diff --git a/src/model.cpp b/src/model.cpp
index 082b6fa..7b35ade 100644
--- a/src/model.cpp
+++ b/src/model.cpp
@@ -14,7 +14,7 @@ Model::Model(Store &store, const std::string &name)
 {
   std::ifstream file(filename(name), std::ios::in);
 
-  Mtllib *mtllib = nullptr;
+  const Mtllib *mtllib = nullptr;
 
   std::vector<glm::vec3> tmp_positions;
   std::vector<glm::vec2> tmp_tex_coords;
@@ -86,7 +86,7 @@ Model::Model(Store &store, const std::string &name)
     else
     if (line.substr(0, 7) == "usemtl ")
     {
-      _material = mtllib->materials[line.substr(7)];
+      _material = (*mtllib)[line.substr(7)];
     }
   }
 
diff --git a/src/mtllib.cpp b/src/mtllib.cpp
index 105153d..65a487f 100644
--- a/src/mtllib.cpp
+++ b/src/mtllib.cpp
@@ -23,7 +23,7 @@ Mtllib::Mtllib(Store &store, const std::string &name)
     if (line.substr(0, 7) == "newmtl ")
     {
       material = new Material();
-      materials[line.substr(7)] = material;
+      _materials[line.substr(7)] = material;
     }
     else
     if (line.substr(0, 7) == "map_Kd ")
@@ -32,3 +32,8 @@ Mtllib::Mtllib(Store &store, const std::string &name)
     }
   }
 }
+
+const Material *Mtllib::operator[](const std::string &name) const
+{
+  return ((Mtllib*)this)->_materials[name];
+}
diff --git a/src/mtllib.hpp b/src/mtllib.hpp
index a0e8291..ab92ecb 100644
--- a/src/mtllib.hpp
+++ b/src/mtllib.hpp
@@ -13,7 +13,10 @@ class Mtllib:
   RESOURCE(Mtllib)
 
 public:
-  std::map<std::string, const Material*> materials;
+  const Material *operator[](const std::string &name) const;
+
+private:
+  std::map<std::string, const Material*> _materials;
 };
 
 #endif // _MTLLIB_HPP_