a glass tube with a first end, a second end, an outer diameter (d1), an inner diameter (d2), and a glass thickness (s1);

a circular glass bottom that closes the glass tube at the first end, wherein the circular glass bottom comprises an inner surface directed to an inside of the glass tube and an outer surface directed to an outside of the glass tube;

a longitudinal axis (L_tube) that passes through a centre of the glass tube and the circular glass bottom; and

a curved glass heel extending from an outer end of the circular glass bottom to the first end of the glass tube;

wherein the outer surface of the circular glass bottom has a topography defined by a function ĥ(x),

wherein ĥ(x) is an azimuthal average of a distance between a contact plane and the outer surface at any given position that is located on a circle having a center that corresponds to the centre of the circular glass bottom and the radius |x|,

wherein the contact plane is the plane on which the glass tube rests,

wherein individual values ĥ for ĥ(x) are determined for a plurality of circles the radius of which increases stepwise by 500 μm, starting with a circle around the center having a radius of 500 μm,

wherein the individual values ĥ are determined in a range from x=0.4×d2/2 to x=+0.4×d2/2, d2 having a size such that at least 4 values ĥ are determined,

wherein the individual values ĥ can be fitted in a least square fit with a curvature function

wherein c and h₀ are free fitting parameters, and

wherein Δc is the standard deviation error for constant c when fitting the individual values ĥ(x) with the curvature function and wherein the relative standard deviation error Δc/c is less than 0.1.