diff --git a/src/easyreflectometry/project.py b/src/easyreflectometry/project.py
index 6ac00ab1..4480b828 100644
--- a/src/easyreflectometry/project.py
+++ b/src/easyreflectometry/project.py
@@ -24,6 +24,7 @@
 from easyreflectometry.model import PercentageFhwm
 from easyreflectometry.sample import Layer
 from easyreflectometry.sample import MaterialCollection
+from easyreflectometry.sample import Material
 from easyreflectometry.sample import Multilayer
 from easyreflectometry.sample import Sample
 from easyreflectometry.sample.collections.base_collection import BaseCollection
@@ -211,22 +212,22 @@ def path_json(self):
 
     def get_index_air(self) -> int:
         if 'Air' not in [material.name for material in self._materials]:
-            self._materials.add_material('Air', 0.0, 0.0)
+            self._materials.add_material(Material(name='Air', sld=0.0, isld=0.0))
         return [material.name for material in self._materials].index('Air')
 
     def get_index_si(self) -> int:
         if 'Si' not in [material.name for material in self._materials]:
-            self._materials.add_material('Si', 2.07, 0.0)
+            self._materials.add_material(Material(name='Si', sld=2.07, isld=0.0))
         return [material.name for material in self._materials].index('Si')
 
     def get_index_sio2(self) -> int:
         if 'SiO2' not in [material.name for material in self._materials]:
-            self._materials.add_material('SiO2', 3.47, 0.0)
+            self._materials.add_material(Material(name='SiO2', sld=3.47, isld=0.0))
         return [material.name for material in self._materials].index('SiO2')
 
     def get_index_d2o(self) -> int:
         if 'D2O' not in [material.name for material in self._materials]:
-            self._materials.add_material('D2O', 6.36, 0.0)
+            self._materials.add_material(Material(name='D2O', sld=6.36, isld=0.0))
         return [material.name for material in self._materials].index('D2O')
 
     def load_experiment_for_model_at_index(self, path: Union[Path, str], index: Optional[int] = 0) -> None: