docs: def refractory_VDI_k(ID, T=None):
      r'''Returns thermal conductivity of a refractory material from a table in
      [1]_. Here, thermal conductivity is a function of temperature between
      673.15 K and 1473.15 K according to linear interpolation among 5
      equally-spaced points. Here, thermal conductivity is not a function of
      porosity, which can affect it. If T is outside the acceptable range, it is
      rounded to the nearest limit. If T is not provided, the lowest temperature's
      value is provided.
  
      Parameters
      ----------
      ID : str
          ID corresponding to a material in the dictionary `refractories`
      T : float, optional
          Temperature of the refractory material, [K]
  
      Returns
      -------
      k : float
          Thermal conductivity of the refractory material, [W/m/K]
  
      Examples
      --------
      >>> [refractory_VDI_k('Fused silica', i) for i in [None, 200, 1000, 1500]]
      [1.44, 1.44, 1.58074, 1.73]
  
      References
      ----------
      .. [1] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
         Berlin; New York:: Springer, 2010.
      '''
      if T is None:
          return float(refractories[ID][1][0])
      else:
          ks = refractories[ID][1]
          if T < _refractory_Ts[0]:
              T = _refractory_Ts[0]
          elif T > _refractory_Ts[-1]:
              T = _refractory_Ts[-1]
          return float(np.interp(T, _refractory_Ts, ks))