  We address the theory of quasi-static crack propagation in a strip of glass
that is pulled from a hot oven towards a cold bath. This problem had been
carefully studied in a number of experiments that offer a wealth of data to
challenge the theory. We improve upon previous theoretical treatments in a
number of ways. First, we offer a technical improvement of the discussion of
the instability towards the creation of a straight crack. This improvement
consists of employing Pad\'e approximants to solve the relevant Weiner-Hopf
factorization problem that is associated with this transition. Next we improve
the discussion of the onset of oscillatory instability towards an undulating
crack. We offer a novel way of considering the problem as a sum of solutions of
a finite strip without a crack and an infinite medium with a crack. This allows
us to present a closed form solution of the stress intensity factors in the
vicinity of the oscillatory instability. Most importantly we develop a {\em
dynamical} description of the actual trajectory in the regime of oscillatory
crack. This theory is based on the dynamical law for crack propagation proposed
by Hodgdon and Sethna. We show that this dynamical law results in a solution of
the actual track trajectory in post critical conditions; we can compute from
first principles the critical value of the control parameters, and the
characteristics of the solution like the wavelength of the oscillations. We
present detailed comparison with experimental measurements without any free
parameter. The comparison appears quite excellent. Lastly we show that the
dynamical law can be translated to an equation for the amplitude of the
oscillatory crack; this equation predicts correctly the scaling exponents
observed in experiments.
