Gypsum plaster (hydrated calcium sulphate)

Physical properties

Laser processing

Laser ablation of plaster generates calcium sulphate particulate and possible calcium oxides. Irritating to the respiratory tract. Ventilation and direct extraction recommended.

CO₂: CO₂ laser engraving on plaster produces a matte mark by ablation of the surface layer. The material is porous and absorbs heat unevenly; the result is a light engraving, suitable for decorations on hardened plaster forms. Low power (20–40% on 40 W machines) to avoid thermal cracking. Cutting is impossible: plaster is too brittle and chips.

Diode: Poor results on white plaster. Blue diode lasers have low absorbance on light surfaces. Possible at maximum power but inconsistent results.

Fiber: Not applicable in standard maker contexts.

CNC / milling processing

Hardened plaster is one of the easiest materials to mill overall: soft (Mohs 2), non-abrasive, does not melt. Standard HSS or carbide bits work excellently. Generous depths of cut are possible but brittleness requires moderate feed rates to avoid workpiece breakage. Dry milling preferred (water reactivates residual plaster). Use dust extraction.

Post-processing

Sanding: Plaster is easily worked with dry sandpaper from 80 to 240 grit. Absolutely avoid wet sanding: plaster absorbs water and softens. For very fine surfaces use fine sanding sponge (240–400). Final polishing is achieved with plaster wax or sanded acrylic primer.

Primer: mandatory. Plaster is extremely porous and absorbs paints directly, causing excessive paint consumption. Before painting apply one or two coats of diluted acrylic primer (1:1 with water) or diluted PVA as a sealer. Acrylic gesso (plaster-specific primer) is the ideal product.

Gluing: Standard PVA works well on porous plaster surfaces. For structural repairs: cyanoacrylate gel or two-component epoxy. Avoid solvent-based adhesives that may damage the surface. The surface must be dry: damp plaster does not bond.

Seasoning: Liquid plaster takes 20–40 minutes to set (initial hardening) and 24–72 hours to reach maximum mechanical strength. Do not work before complete hardening.

Over time: Plaster is moisture-sensitive: environments with relative humidity >70% progressively weaken it and may cause swelling or surface detachment. In dry environments it is stable for decades. Not suitable for outdoor applications or damp environments without specific waterproofing treatment.

Common defects

• Fracture and chipping on sharp edges and thin sections during milling
• Swelling and loss of strength from ambient moisture absorption
• Shrinkage and surface cracks if the liquid gypsum was mixed with too much water

Safety

Recommended PPE: mask_ffp1, eye_protection, gloves_disposable

Plaster dust (during dry plaster mixing or mechanical processing) is irritating to eyes, nose and throat. Not classified as carcinogenic but prolonged exposure is discouraged. FFP1 mask for occasional work, FFP2 for prolonged work. Quick-setting plaster generates heat during the exothermic reaction — do not immerse hands in large quantities of setting plaster.

Availability and formats

Commercial formats: powder in 1–25kg bags, pre-cast blocks for sculpture, plasterboard panels, ready-made moulds

Related materials

Sources

• EN 13279-1 — Leganti a base di solfato di calcio — Definizioni e requisiti
• Smooth-On — Plaster of Paris Technical Data Sheet
• Epilog Laser — Engraving on Plaster/Gypsum Application Note