The efficiency and the lifetime of a solar pond depends mainly on the behavior of the gradient Non-Convective Zone (NCZ), embedded between an Upper Convective Zone (UCZ) and a Lower Convective Zone (LCZ). The NCZ is often the siege of instabilities. These instabilities can destroy the linearity and transform it to a well mixed layer leading the pond to loose its main role of storing. Series of laboratory studies have been carried out to assess the stability of a linearly salt-stratified system (simulating the NCZ of a solar pond) heated from below. An assortment of flow visualization by Shadowgraph and Particle Image Velocimetry (Ply) techniques was employed to provide a phenomenological description of flow convection. This experimental characterization reveals three specific stages. The first stage corresponds to the onset of a non periodic oscillation in space and time of the bottom flow. In a second stage a well mixed layer is born and an oscillatory movement appears at the free surface whereas the last stage is relative to the transition from linear stratification to non-linear one (two superimposed layers separated by a well thin interface), before the homogenization of the whole system is accomplished. Most theoretical and experimental studies in the literature, considered a gradient layer heated from below at constant heat flux, whereas in this work the free surface system is heated from below at constant temperature. In addition, we will focus on the onset and development of the first mixed layer for small values of stability parameter Lambda (values that can be encountered in a real solar pond. In the vicinity of our laboratory a 3 m-deep solar pond of 1500 m(2) area is available). To our knowledge, no investigations has been performed, on the birth and self-organization of the first layer for small values of Lambda. (C) 2010 Elsevier Ltd. All rights reserved.