Roll the gather on the marver into a cylinder, blow one short, sharp puff into the pipe to start a bubble inside the glass, then enlarge it with gentle breath and reheating while rotating. The bubble is called the parison, and the aim is an even wall: thin spots blow out and thick spots do not stretch. As a bubble grows its wall thins in proportion to the inverse square of its diameter, so doubling the diameter cuts the wall to a quarter.
Part of Glass Blowing and Forming
Starting a hollow shape in a gather and growing it into the parison from which the vessel is formed.
The First Bubble Fixes The Wall Thickness You Will Have To Work With
The carrier’s six steps start from a gather and end with an evenly walled parison. This article explains why even walls are the whole task.
What You Need
- A centred gather from Gathering, at working temperature.
- A marver and the pipe held horizontal.
- Reheating available at once in the glory hole.
Method: From Gather To Parison
Step 1: Roll on the marver into a cylinder.
Step 2: Blow a short, sharp puff. The glassblowing article says to seal the far end of the pipe with a thumb or cork; you should see the gather expand slightly and feel resistance followed by give.
Step 3: Check the parison. For a typical vessel the first bubble is small and centred, about 3-5 cm in diameter per the glassblowing article. If it is off-centre, marver it back before inflating further.
Step 4: Enlarge by gentle blowing. Never blow hard; steady pressure gives even walls.
Step 5: Reheat as needed. The glassmaking article says 5-10 reheats are common for a job.
Step 6: Aim for even wall thickness. The carrier states thin spots blow out and thick spots do not stretch.
Worked Example
A thin shell of glass with volume V (the gather, 220 cm³ for a gather about 7.5 cm across, as 4/3 × π × 3.75³ = 4.19 × 52.7 ≈ 221 cm³) blown into a sphere of outer diameter D has a wall of about t = V ÷ (π × D²), as a thin-shell approximation.
- D = 10 cm: t = 221 ÷ (3.14 × 100) = 221 ÷ 314 ≈ 0.70 cm, or 7 mm.
- D = 20 cm: t = 221 ÷ (3.14 × 400) = 221 ÷ 1,257 ≈ 0.18 cm, or 1.8 mm.
Doubling the diameter divides the wall by four (7 mm to about 1.8 mm). That is why the carrier’s advice to blow gently and reheat matters: the glass has no spare thickness at large sizes. The thin-shell formula is an approximation that gets worse for thick walls.
Common Mistakes
- Blowing a large first bubble. The wall is already thin before the shape is set.
- An off-centre parison. Marver it back before inflating further.
- Blowing while the glass is too cool. The carrier says glass below its working range cracks when forced.
Safety
A bubble can burst
A thin parison that bursts throws hot fragments and can burn the face. Keep the face to the side of the pipe’s axis and wear eye protection.
Hot glass does not look hot
Glass fragments below about 600 °C look cold while still burning skin.
What’s Next
- Learn next: Shaping and Finishing — forming the parison into a vessel
- Combine with: Mold Blowing — inflating the parison into a mould