A brown or yellow mark on a pressed cotton or linen sample is scorch: heat, contact time and pressure have chemically degraded the cellulose. Shine is different: the fibre surface has been flattened, so it reflects light like a mirror. A tech pack lowers both risks by stating the iron setting, steam, press cloth and pressing side.
This post covers the fault, not the crease. Why cotton, linen and viscose hold or lose a pressed shape is covered in Pleats That Hold and Pleats That Fall Out. Here the subject is what heat does to the colour and surface of the cloth on women’s dresses, tops, kaftans, linen co-ords and men’s shirts.
Why does a cotton or linen panel brown under a hot iron?
It browns because cellulose, the polymer in cotton, linen and viscose, starts to break down chemically once heat, pressure and contact time add up. The colour is a by-product of that breakdown, not a stain sitting on top.
Three factors decide when it starts:
- Temperature. Hotter scorches sooner.
- Contact time. A held press scorches at a lower temperature than a moving iron.
- Pressure. A heavier press scorches at a lower temperature than a light one.
The clearest measurement is old but direct. A 1936 US Department of Agriculture study ironed cotton sheeting on a controlled ironer. For contact times from 2.4 to 30 seconds, the scorching temperature fell from 254.5 °C to 141.5 °C at low pressure, and from 198 °C to 132 °C at high pressure (Downey and Elmquist, USDA Technical Bulletin 517). So more time and more pressure both lower the temperature at which cotton scorches.
The colour comes from oxidation and dehydration of the cellulose:
- Below about 300 °C in air, oxidation speeds up the breakdown of cellulose and forms hydroperoxide, carbonyl and carboxyl groups on the chain (Shafizadeh and Bradbury, Journal of Applied Polymer Science, 1979).
- Carbonyl groups are the smallest unit that produces colour in cellulose. Carboxyl groups alone do not, but they strengthen the effect when carbonyls are present (Ahn et al., Cellulose, 2019).
- On the other route, cellulose heated between 150 °C and 250 °C starts down the path to char, releasing water, carbon monoxide and carbon dioxide. Infrared spectra of treated fibres show carbonyl and carbon–carbon double bonds (Fliri et al., Journal of Analytical and Applied Pyrolysis, 2023). The exact chemistry is still debated in that literature.
Scorch is not only cosmetic. In the USDA study, cotton whose strength had been cut by 10 percent through heat and pressure lost about twice as much after one more laundering (USDA Technical Bulletin 517). The authors also suggest that protruding surface fibres scorch first.
Viscose needs extra care. It is cellulose too, but its care-label limit is lower. GINETEX, the body behind the care symbols, places viscose at two dots (160 °C maximum soleplate) and cotton and linen at three dots (210 °C) (GINETEX care symbols). A viscose kaftan pressed on the cotton setting is outside its label limit. For the difference between the two names, see Rayon vs Viscose.
Why does a pressed garment go shiny, and does the shine stay?
It goes shiny because heat and pressure flatten the fibres on the face of the cloth, and a flat surface reflects light in one direction instead of scattering it. Whether it stays depends on how far the flattening went.
A US patent on rayon fabric describes the mechanism. Under heat and pressure the fibres deform and form flat spots. When there are enough flat spots, the fibre “begins to act like a mirror” (US Patent 6,511,928). In the patent’s own test, pressing shone two fabrics, and the shine was more noticeable on the black one. In severe cases, it says, the shade changes completely.
That is why dark colours show shine most. A navy linen co-ord, a black viscose dress or a dark cotton shirt shows a polished patch that a white or ecru version would hide. A university extension guide says dark cottons should be pressed on the wrong side to prevent shine. It says the same for linen, and warns that too much heat, steam or pressure can dull or flatten the fabric surface (New Mexico State University, Guide C-309).
Does the shine stay?
- Light shine on cotton, linen or viscose can sometimes be lifted. Steaming may lift slightly flattened fibres, according to a fabric-care article from a silk supplier.
- Heavy shine on a dark panel can change the shade outright, as the rayon patent describes. Treat that panel as a reject.
- Washing can raise the risk. The same patent notes that rayon fabrics that are washed, and so lose their resin finish, shine badly under a hand iron. A viscose top that has been garment-washed may shine more easily than the unwashed fabric did. Garment washing itself is covered in Enzyme Washing Cotton and Viscose Garments.
How is cellulose scorch different from polyester glaze?
Cellulose discolours because it degrades chemically; polyester glazes because it softens and its surface flattens or fuses under the iron. One is a colour change, the other mostly a gloss change.
Cotton, linen and viscose are not thermoplastic, as our pleating post explains. They do not soften under the iron. They take heat until they start to break down, and the breakdown shows as yellow, then brown.
Polyester is a thermoplastic. Its glass transition is around 67–81 °C and it melts above 250 °C (Wikipedia, polyethylene terephthalate). An iron is well above the first figure. Heat-sensitive synthetics can flatten or slightly melt under a hot iron, and melted fibres cannot be restored (Pandasilk). GINETEX places polyester at two dots, 160 °C, the same as viscose (GINETEX). How polyester behaves in a finished kaftan is covered in Polyester Georgette Kaftans.
Blends follow the weaker fibre. Set the iron for the fibre that needs the lowest temperature (NMSU Guide C-309). A polyester-cotton shirt is pressed at the polyester setting, not the cotton one.
| Cellulose scorch | Cellulose shine | Polyester glaze | |
|---|---|---|---|
| Fibres | Cotton, linen, viscose | Cotton, linen, viscose | Polyester and other thermoplastics |
| What happens | Oxidation and dehydration of cellulose | Fibre surface flattened by heat and pressure | Fibre surface softens, flattens or fuses |
| What you see | Yellow, then brown; the fibre is also weaker | Polished patch, most visible on dark shades | Glossy, flattened patch |
| Driven by | Temperature, time and pressure together | Heat, steam and pressure on the face side | Heat that softens the fibre surface |
| Reversible? | No; the fibre is also weakened | Light shine sometimes, with steam and brushing | Melted fibre cannot be restored |
| Label limit (GINETEX) | Cotton, linen 210 °C; viscose 160 °C | Same as scorch | 160 °C |
What should the tech pack say about pressing?
It should state the maximum iron setting as a care symbol and temperature, whether steam is allowed, whether a press cloth and wrong-side pressing are required, and how the pressed sample will be judged. Write it per fabric and per shade, not once for the whole range.
The lines worth including:
- Maximum setting, as a symbol and a figure. GINETEX gives the maximum soleplate temperature for each dot: one dot 120 °C, two dots 160 °C, three dots 210 °C (GINETEX). Some references, including Wikipedia, list 110, 150 and 200 °C, so write the symbol and the temperature you mean side by side. Cotton and linen dresses and shirts: three dots. Viscose dresses, tops and kaftans: two dots. Blends: the lower fibre.
- Steam. For three dots, GINETEX says to iron damp and allows a steam iron. For two dots, it allows steam under a moderately moist cloth and says to avoid heavy pressure. NMSU’s guide says not to put steam directly on rayon (NMSU Guide C-309). So for viscose, write “steam through a press cloth only”.
- Press cloth and side. For dark shades, write “press on the wrong side; right side only through a press cloth”. That follows the NMSU guidance for dark cottons and linen, and AMANN’s advice to use a pressing cloth on delicate materials (AMANN).
- Pressure and dwell. The USDA data shows that pressure and contact time both lower the scorch point. For viscose and dark linen, ask for light pressure and a moving iron rather than a held press.
- Trims and prints. GINETEX advises extra care with embossed or applied decorations. Note any embroidery, print or trim that needs a lower setting than the base cloth.
- Test before bulk. Ask for a swatch of the bulk fabric, in each dark shade, pressed at the stated setting and returned with the pre-production sample.
- How the sample is judged. State that pressed samples are checked for yellowing on light shades and for shine on dark shades, viewed at an angle to the light.
A short block for the tech pack might read:
Pressing: 100% linen, navy. Max iron 3 dots (210 °C soleplate). Press damp or with steam. Wrong side only; right side through a press cloth. Light pressure, moving iron, no held press on seams or pockets. Reject any panel with visible shine or yellowing. Pressed swatch in bulk shade to be approved before cutting.
Fusing presses also combine heat and pressure; their settings belong in the interlining spec, covered in Fusible Interlining: Why It Bubbles, Strikes Through or Lets Go. For the rest of the document, see How to Create a Tech Pack for Manufacturers, and for linen more broadly, Sustainable Linen Trends in European Fashion.
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