point gets tested. Expect questions that ask whether a task is in a dental nurse’s core role, an additional skill, or outside their scope altogether.
Bite registration: recording how the arches meet
A bite registration records the patient’s occlusion, meaning how the upper and lower teeth fit together when the patient bites. It can be taken before or after the alginate impressions. The materials vary:
- Wax: a softened wafer the patient bites into.
- Bite registration paste: often a fast-setting silicone, syringed onto the occlusal surfaces while the patient holds their bite.
The video answers a question patients often ask. Why take impressions of both arches for an appliance that only fits the upper teeth?
The lab needs to see how your top and bottom teeth match to fabricate a properly fitted night guard.
What the video calls a night guard is usually called an occlusal splint or bite guard in the UK. The reasoning holds for any appliance or restoration that touches the opposing teeth. The opposing model and the bite registration let the technician articulate the models and build the work to the patient’s actual occlusion.
Tray selection and patient preparation
Before any material is mixed, try in stock trays to find the size that fits each arch. The tray should cover all the teeth and the relevant soft tissue, with room for an even thickness of material. It should not press on the soft tissues.
Gagging is the patient’s main worry. The video’s advice is simple and sound:
If you’re nervous about gagging it truly helps if you breathe through your nose and even wiggle your toes.
Other practical points:
- Explain what will happen. Patients cope better when they know it will only take a few minutes.
- Sit the patient upright, not reclined. This keeps material away from the back of the throat.
- Don’t overload the tray. Excess material running posteriorly is the usual trigger for gagging. It is also a reason to watch the airway throughout.
- Check the medical history for anything relevant, and keep suction to hand.
For tray try-in, mixing and seating in more detail, see our guide on taking alginate impressions.
Alginate: fast set, regular set, and what controls it
Alginate is an irreversible hydrocolloid. You mix the powder with water to the manufacturer’s ratio. Once the reaction starts it cannot be reversed. The video uses a fast-setting alginate. Regular-set products give more working time.
Setting time also depends on the water you use. Warmer water speeds the set and colder water slows it. That gives you some control, and it is a classic exam question.
The usual order is upper (maxillary) arch first, then lower (mandibular). The tray is held still in the mouth until the material has set. The video describes the clinician feeling the material until it is firm and no longer “squishy”. Checking the excess material is common practice, because it shows you how the material in the mouth is setting.
Removal often feels like breaking a suction seal. A firm, quick movement along the long axis of the teeth reduces tearing and distortion. After both impressions, rinse the patient’s mouth and remove any fragments of material.
| Factor | Effect on alginate |
|---|---|
| Warmer mixing water | Shorter working and setting time |
| Colder mixing water | Longer working and setting time |
| Left exposed to air | Shrinks as water is lost (syneresis) |
| Left soaking in water | Swells as water is absorbed (imbibition) |
| Removed before fully set | Distortion and tearing |
The step the video skips: disinfection and transport
The video moves straight from the mouth to pouring. In a UK practice there is a step in between that you must know.
Every impression is a contaminated item. Before it is poured or sent to a laboratory:
- Rinse it under running water to remove saliva and blood.
- Disinfect it with a product suitable for alginate, following the manufacturer’s contact time.
- Keep it moist and covered as directed, usually sealed in a bag, so it doesn’t shrink or swell.
- Label it as disinfected and send it with a complete, signed laboratory prescription.
The aim is to protect laboratory staff and anyone else who handles the impression. This follows the same infection-control logic as instrument reprocessing. Our post on decontamination and one-way flow covers how cleaning differs from disinfection.
Pouring the model in dental stone
Once the impression is disinfected, it can be cast. Dental stone is mixed with water to the right consistency. The ratio matters: extra water makes a weaker model. The impression is held on a dental vibrator while the stone is added a little at a time from one end, so it flows through the whole impression.
The goal is to not have any bubbles because that can ruin the model.
An air bubble on an incisal edge, cusp tip or preparation margin can leave the model unfit for its purpose. Then the patient has to come back for new impressions. A base is added, the stone is left to set fully, and the impression is separated from the model. The model is then trimmed.
Separating too early risks breaking teeth off the model. Leaving the stone in the alginate too long can affect the surface as the alginate dries out.
What the models are used for
Study models and working models support a wide range of work. The video mentions splints, whitening trays, crowns and dentures. Study models are also central to orthodontic assessment, where the occlusion is examined in three dimensions. If you are studying orthodontics, our guide to incisor classification from the cingulum plateau shows the kind of assessment that relies on accurate models.
One UK point: tooth whitening may only be provided by registered dental professionals, on a dentist’s prescription. Making a whitening tray is part of a regulated treatment, not a cosmetic add-on.
In the assessment. When a question describes a distorted model or a bubble-filled cast, work backwards through the steps. Ask whether the impression dried out, sat in water, was removed too early, was mixed wrongly or poured without vibration. The answer is almost always in the step that was skipped.
Revise it properly
Impressions and models come up throughout post-registration dental nursing: additional skills, orthodontic nursing, and questions about materials. Dental Post-Reg Mastery has 1160 questions across 26 topics for the NEBDN post-registration certificates, including 464 extended matching questions. 72 questions are free to try. It is an independent revision tool built to mirror the assessment format. It is not endorsed by the NEBDN or the GDC.
Nothing on this page is clinical guidance. It is written to help you revise for an assessment. In practice, follow your training, the manufacturer’s instructions, your practice protocols and the GDC Scope of Practice.
- Impressions
- Dental materials
- Additional skills
Frequently asked
Can a dental nurse take impressions in the UK?
Yes, as an additional skill. The GDC Scope of Practice lists taking impressions to the prescription of a dentist (or a clinical dental technician, where appropriate) as a skill dental nurses can develop. You need training and competence first. Qualifying as a dental nurse does not include it automatically.
Why do you need upper and lower impressions for an appliance that only fits one arch?
The technician needs to see how the two arches meet. An occlusal splint made from one model alone may stop the teeth meeting properly. The opposing model and the bite registration together let the lab make it to fit the patient's occlusion.
Why should alginate impressions be poured promptly?
Alginate is dimensionally unstable. Left in air it loses water and shrinks (syneresis). Left soaking it absorbs water and swells (imbibition). Either way the model is distorted, so keep the impression moist and covered as the manufacturer directs and cast it as soon as practicable.
Do impressions need to be disinfected before going to the lab?
Yes. Rinse off saliva and blood under running water, then disinfect with a product suitable for that impression material, following the manufacturer's instructions. Label it as disinfected before it leaves the practice so the lab knows how it was handled.
Why is a vibrator used when pouring a stone model?
Vibration helps the stone flow into every detail of the impression and brings trapped air bubbles to the surface. A bubble on a tooth surface or margin can make the model unusable.
