Multi-colour (red, green, blue, yellow, gold) or a single color in “colour sparklers” versions.
30 seconds (for 7 cm models) according to TradeIndia listing.
Paper / wire core + chemical coating (oxidizers, fuels, binders, and metal salts for color)
“Long stick-like structure coated with sparkling composition … produces amazing colourful electrical stars” (Sakthi Crackers)
“Emits showers of sparks with color … like the stars have landed in your hand” (OS Crackers)
The sparkler consists of a metal or steel wire (or iron/steel rod) coated with a pyrotechnic mixture.
That coating contains:
Oxidizer (e.g., nitrate / perchlorate) to provide oxygen for combustion
Fuel / binder (e.g., a combustible mix + binder to hold everything together)
Colorant / metal salts (e.g., for green: barium compounds; for red: strontium compounds; for blue: copper compounds; etc.)
When ignited, the oxidizer helps the fuel burn, causing small bits of metal to heat, incandesce, and eject as glowing sparks, colored by the metal salts.
Because the sparkler is short (7 cm), the coating is also thin and burns quickly, giving a short but bright colorful spark display.