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Material guide · K9 optical glass vs PMMA · 12 min read
Two clear blocks can sit side by side on a shelf and look like the same material. One is K9 optical glass; the other is acrylic (PMMA) — a thermoplastic. They differ in almost every property that matters: how much they weigh, how they bend light, what scratches them, what dissolves them, how they age, and which one can be engraved on the inside.
Neither material is "better" in the abstract. They are different materials, and the right choice depends on what the part has to do. This is the comparison we wish every buyer had read before approving a sample photo.
On this page
First: "Crystal" Is Not a Protected Word
In international trade, the English word "crystal" has no mandatory legal definition. Nothing requires a supplier to reserve it for glass, and many do not: acrylic parts are routinely listed as "crystal," and buyers who approve from photographs discover only on arrival that the carton holds plastic.
Only one phrase actually means optical glass: K9 Optical Crystal. If that phrase is not on your paperwork, you have no material claim when a shipment is wrong.
Put it in writing. On the PI (Proforma Invoice) and sales contract, specify:
Material: K9 Optical Crystal Glass, NOT acrylic
Put it on the product specification line, not only in an email. An email is a conversation; a PI line is a claim. It costs nothing to add and it is the only thing that holds if the material is substituted.
What Each Material Actually Is
K9 crystal is a lead-free borosilicate crown glass — silica, boron and potassium/sodium oxides with a small barium content, and no lead oxide. It is the Chinese standard optical glass equivalent to Schott BK7: refractive index nd ≈ 1.517, Abbe number ≈ 64, density ≈ 2.52 g/cm³, light transmission >92% in the visible range, Mohs hardness around 6. It is melted, annealed, then cut, ground and polished into shape.
Acrylic (PMMA) is a thermoplastic polymer: refractive index ≈ 1.49, density ≈ 1.18 g/cm³, transmission ≈ 92% when new, Mohs hardness only 2–3. It is moulded, extruded or cast — not cut — and parts are produced by injection moulding, extrusion or CNC routing from sheet or block.
That single difference — inorganic glass network versus long-chain polymer — drives every number below.
Property Comparison
| Property | K9 Optical Crystal | Acrylic (PMMA) |
|---|---|---|
| Material class | Lead-free borosilicate glass | Thermoplastic polymer |
| Density | ~2.52 g/cm³ | ~1.18 g/cm³ |
| Refractive index (nd) | ~1.517 | ~1.49 |
| Light transmission | >92%, stable for decades | ~92% when new, declines with UV |
| Mohs hardness | ~6 | 2–3 |
| Scratch resistance | Resists coins, normal wiping | Scratches from wiping |
| Impact resistance | Brittle, shatters | High — 10–20× glass |
| Continuous service temp. | Several hundred °C | ~80 °C (softens near 100 °C) |
| Thermal conductivity | ~1.1 W/(m·K) — feels cold | ~0.19 W/(m·K) — warms to touch |
| UV / weathering | None — permanent | Yellows and hazes over 5–15 years |
| Chemical resistance | Inert to common solvents | Attacked by alcohol, acetone, etc. |
| Subsurface laser engraving | Yes — inside the material | No — surface marking only |
| Typical forming method | Cut, ground, polished | Moulded, extruded, cast |
| Combustibility | Non-combustible | Combustible |
| Static / dust attraction | Negligible | Attracts dust |
| Relative material cost | Higher | Lower |
Optics: Why One Sparkles and the Other Doesn't
K9 bends light more strongly (1.517 vs 1.49) and, more importantly, it is cut and polished. A polished bevel is a real prism: light entering one face leaves another at an angle that depends on wavelength, so white light separates into visible colour. That is the prismatic "fire" people notice at an edge.
Acrylic is moulded. Moulded edges are rounded at the micro scale and reproduce the tool surface, so they glint rather than refract. (PMMA's Abbe number is around 57 versus ~64 for K9, meaning the polymer disperses slightly more per unit of bending — yet in practice it shows less fire, because edge quality dominates dispersion.)
Internal clarity is the other difference. Optical glass is annealed and inspected for bubbles, striae and inclusions; a clear block reads as deep. Cast acrylic can be very clear when new, but it is the material that hazes with UV and micro-scratches over time.
Mechanical, Thermal and Chemical Behaviour
Hardness
At Mohs ~6, K9 shrugs off a coin, a key and routine dusting. Acrylic at 2–3 takes a visible mark from a fingernail dragged firmly across it, and a year of dry wiping leaves a fine network of scratches across the whole surface.
Impact
This is acrylic's win, and it is a large one. PMMA absorbs 10–20 times the impact energy of glass before failing. Where a part will be dropped, knocked, shipped through rough freight, or used around children, acrylic is genuinely the safer specification.
Heat
Glass is unaffected by temperatures that destroy acrylic. PMMA's glass transition is around 105 °C and continuous service is roughly 80 °C — hot water, a closed car in summer, or a lamp enclosure can distort it. K9 is stable to several hundred degrees. Conversely, that also means K9 cannot be thermoformed and acrylic can.
Touch temperature
With roughly six times the thermal conductivity, K9 pulls heat from your hand and stays cold; acrylic is an insulator and warms to body temperature quickly. This is the most useful identification test in the field.
Chemicals
Glass is chemically inert to almost everything a part will meet. Acrylic is not: alcohol, acetone, glass cleaner and many solvents attack the surface, producing crazing (a fine internal crack network) and permanent haze. Stress-cracking can appear days after exposure, away from any obvious cause.
UV and weather
Mineral glass does not age under ultraviolet light. Acrylic yellows and hazes, fastest on a windowsill or outdoors. UV-stabilised cast grades last longer, but none of them match glass for permanence.
Static
Acrylic builds a static charge and collects dust; K9 does not.
Processing: What Each Material Can and Cannot Do
Shaping
K9 is cut, ground, then polished — which is why complex geometry costs more and why sharp, crisp edges are achievable. Acrylic is moulded or extruded, so it is cheap at volume, easy to thermoform, and poor at holding a truly sharp edge. Acrylic also machines easily on a router or laser cutter (a CO₂ laser leaves a flame-polished edge); glass does not.
Laser engraving — the hard boundary
This is not a matter of degree.
- Subsurface (3D) engraving works by focusing a pulsed beam inside the material. In glass, the focused energy creates tiny, clean micro-fractures — leaving the surface untouched. That is how a photo, a portrait, a logo or a true 3D figure is suspended inside a solid block.
- In acrylic, the same beam melts and clouds the polymer. You get a smear, not a point.
Acrylic is therefore limited to surface screen printing, surface laser marking and shallow flat engraving — two-dimensional, on the outside.
Any job requiring 3D photo engraving, internal portraits, or a logo inside the material can only be done in K9. There is no acrylic workaround.
Marking permanence
Ink-based decoration inside or on acrylic fades within 5–15 years. Subsurface engraving in glass is structural — there is no ink or coating to fade, because the mark is the material.
Bonding and assembly
Acrylic welds and solvent-bonds easily, and takes UV-cured adhesive well. Glass is bonded with UV adhesive or optical cement, and requires clean, dust-free surfaces and correct curing; it also needs annealing control during manufacture to avoid internal stress.
6 Field Tests — No Lab Required
1The Weight Test
Pick it up. K9 is ~2.1× denser than acrylic — noticeably heavier than it looks, and clearly heavier than an identically sized plastic part. Acrylic feels light, almost hollow.
2The Scratch Test
On an inconspicuous spot, press a coin edge or fingernail and drag firmly. Acrylic marks (Mohs 2–3); K9 does not (Mohs 6). On a used part, look for the fine web of wipe-scratches that acrylic develops within a year.
3The Edge / Dispersion Test
Look at a bevelled edge under a light. Cut-and-polished K9 throws sharp, coloured prismatic flashes. Moulded acrylic gives a flat, glassy glint with almost no colour separation. Photographs hide this completely; in person it takes two seconds.
4The Tap Test
Flick it with a fingernail. Glass rings with a bright, high "ting"; acrylic answers with a dull plastic "tick." With a little practice you hear it before you think about it.
5The UV / Age Test
Ask what the part looks like in five years, and look at a sample that has already lived on a windowsill. Acrylic yellows and hazes. K9 looks identical. For anything intended to last a decade or more, this is the test that decides it.
6The Temperature Touch Test
Hold it in your palm at room temperature and wait. K9 keeps drawing heat away and stays cold. Acrylic warms up and reaches body temperature quickly. Fastest test available when you cannot scratch, tap or weigh anything — a trade show, a supplier's showroom, a client's desk.
Shipping and Handling
K9 is hard but brittle. Large or heavy pieces need thick foam cushioning plus a wooden crate (or at minimum a wood-framed carton) for sea freight. Corner and edge protection matter far more than carton thickness — breakage happens at edges, not faces.
Acrylic is drop-resistant, and breakage over long sea shipments is rare. Its shipping risk is the opposite one: surface marring. Sheets and polished faces arrive scratched if they are stacked without interleaving or film.
So match the material to the logistics. If the budget is tight, the application is short-term, or the freight chain is rough, specify acrylic openly — that is a better outcome than shipping glass into a chain that will break it, and far better than substituting acrylic while the paperwork says crystal.
Care and Cleaning: Opposite Rules
The two materials want opposite treatment, and acrylic is the demanding one.
K9 optical crystal
- Plain water and a soft cloth for routine dusting.
- Standard glass cleaner is safe.
- Normal wiping will not scratch it (Mohs 6).
Acrylic
- Never alcohol, acetone, or glass cleaner — these attack the surface and cause crazing and haze.
- Water and a microfibre cloth only.
- Any hard or rough cloth leaves visible scratches, even with water alone.
Put these two rules on the care card. A large share of "the part has gone cloudy" complaints are solvent damage on acrylic, not a product defect.
Choosing by Application
| Requirement | Specify |
|---|---|
| 3D photo, portrait or internal logo engraving | K9 only |
| Decades of clarity, no yellowing | K9 |
| Sharp prismatic edges, real depth and weight | K9 |
| High heat, or a lamp / light-source enclosure | K9 |
| Will be dropped, knocked, or used around children | Acrylic |
| Tight budget, short-term or one-time use | Acrylic |
| Large, light parts where weight matters | Acrylic |
| Complex moulded shapes at high volume | Acrylic |
| Rough freight, poor handling at destination | Acrylic |
Where budget allows, K9 is usually the "buy once" choice; acrylic is the "buy for now" choice. Both are correct — as long as the material on the quotation is the material in the carton.
FAQ
Do suppliers really ship acrylic when the order says crystal?
Yes, particularly on low-priced listings with flattering photographs. The weight and temperature tests expose it in seconds.
How do I prevent it?
Write "Material: K9 Optical Crystal Glass, NOT acrylic" into the PI and contract specification line. "Crystal" on its own is not enforceable — it has no legal definition in trade.
Can acrylic be 3D laser engraved inside the material?
No. A focused internal beam melts and clouds PMMA instead of forming clean micro-fractures. Acrylic can be surface printed, surface laser-marked or shallow-engraved only. Anything suspended inside the block requires K9.
Is K9 the same as lead crystal?
No. K9 is a lead-free borosilicate optical glass (n ≈ 1.517). Lead crystal is a different glass family with higher refractive index and higher density. K9 is the standard optical glass used where clarity and consistency matter.
Which handles heat better?
K9, by a wide margin. Acrylic begins to soften around 100 °C and is not rated for continuous service much above 80 °C.
Why does acrylic go cloudy after I cleaned it?
Almost always solvent damage. Alcohol, acetone and glass cleaner craze the surface of PMMA. Clean with water and a microfibre cloth — the clouding is permanent.
Is acrylic unsafe?
No — PMMA is safe and stable in normal use. It is simply not glass, and it will not age like glass.
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