Nickel Crucibles for Alkali Fusion: Grades, Fluxes and Care

What a Nickel Crucible Is and Where It Fits in Sample Decomposition

A nickel crucible is a cup-shaped vessel, normally supplied with a close-fitting lid, produced by spinning or deep drawing from commercially pure nickel sheet in the UNS N02200 or UNS N02201 family. Its value in a laboratory or a process control room has nothing to do with high-temperature strength. It is chosen because the metal is resistant to fused alkalis at the temperatures where alkali fusion actually happens, roughly 500 to 700 degrees C, and because the crucible can be made thin-walled enough to heat and cool quickly without cracking.

The practical division of labour is easy to state. A platinum crucible is destroyed by sodium peroxide and by sodium hydroxide at fusion temperature. A silica or porcelain crucible is consumed by the same reagents within a single fusion. A nickel crucible tolerates both, costs a fraction of a platinum dish, and can be re-used dozens of times if it is handled within its limits. What it cannot tolerate is an acidic flux, a sulphur-bearing sulphide flux, borax, or a molten heavy metal. Those are the four failure routes that account for most premature scrapping.

Buyers entering this category usually start from a laboratory glassware catalogue and treat the crucible as an accessory. It is better treated as a small pressure-retaining consumable with a specification: nickel grade, carbon level, wall thickness, base flatness, lid fit, and trace element control. Two crucibles of identical nominal capacity can differ by a factor of five in service life depending on those five variables. The sections below set out what the metal can do, which reagents are compatible, how the geometry is chosen, and how to write an acceptance specification that a supplier can actually meet.

Nickel Grades Behind the Crucible: Chemistry and Mechanical Limits

Almost all crucibles in this class are made from one of two grades: Nickel 200, designated UNS N02200, and the low-carbon variant Nickel 201, designated UNS N02201. The material specification normally referenced is ASTM B162 for plate, sheet and strip, with ASTM B160 covering rod and bar if a wrought blank is machined rather than drawn. Both grades are commercially pure nickel with a minimum of 99.0 percent nickel, and the difference between them is almost entirely the carbon ceiling: 0.15 percent maximum for N02200 against 0.02 percent maximum for N02201.

That single number matters more than any other line on the certificate. Carbon in nickel is practically insoluble, and above roughly 315 degrees C in long service it migrates to grain boundaries as graphite, leaving the metal weak and brittle at the very boundary where a fusion crucible carries its thermal stress. Short fusion cycles are tolerable in either grade, but a works that runs a muffle furnace for eight hours a day will see the low-carbon grade outlast the standard grade by a wide margin. Where repeated heating above 600 degrees C is expected, specify N02201 without exception.

Element	Nickel 200, UNS N02200	Nickel 201, UNS N02201
Ni, minimum	99.0 %	99.0 %
C, maximum	0.15 %	0.02 %
Mn, maximum	0.35 %	0.35 %
Fe, maximum	0.40 %	0.40 %
S, maximum	0.010 %	0.010 %
Si, maximum	0.35 %	0.35 %
Cu, maximum	0.25 %	0.25 %

The physical constants that govern furnace behaviour are consistent across both grades. Density is about 8.89 g per cubic centimetre, the melting range is approximately 1435 to 1446 degrees C, thermal conductivity is roughly 70 W per metre-kelvin at room temperature, mean coefficient of thermal expansion is about 13.3 micrometres per metre-kelvin over 20 to 100 degrees C, and electrical resistivity is around 0.096 micro-ohm-metre. The Curie temperature sits near 354 degrees C, which is why a used crucible may lose its attraction to a magnet after repeated heating while remaining perfectly sound.

Two consequences follow from the numbers. First, the thermal conductivity is high enough that a thin-walled crucible equalises temperature quickly, so a well-controlled muffle furnace will not produce the local overheating that cracks thicker ceramic ware. Second, the expansion coefficient is high for a metal used against ceramic hearths. A crucible that is allowed to sit in a hot furnace and then dragged across a firebrick will pick up scratches that become crack initiation sites, and a crucible quenched from red heat into cold water will warp at the base and lose its seat on the furnace floor.

Mechanically, annealed Nickel 200 sheet in the thickness range used for crucibles, 0.5 to 1.5 mm, offers tensile strength of roughly 400 to 460 MPa and yield strength near 150 to 180 MPa with elongation above 35 percent. Those figures are why the article can be spun and why it deforms rather than shatters when it is dropped. They also explain why a crucible is a soft item: it will dent, and a dented base will rock, and a rocking base will heat unevenly for the rest of its life.

Which Fluxes Are Compatible and Which Attack the Crucible

The compatibility question is chemical, not thermal. Nickel forms a protective oxide film that is stable in air and reasonably stable in fused alkali, but that film is destroyed by acidic fluxes and by anything that introduces sulphur into the metal. The table below reflects long-established laboratory practice for alkali fusion in nickel ware.

Reagent or melt	Compatibility with nickel	Practical note
Sodium hydroxide, NaOH	Suitable	Standard fusion medium for silicate, alumina and slag decomposition; melts near 318 degrees C, worked at 550 to 650 degrees C
Sodium peroxide, Na2O2	Suitable with control	Strong oxidiser; use a cool crucible, add in small portions, keep organic matter out of the charge
Sodium carbonate and bicarbonate, Na2CO3, NaHCO3	Suitable	Often blended with NaOH to lower the working temperature
Alkaline flux containing potassium nitrate, KNO3	Suitable	Nitrate additions raise oxidising power; keep the charge free of filter paper and grease
Potassium bisulphate, KHSO4	Not suitable	Acidic pyrosulphate melt attacks the metal rapidly and contaminates the sample with nickel
Potassium or sodium pyrosulphate, K2S2O7, Na2S2O7	Not suitable	Same acidic mechanism; use platinum, silica or ceramic ware instead
Borax, sodium tetraborate	Not suitable	Dissolves the protective oxide and alloys with the crucible wall
Alkaline sulphide flux containing sulphur	Not suitable	Sulphur embrittles nickel at grain boundaries; the crucible will crack without warning
Molten Al, Zn, Pb, Sn, Hg and their low-melting salts	Not suitable	These metals alloy with nickel and produce a brittle intermetallic layer through the wall

The embrittlement row deserves emphasis because it is the least intuitive. A drop of molten zinc from a galvanising line, a splash of lead from a solder pot, or a trace of tin from a tinned scoop will combine with nickel to form a low-melting phase that penetrates along grain boundaries. There is no visible warning: the crucible looks normal, then fractures into several pieces the next time it is lifted. The same mechanism applies to mercury and to mercury salts that may be present in environmental samples. A nickel crucible used for samples containing these metals should be retired from general service or reserved for that single duty.

There is also a contamination angle that matters for trace analysis. Commercial nickel sheet is not a spectrographic standard. It can carry several hundred parts per million of cobalt, and residual chromium from scrap-based melting is common. A crucible that is perfectly acceptable for a major-element silica determination can be unacceptable for a trace chromium or cobalt determination at the low parts per million level. The remedy is procedural rather than metallurgical: run a full reagent blank through a fresh crucible before trusting it, and keep a dedicated crucible for the trace work.

Dimensions, Capacity and Wall Thickness for Practical Fusion Work

Capacity is nominal, and the useful working volume is smaller, typically 40 to 60 percent of the nominal figure once the charge and the boil-up space are accounted for. A fusion that foams, which sodium peroxide charges frequently do, needs a crucible one size larger than the arithmetic suggests. The table gives the geometry that is normally available and the duty each size suits.

Nominal capacity	Typical top diameter	Typical height	Wall thickness	Typical duty
15 to 20 mL	30 to 34 mm	28 to 32 mm	0.5 to 0.7 mm	Small trial fusions, single sample checks, teaching work
30 to 50 mL	40 to 46 mm	36 to 42 mm	0.6 to 0.8 mm	Routine 1 to 3 g charges, ferrous and non-ferrous ores
80 to 100 mL	55 to 60 mm	48 to 55 mm	0.8 to 1.0 mm	Silicate rock, slag and refractory decomposition, 3 to 5 g charges
150 to 200 mL	65 to 75 mm	60 to 70 mm	1.0 to 1.2 mm	Large batches, alumina and bauxite, cements, multiple-sample fusion
250 to 500 mL	80 to 95 mm	75 to 95 mm	1.2 to 1.5 mm	Pilot work and process control in plant laboratories

Wall thickness is the specification item most often omitted and most often responsible for short life. Below about 0.5 mm the crucible deforms under its own weight at fusion temperature and the base loses flatness. Above about 1.5 mm the wall carries a steep temperature gradient during heating and cooling, and the extra mass simply absorbs furnace time. For a 100 mL crucible, 0.8 to 1.0 mm is the sensible band, and a tolerance of plus or minus 10 percent on nominal thickness is a reasonable acceptance condition.

Lid fit is the second quiet variable. A lid that seats with a slight taper and a small vent relief keeps dust out, limits oxidation of the charge, and still allows gas to escape. A lid that seals tightly will be blown off or will pressurise the crucible when a carbonate charge evolves carbon dioxide. A lid without any locating feature will slide off during transfer from the furnace to the desiccator, which is where most spills in a fusion laboratory originate. For crucibles above 150 mL, a lid with a lifting lug or a knurled rim is worth the small price difference, because tongs on a smooth polished rim at 650 degrees C are a genuine hazard.

Conditioning, First Use and Between-Use Handling

A new crucible carries drawing lubricant, oxide scale from the mill, and a work-hardened surface layer. None of these belong in a fusion. The standard conditioning sequence is: wash with a mild alkaline detergent and a soft brush, rinse, dry, then heat gradually in a muffle furnace to between 700 and 800 degrees C and hold for 10 to 15 minutes until the whole surface takes on a uniform blue-purple temper oxide. That oxide is the working surface. It is thin, adherent and stable in fused alkali, and it is the reason a conditioned crucible resists attack far better than a freshly polished one.

After the oxidation step, cool the crucible slowly on a clean firebrick rather than in a cold draught, then boil it for a short period in dilute hydrochloric acid prepared as one volume of concentrated acid to twenty volumes of water, and finish with a thorough rinse in deionised water. The acid boil removes residual oxide dust and any metallic smears from tooling. Dry in an oven at around 105 degrees C and store inverted, in a dust-free cupboard. Never store a nickel crucible with its lid closed on a damp shelf: trapped moisture plus residual alkali produces a corrosive cell that pits the rim.

Between fusions the handling rules are short. Do not pick a hot crucible up with bare steel tongs; use nickel or stainless tongs with the tips kept clean, because a steel tip leaves a transferable iron smear that shows up in the next analysis. Do not quench. Do not scrape the base with a spatula to dislodge a stubborn button; instead soak the crucible in warm water until the melt lifts, which for a sodium hydroxide charge takes minutes and for a peroxide charge may take an hour. Do not use the crucible as a weighing vessel for acidic sample solutions, and do not leave acidic residues in it overnight.

Where a fusion has left a dark adherent film, a short soak in warm dilute acid followed by a soft nylon brush is enough. Abrasive paper and wire brushes remove the temper oxide and expose fresh metal, which then has to be re-formed, so they should be a last resort. A crucible whose base has been ground flat on abrasive paper will hold a polished, oxide-free patch that fuses with the charge on first use and thereafter sticks.

Failure Modes and How to Recognise Them Early

Nickel crucibles fail in a small number of recognisable ways, and each has a signature. Knowing the signature turns a mysterious breakage into a correctable process fault.

Intergranular cracking from sulphur is the most dangerous because it is silent. It begins as a network of fine cracks visible only under low magnification at the base-to-wall radius, and it ends with the crucible separating into fragments while being carried. Any plant that fuses sulphide-bearing ores, or that shares a furnace with sulphur-bearing work, should inspect crucibles at the radius every twenty fusions with a magnifier and a bright light.

Brittle alloying from molten metals shows differently: a silver-grey skin on the inner surface, a dull ring instead of a clear one when tapped, and a crack that runs straight through the wall rather than along a boundary. Once alloying has occurred the crucible cannot be recovered.

Wall thinning from over-vigorous peroxide fusion appears as a gradual loss of mass and a change in colour towards matte grey-black on the inside, with the base becoming noticeably flexible under thumb pressure. Weighing crucibles at commissioning and every fifty fusions is a cheap way to trend this; a loss of more than 5 percent of the original mass is a reasonable retirement trigger.

Warping from thermal shock shows as a rocking base and a run-out of light under a straight edge. It does not stop the crucible from working, but the uneven contact with the furnace floor creates a hot spot that accelerates every other failure mode, so a warped crucible should move to non-critical work.

Finally, contamination is a failure of the analysis rather than of the article. A crucible that has been used for a high-chromium slag will release chromium into the next fusion for several cycles. Segregating crucibles by sample type, and marking the base with a stamped number rather than a painted label that burns off, prevents the confusion that produces an impossible assay result.

Selection and Acceptance Points for Purchasing

A workable purchase specification for a nickel crucible contains seven items and fits on one page. First, the grade with its UNS number, stated as Nickel 201 UNS N02201 for any duty above 600 degrees C. Second, the material standard, ASTM B162 for sheet-derived ware. Third, nominal capacity and the principal dimensions with tolerances of plus or minus 1 mm on diameter and height. Fourth, wall thickness with a plus or minus 10 percent band, measured at three points on the wall and one on the base. Fifth, a certificate of analysis showing nickel, carbon, iron, sulphur and copper, with carbon reported to two decimal places because the difference between 0.02 and 0.15 percent is the whole reason for choosing one grade over the other. Sixth, a statement of surface condition: annealed, descaled, free of laps, seams and die marks, with no weld repair. Seventh, mass consistency, since a set of crucibles that varies by 15 percent in mass will heat at different rates in the same furnace.

On receipt, check the base for flatness against a straight edge, check that the lid seats without rocking and lifts without binding, and check the inner surface at the base radius under magnification. Then run a blank fusion before the crucible enters routine work: a charge of the same flux, carried through the full cycle and analysed, will reveal both contamination and any tendency to stick. A supplier who cannot state the carbon level of the sheet is not supplying a specified article, whatever the label says.

Storage and transport are simple but not trivial. Crucibles should be packed nested with paper interleaves so that rims do not rub, kept dry, and never stacked in a way that puts load on the wall rather than the rim. In a humid climate, a desiccant sachet in the storage box prevents the rim pitting described earlier. Because the article is thin-walled and soft, transit damage usually appears as a flattened rim or a dished base rather than a visible break, so incoming inspection should include a straight-edge check on every unit, not a sample.

FAQ

Q: Can a nickel crucible be used for potassium pyrosulphate fusion?
No. Acidic pyrosulphate melts attack nickel quickly, dissolving the wall and loading the sample with nickel. Use platinum, silica or a suitable ceramic crucible for K2S2O7 and Na2S2O7 fusions, and reserve nickel ware for alkaline fluxes.

Q: Why must a new nickel crucible be oxidised before its first fusion?
Heating to 700 to 800 degrees C in air forms a thin, adherent blue-purple oxide that acts as the working surface in fused alkali. It also burns off drawing lubricant and mill scale that would otherwise contaminate the first analysis.

Q: What is the difference between Nickel 200 and Nickel 201 for a crucible?
Both are at least 99.0 percent nickel under ASTM B162, but Nickel 201, UNS N02201, limits carbon to 0.02 percent against 0.15 percent for Nickel 200. The low-carbon grade resists graphitisation at grain boundaries and lasts considerably longer under repeated heating above 600 degrees C.

Q: Can borax be melted in a nickel crucible?
No. Borax dissolves the protective oxide film and alloys with the crucible wall, producing both a contaminated melt and rapid local thinning. Borax and borate fluxes belong in platinum or high-alumina ceramic ware.

Q: What happens if molten zinc or lead contacts the crucible?
The molten metal alloys with nickel and forms a brittle intermetallic layer that penetrates along grain boundaries. The crucible gives no visible warning and then fractures, so any crucible exposed to these metals should be withdrawn from service immediately.

Q: How is a nickel crucible cleaned when the melt will not release?
Soak it in warm water until the fused button lifts by itself, then brush with nylon under running water. Do not pry with a spatula or grind the base, because both remove the temper oxide and create sites where the next charge will stick and pit.

Q: Is a nickel crucible suitable for trace chromium determination?
Only with care. Commercial nickel sheet can carry residual chromium and cobalt, so run a complete reagent blank through the specific crucible first, and keep a dedicated unit for trace work rather than sharing one with high-chromium slag fusions.